Solidity : learn the fundamentals to become a smart contract developer - 2022 - | Dave Slater | Skillshare

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Solidity : learn the fundamentals to become a smart contract developer - 2022 -

teacher avatar Dave Slater, Develop the World that you dream of

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Taught by industry leaders & working professionals
Topics include illustration, design, photography, and more

Watch this class and thousands more

Get unlimited access to every class
Taught by industry leaders & working professionals
Topics include illustration, design, photography, and more

Lessons in This Class

    • 1.



    • 2.

      Lesson 1 - Remix IDE & Docs


    • 3.

      Lesson2 - Variables & Functions


    • 4.

      Lesson 3 - Arrays


    • 5.

      Lesson 4 - Mappings


    • 6.

      Lesson 5 - Conditionals & Loops


    • 7.

      Lesson 6 - Enumerated Lists


    • 8.

      Simple Storage


    • 9.

      Rent a Car


    • 10.

      Coin Minter


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About This Class

Who is this class for?

This class is for anyone who wants to start learning smart contract development using the Solidity language! Here we will cover the fundamentals in a step by step method.

Most of the tutorials and courses I followed focus on creating some type of project from the start, which is a great way to learn, but they also go very fast through the basic and I used to find myself not understanding why they write code in a certain way. 

Instead, in this class you will learn the basic concepts of the Solidity coding language and the basic structure of simple smart contracts, as well as learning how to read the solidity documentation.

The goal is for you to complete the class with the ability to understand the basic concepts so that everything will make more sense to you when you dive into more complicated subjects.

What you will learn:

  • Remix IDE, Solidity versions and documentation
  • Data types, variables, and Structs
  • Function
  • Arrays and Mappings
  • Loops if/else statements
  • Enumerated lists and function modifiers

What you will create:

Along the way, we will create some basic smart contracts:

  • Simple Storage contract;
  • Control a car rental
  • Mint and transfer coins

The class is made for anyone with little to no knowledge of coding!

Find the files about this class in the GitHub Repository for Solidity

Solidity Docs: Solidity Docs

Remix IDE: Remix IDE

Music in the intro:

Meet Your Teacher

Teacher Profile Image

Dave Slater

Develop the World that you dream of


Hello, I'm Dave. I am a developer advocate, tech enthusiast, pilot, and flight instructor! I have an extensive teaching experience from my flight instructor background. During my programming journey, I learned so much from the community, and I want to do my part to give something back!
Here I will focus on the Solidity language to create smart contracts and python, especially python, to interact with the blockchain.

I have worked on many projects and helped many teams launch their DApps.

What can you expect from me?
- Clear and step-by-step explanations.
- Support and interaction if you have doubts or questions.
- Enjoyable classes that will help you during your coding journey!

I structure my courses to be beginner friendly; if you have zero... See full profile

Level: Beginner

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1. Intro: The blockchain is an amazing technology. When I started to learn solider, there was very excited about all the opportunities are all the ideas that I had in my head. But then I started noticing that all the courses and all the tutorials, they teach the fundamentals while they called some smart contract. And they're just really scheme a little bit unfair to paste it very quickly. That makes it difficult to learn this language unless you have some other type of programming experience. Because of that, I decided to make this course to help everyone who wants to start learning how to code smart contract in the solidity language, even without any programming experience, we're gonna go over the business structure of a smart contract and the tools are simply gives us. You will learn how to use the remix IDE and how to leak through the documentation. The main roadmap consists in all the fundamentals. At the end of this course, you will be able to understand variables and functions, arrays, mappings, conditionals, loop, as well as an enumerator, list AND function modifiers. Then along the way, we would also code some more practical smart contracts to help to solidify what you've learned during the course, we will make a simple storage contract, a contract that allows you to keep track and rent a car, and even a contract that allows you to mint and send coins to different addresses. My name is Dave, and I hope you will enjoy this course. 2. Lesson 1 - Remix IDE & Docs: Let's start right away. Solidity is a language you need to know if you want to become blockchain the middle there. Now, solidities the language that interface with the Ethereum Virtual Machine. And that's the software that then is going to ultimately go and interact with the theorem blockchain. Now, we can start learning about solidity from the documentation they release is actually very extensive and very useful. You can find it at If we explore, this website has a lot of information that we don't have to cover right now and it's introduction. But one thing that I want to point out is the version that's very important. If you go down here, you can identify every version that they release. Right now we are selected on latest. So we are looking at the documentation for the latest version possible, which is this 0.80.13. This one here. If we go and look at the other versions, for example, version in four, here, we can see that it's actually very different. It is from a few years ago. Everything is so different, there's way less information. Now this is to highlight that it's very important to use the latest version because they actually change syntax. So for example, syntax that you do you are using on version eight will know work if you're trying to compile it with a version for compiler. And we will see more about this later. But to go and talk more about solidity. Solidity is a high-level language and it is taking inspiration from C Plus Plus Python and JavaScript. So you are already familiar with these languages is going to greatly help you because the syntax are very similar, especially to JavaScript. Now, where do we write solidity? We actually don't have to install anything on the computer, which is great. There is a great website,, that gives you all the tools to do everything. We can write the code, we can compile it, we can deploy it to the blockchain and we can test it. We can test it in a similar than environment or even on a magnet. So this is the remix IDE. Here. What is an ID? Id stands for Integrated Development Environment. And compared to just a regular cold that it'll gives a little bit more tools. For example, if you're already familiar with programming, you might know other softwares like Atom and VS Code. Atom is considered regular code editor because you can code, yes, it gives you a few extra things compared to a text editor. Gives your syntax highlight and things like that. But it doesn't give you any extra tools. For example, VS Code. Vs code gives you a virtual terminal where you can test yourself to runaway and remixes. Pretty much the same thing for solidity. It has a free irregular interface when you click on their main icon where you can notify open files and things like that. If we're trying to get a new file, here is already up and it gives us an example. Here. This is a contract. Right? Now. We can cope in this environment. We can also choose a compiler and compile it. And here, for example, is where the versions come into, come into play. As well as compiling it. We can deployed and tested. Now let's talk a little bit more about the versions that I want to cover it today. Here. Every contract, every time you start coding solidity, you will start with these two lines. This is a comment to identify the license of this smart contract that you are coding. So to give a little bit of history here and why the compiler likes to have that license identifier. And top is because the blockchain is essentially an open source environment. And having a source where your code is coming from is very important. If you go and look for the ESP edX license identifier on Google is going to bring you to the website that gives you examples of all the different licenses that you can have. When we're gonna call it our things. In this course, we're going to use the MIT license because it's the most open source. You can. If we can take this code and do absolutely whatever you like with it. The circle line that is always going to be there is the pragma solidity. Solidity gives the compiler the version of the code that you're using. For example, these lines that they have here where they have leader to equal 40416 less than 0.9 means that these contract works with the compiler's from version 0.4.1 up to burn in 0.9. So essentially the last possible version. We always gonna see some free like this or for these contrasts are we going to write during this course? I am going to type something a little bit different than the first version. And I'm just gonna leave it like this, whereas says, Actually we did eight. This format here tells the compiler that all of the compiler version late work. But now you're saying how do I choose the compiler? Well, Rubrics has it already in the interface. So if we go on to the compiler page, you will see here that you can pick the pilot version automatically is always peaking the 0.6.6. So many times we might want to code like this because then it avoids this step here. But if we want to compile with a different version, we just selected. These works with all of the version from eight. If we pick 0.80.13 and compile, it should work. And indeed shows up the icon that says recompile it correctly. Now, why I was mentioning that is important to remember which version you're using is because if we try to compile it with a compiler that is outside of this version, then it's not gonna work. Now this is gonna be the main environment where we are going to be coding our contract. But the second part that is very good about remix is that it has a section where you can deploy it wrong transaction. This is great because it allows you to test the Congress immediately on either a virtual machine or on a test net, and even using MetaMask on the test. So it's a great tool. Now, you will see how is the buyer here. You pick the environment up here on top. And usually the false. So this JavaScript Virtual Machine in loans on Oberlin, JavaScript Virtual Machine, VM means that we are trying and testing the code in a virtual environment. These connected from testis. If we, for example, want to use our mathematics to try, I don't know, on the RNC be testlet. Here I have it selected main account by away. It's just the account that I use for testing. Issues. Show any here. If we select the ring B test neck, you'll see I have some eith on the test. Then if we picked environment of injected with three, then you will see that here shows our brain PI network. We can even use mathematics to deploy and try and test our transactions. In. This system is very good for our development then is the, pretty much the only thing that we're gonna use. Then in addition to in addition to these couple of things online, there are so many good resources to learn solidity as is a relatively new language. And it's very open source. This website here to tolerance is very good at using freely, often has a very good layout of everything that all the basic syntax that you need any shows. Many examples were many explanations as well. So really recommend too deep for this one too. These are the article or website here. Blog, and guys solidarity is very good. So don't be afraid of looking out for resources and other tutorials it will help you learn. Today, we went through the very basics of the language is have an idea what the language of solidity means. And we explored a little bit remix interface we're going to use during this course. So in the next lesson, we're going to start basically over recording. In the next lesson, you will code your first smart contract. We are going to go over all the variables that we are primarily going to use, as well as an introduction to functions. Functions are very important in solidity or use, I use a lot. I want to introduce them early to start having a pretty good understanding of them. See you in the next one. 3. Lesson2 - Variables & Functions: Hi and welcome back. Today is the first actual lesson where we will start coding and writing something in the remix ID. We'll go over variables and functions in Solidity. Solidity, just as any other language has many variables available. Today we're going to explore the syntax, the different types, and I'll show you some pretty good example. So Let's go. Okay, let's start looking at some code. So once you open the really exciting is always going to show up with this example contract the way right now can just close. If we go look at the regular interface here is pretty straightforward. We can just click here on new file. Like any other code editor workspaces available. And I have just created one for these course. You can, you can either use one of the sample environment or make your own. It's up to you. It doesn't really make any difference. Now, here, we'll have a little button to create a new file or we can create reform greatly from here, new file. And we can start with a name. So in solidity, it's a common procedure to name the file with a capital initial. For example, we're going to name this one here, variables, variables, those salt. So every Solidity file is going to have the extension. By the way, in the description of this course, you will find the link to my GitHub repository where you will find all of the files that we're going to write down here, which is for many more comments. Comments usually our, not, our comments are very good practice, but you don't want to overdo them. And in these files that you'll find in the repository the way overdone just because I want to have a way to have a static file without the video that explains many things. Once we save the file is going to be MTN, as we mentioned yesterday. Every solidity smart contract is always going to start with the same two lines. The nicest certainty, fire, and the solidity, the rectum. The identifier is going to be written with the two forward selection from the two forward slashes solidity are the characters for comments. When you want to comment something, you always going to start it with the two forward slash. Here we'll do the two hormones. Pdx identifier. As we mentioned yesterday, will always gonna use. In this lesson. We always are going to use the MIT licenses is the most often one. Then the pragma directive is gonna come next. And as we mentioned in the last slide, a version. Here we'll use the keyword pragmas solidity. And as you can see, the code editor actually gives a suggestion. So this is a very efficient way to cope. And you can just press Tab and it's gonna compare it out solidity. Have the version. So in this case we're just going to use the, all the compilers conversion eight, so we can use the character than the little top accent. And then 0 dot eight dot 0, and end with a semicolon. Every instruction in Solidity ends with a semicolon, just like in JavaScript. This first instruction tells the compiler that all dead version eight should work. So we can write, for example, if you want to code this, sorry, if you want to comment these two have a reference for later so you don't forget, you can do the two horror slash and thereby whatever you want, whatever you put in the comments is now gonna be executed as code. So here for example, is say tails, the solidity version. You will have a reminder for later. Next, it comes to the actual contract salt the instruction in the contract encompassed in the contract keyword will do a contract. Then after the keyword contradict comes the name of the contract. In solidity is also a great practice to name the contract the same as the file. So in this case we'll just name it capital V, variables. Variables. The two brackets here. So every time the structures are inside the two brackets in the contract. Now let's start talking about variables. So just says in any other language there are many variables are the variables can be numbers, which in the programming world that are commonly referred to as integers, there can be strings that are just a collection of characters, like letters, names, but numbers can also be used as strings. They are just not going to be recognized as numbers. And so you're not gonna be able to use those numbers to do mathematical operations. There are Boolean variables. Boolean variables is just a true or false. So you can set up a variable to be true or false, and that can be used to maybe trigger another instruction later on. Solidity has an actual, actually a few special variables that are a little bit more specific to the blockchain. So it has an address variable that only accept the address numbers. And it has by 32, which is just strings, but is convert into bytes, and it's a little bit more efficient. We will talk more about later. Now, let's start exploring the first cause the syntax. So how do we write a variable, the oldest bride that variable type first, then the name equal to a value that can be a string, a number, or whatever the variable is above. And then the semicolon. Now in solidity, if we only type the type and the name like this, gonna be initialized directly to 0. So for example, if you want to have a integer variable that starts at 0 when the contract is, is launched or when the instructions com, then we'll just put it out like that. Now let's go over the actual syntax. So if we want to make a variable for an integer, we're going to write something like int, the name number. Let's call this variable number equal ten. In this case, we just created integer variable name number, which is equal to ten. So one thing to explain about the equal sign here. If you are not familiar with programming languages, equal, It's not really a new colon, this case, unequal here is more like an association. We are associating the number ten to the variable called number. Now, in Solidity, numbers have different sizes. And as you might know for from general computer architecture, data in computers is more than bids. And so when you have an integer like this name, just int, it means that this variable can accept numbers with a sign that can be minus or plus up to 256 bits. Why would you want to specify the size of the number? Well, because in crypto and blockchain, we've worked with very big numbers in all your work with numbers and maybe have 18 decimals and things like that. You'll want to make sure that the variable that you use except the right size. Now, if you know the, you are going to work with a smaller number. You don't have to use the 256 bits. You can use a smaller one. You can create an int where there is only eight bits and you will the syntax that you can have increments of eight. So you could have in aid, it can have in 16, and then you will name it another number equal to one, for example. Now one thing that I want to explain a little bit more is you see how I wrote the num, the name here, another starting with the non-capital initial and then the next word. It's with a capital letter. This is another convention in programming. It's called camelCase, playing camelCase typing, where you basically make separations between different words just from the initial. So you will see most of the variables, most of the variable name. You will see typed in a way similar to this, where the first word is lowercase initial, and then it goes to the uppercase initiative. For example, is an integer variable. Salinity can also have another integer type that is called an unsigned integer. N is usually the one that you will see the most insulated it. And you write it, typing you and then the name, unsigned, unsigned number. Equal 50, for example. This basically tells us that we can only use positive numbers in this one here, for example, int number we can call it. We could associate it to negative 10th and it will work just fine. But the uint8, you can only associated with positive numbers or unsigned numbers. Then next time, when we mentioned earlier, it will be a Boolean. Boolean is start with the rule keyword. We can call this one a Boolean name equal to true. When we're going to recall these variable, for example, in a function later on during the contract. The function knows that this value is true and so we can use this. Have the function do something. I don't know if the variable Boolean is true, then write something. Next variable that we can look at his address. Addresses are very common in solidity. S is the way to transfer currencies. How to work with the contrast. So same syntax. Address, my address. As we mentioned, this one only accepts addresses. We're going to put it between the double-quotes. Let's say I want to use, in this case, I'm just going to use the address that I have here in my mathematical copy. Now this variable address called My address is associated to this theorem address right here. And then later on we also will, we will also see how to, for example, have the contract pick the others automatically when it's deployed and reuse later. Things that is, the last variable do not want to mention right now is the Thirty-two. Thirty-two is a type of variable. We can just type some bytes, equal a string. That can be here we have a regular string. Actually, I forgot to mention the regular string that we'll talk about that after this. So here we have a strain. As we mentioned, a string is just a collection of characters. The difference is that in this case, buys 30 to convert these strings in bytes. And just basically for this software is a little bit more efficient. It's usually a good practice because every time that your code in, in solidity, more code you have an heavier the numbers are and the characters are, costs more gas to use my transaction. If you can make it better, efficient way, it's good because then you will the customers or the contract, we will use less gas. Then this is gonna be converted in bias and you will need to have a front-end to convert it back into string. But anyway, the last one actually here, it's a string type. That is just a string. And we said a string is a collection of characters. So just this can be a word or a letter or anything with a character. Here are the main variables that you can have in solidity. And just in a little bit after this, we will explore. Other variables are able to now assume a, as you may see here, these variables here are all right in the contract, they're not inside the functions. So these variables here are called state variables. State variable is a variable that is available to the entire contract. We can, for example, call these function. Calls are called these variables perma function without having to maybe write it down again or without having to specify it again, because it's already available to the entire contract. If we have variables that are inside of function, those are called local variables. And these variables are only available inside the function, so you will have to be careful with where you specify your variables right? Now in Solidity has an extra, let's say an extra instruction for variables because variables can be looked at from inside the smart contract or from outside the smart contract. So when we type a variable here, let's say that we want to write on the right. So we do int second number equal to 20. Now we can specify if this number can be visible from outside the contract. So by everybody or just inside the contract. In here between the name and type of variable. We can add another keyword. In this case, we cannot public. And public means that this variable can be called or can be read from outside the contract where it can be an application. It can be another contract or some other type of software can access these variable, read the content. In this case, you can even modify. So everyone can see and change, which isn't always ideal. To visibility. Visibility types that we can have internal and private. Now if we make a u that is internal. Now these internal keyword means that this variable can only be modified by the contract n by the right contracts. Contract is just another contracted is inheriting the values from this contract that I mentioned a little bit more about that later. Internal can only be modified by this contract. Contract. The last one that we have available for variables is private. Misspelled it, Let's call the private. It's a little bit see moral, internal. But this means that the private, only exclusively this contract can modify these contracts. This means that these variable cannot be narrowed headed by another contract or product. You could, but then the other contract cannot change it. These are called disabilities. State can be applied to variables and functions. When we write down our variable, we can apply a visibility state to make sure that it does the things that we want them to do. Now. Now that we've talked about variables and we know how to type a variable. Let's briefly talk about functions. Might feel like we are introducing functions a little too early but insulated it that are very important because you basically do everything just with the functions. I want to start going through them soon. When we do some finalists you understand and have an understanding of what's going on. So let's go through the syntax. So the layout of a regular function will be starting with the type or with the keyword function. We always started with the keyword function, then the name of the function, parenthesis. The parenthesis, we can type parameter. Parameter. A parameter in a function is simply something that will pass. The function, I always had troubles understanding this concept in the beginning. Let's say that we want to make a function that takes a number and add ten to it. Somewhat can be somewhat useless. But it gives the example. If we take, if we want to give you a number and then add ten, we will pass the number here. Dislocation as the parameter. When you specify a parameter and a function, is there anything you always want to put the parameter type? Then the parameter name. For example, let's say we want to pass in a number and we will pull you in if you want an unsigned integer. And then the name of the variable. In this case, the name is not extremely important in the sense that unless we want to pass it something that is already known, we can just call it whatever we want. Then after the parenthesis and the parameter will pull the visibility state. That for functions can be just variables and it can be public, internal, private. Then we'll have the state mutability. Functions can do things on the blockchain. The statement abilities specifies what the function can do. And we have, we have different options. We can understand usability. Usability can be pure, which means that the function does not do anything on the blockchain. It doesn't have access to it. It does not read anything of the blockchain, does not write anything on the blockchain. So when would you use this? We will use these in case we are just doing something internally. For example, this function that I mentioned already that we pass in a number and we want to add a number to it. There will be a pure function because it just does all the computation internally without ever touching the blockchain. Then another mutability state that we have is view. View means that the function read something of the blockchain, but it doesn't change. For example, if you want to call a function that reads variables that we wrote, we want to take a function you can access from outside the contract and call it and read what this, what's in this variable and other number, then there will be a view function because we are reading something off the blockchain. Then after we specify the mutability state, we can have our return keyword. Now return is used in case you want the function to return some type, some type of value. When you use the return, actually here is returns. And when you use the return here in the header of the function, you will have to specify the type that you want to return. It can be want to return an under, then you will write in or you it. Then after here, we'll have the brackets. And inside the brackets we will have the actual function console is just a general layout syntax for a function. Here. Before we type the actual function, I noticed that I had a few mistakes and what I wrote before here where we wrote this train, I realize that before I put string as a name. So it didn't, doesn't like that because string is a type of variable, so it will recognize it. It will expect a name, but he thinks polluted the things that you tried to write another typing, just change the name to something else characters or something that is not Up type of variable. And another mistake that I noticed was that the address was type between coats, which will not work. Because what do you tie between quotes like this? String is not going to use the string as an address. So what happens if you leave, leave it like this when you try to compile it, it's going to give you an error. There's a pretty good example of how re-mix can help you because it identifies everybody tells you even volts the areas for example, in this case. It says that the string is, the type is a string and it cannot be converted to an address. So what do you have to do to fix this? We just remove the quote. Now, let's continue with the function. Type. The function. We'll start with the function keyword. We want to make a function, as we mentioned earlier, that we pass a number through and it gives us a value of the number plus something else. For example, we will name it, add a number. Always do the camelCase conversion. We are going to pass a parameter. We want to pass a number type. We pass our view int name. In this case, given number. Here I put an underscore in front of the name. So one of the other conventions in solidity is to name a parameter is used in setup variable with an underscore in front of it. You might see it is. If you explore our contract, That's just what it means, is just a convention to name the parameter. Then we want this function to be public. Because we want to access this function from outside the blockchain. Now, this mutability is going to be pure because this function is not going to read or change anything from the blockchain. This is just going to take a number the way give in to another number that the function already has. So it does everything internal. And then this function is going to return Ru it because we want to be to take another number back. Now the keyword here is returns with the S. Remember, then after the parenthesis pace in brackets, the space in brackets after the definition of the function is just not a conviction in solidity. If you go through the documentation is going to explain all these things under the style. I believe it's just a common way to do it. Now how do we tell the function that we want to take the number that would give, throw them. Do sampling. First of all, we can add another variable in it. Then let's add the full number inside the variable can be called value. We assign it the value of ten. Now, these uint8 inside the function, some local variable. This can only be used inside this function if you try to call it from outside, the function is not gonna work. There's not gonna be able to see, to give instruction to the function. We want to take the value of this number here and add it to whatever number that we pass through. So how do we do that? We type the name of the variable that we want to change, equal to and then instructions. So what do we want to do? We want to take value, the number that would pass through to it, plus number semicolon. This is actual operation, mathematical operation that is gonna be done. And then how do we tell the function that we want to have a way to see what the result of that is. We use the keyword return. Return. What do we want to return value? So now this function, it's going to take a number that we give ten to it and then return the result that we can see. Once we're complete, we're done with this variable. We can try to compile it and test it. So now, how do we do that? We go on the compiler page. Here. We have to decide what compelled to use the standard compiler that issue show up if you didn't do anything else before it is 6.6. Well, we're using the version eight. We will have to change to one of the version a. I'll use 8.13. And then this button you can compile, the file, could compile. And if everything goes right, which doesn't show up with a little green tape, but it gives us an error. So let us see what these iterates. And it tells you that it expected a semicolon got brackets. So can you spot what I miss here? Yes, it was a semicolon here. Now, these will compile or should compile. So we're trying again, if you don't want to go on the page and click Compile variables every time you can just do Control S, which is what I usually do. Control S, it saves it a composite and as you can see this time it is the little green tick. That means that the compiled correctly and there were no mistake to test it. We can go on to deploy and run transaction page. We mentioned this previous lesson, something about this. Test it like this. We are going to use arbitrary environment. So we can just keep the JavaScript VM that we have here. Who's going to give you a bunch of accounts available. So you have all these accounts are available for you to get to the point contracts and test them. And each account has a 100 ether. Will do slip the standard 1 first. And here we can depart. If we click Deploy, we can see here in the dashboard here that there is a green tick. That means that the control was deployed successfully to look at what we've done here. And it's gonna show what the public is going to basically show all the public functions and variables. For example, here we can see that we have a public number, which is a function, which is our function here. And number, we have something else that we can see. Second number, why is that? It's because here we specify a variable that is public. So we can see what happens when I look, when I click here, it's going to tell me what's inside the variable. Here. You can see it's telling me that variable is associated. We wanted to look at our function. Let's expand this so we have a little bit better. What does this do? We can call this function inserting a number here, see if it works. We said that we wanted to add a number that we'd pass to this tab. So if I add another ten, it should give us a value of 21. We call it, we call it here, it gives us 201525 and saw these very simple and basic example of how a function could work. Then once we read this contract, testing, if we can just destroy it and it's gonna go back, gonna go away. And we could redeploy moment quantification report and so on to the students. One thing that I want to highlight here, It's here in that account. Remember that when we started, we had 108 and now we have 99.9 something. Why is that? Remember that every time that you make a change on the blockchain, which it can be just even read from it. It's gonna use some gas. Remember, we specify that our function is pure, so the function doesn't do anything on the blockchain dysfunction should not use gas. But we called the value from these public state variable here. That's what it consumed over gas. So just keep that in mind. More things you colon here higher the gas is going to be. Now before we end the lesson, I want to mention one more type of variable that I didn't mention with all the other ones. Just because there's a little bit more complicated and confusing, birth is going to be very, very useful for many things. These variables called a struct. Struct is a type of variable that allows you to store multiple variables in it. That then can be called layer from functions and other things. It's very useful. Now how do we call distract is just a variable, right? So there's going to be, the syntax is gonna be like they are variables. We'll start with the type of variable which the type is struct name. Let's say that we want to create a struct. Inside has names and February colors of people. We can call this name, sorry, we can call this truck peoples. People struct. We call it people struct. Now, conventionally name instruct the capital letter for the whole, for the first word. Just as a reminder there. Now, we'll have the brackets and inside the brackets, we can type the variables that we have inside. So we said we wanted to have a struct with the names and February colors. Those are two strings. So we'll have string name, semicolon, and then stream. Call. Semicolon. Here, we've just created a struct. Struct allows us to hold this information inside. Now this struct here doesn't have any information because we didn't pass it through. And one of the next lessons we're going to check or rather see how to add information to the stretch from functions. But we can initialize this trucked pre-recorded information. The syntax to do that is starting with the name of the stretch. So the pupils strapped. We can do it public so we can see what information is in there. Then we'll add the name. We're doing now is just basically keeping our variable inside the struct with the information that we have in there. We wanted to have a name, so let's call the variable Dave. These variable is going to have, is going to be people's strengths. So we put the name of the struct again to tell that this is what's in there. And then we pass the parameters through parentheses. There are two strings. The first one is going to be a name. The second one is going to be a color, orange. Let's say. Then let's not forget the semicolon. So now we have initialized the struts. So that means that inside the strategies are variable called day. It has associated the string name today, even the string color to orange. We can test it and try to deploy it. So if you try to deploy Control S, we see that there are no errors. We did want to take. We can go back into the deploy page and deploy it. We can see the successful can go check. Now we have an extra natural variable here called Dave. If we call it, is going to show that we have a name, variable name called Dave in it, and string variable called or a color, orange. And we can just add more information to this tract like this. You can copy paste it. And another one, for example, Charlie. Let's say the Charlie's favorite color is red. Recompile it again with Control S, We Cubans contract here to deploy it. We check. We have Charlie. Charlie is associated with Charlie and red. Now this is a very inefficient way to use a strep. And in the next lesson we will see a more efficient way to do it. In conclusion here today, we went through the different types of variables. We went to state and local variables. This friend visibility states. Then we also went through function, basic function. I wanted to go over this mostly for the syntax and just the basics under. In the next lesson we will go over arrays, how we can use them in different situations. 4. Lesson 3 - Arrays: Hello and welcome to the lesson about a race. So today we're going to talk about what arrays are and how we can use them in Solidity. Here we have remakes open and we'll start with the usual two line with the license identifier and the solidity version. I also created a new file here called the race in case, in case you want to type it in a new file, otherwise you can just type it in the variable file that we created. What are the race? A rays are actually a very common object in programming, and they are essentially just a list of elements. We can use arrays to store multiple objects in them. It can be numbers, it can be strings. And in Solidity can even be struct, or other types of variables that we have. In solidity. We have two different types of arrays. The first one is called the fixed size array. Dynamic size array. The dynamic size. So what's the difference? The fixed size it has declared size. So it means that it can only contain the specified amount of elements that we declared in the beginning. If we declared that the arrays only made of three elements, let's say three u, and then we cannot put four in them because it's going to give you an error can only be three. While the dynamic size means that elements can be either as we want them, we will explore and show a function of how to add the elements. How do we start this contract we're going to type, we're going to write a contract today called the arrays. Here our control is done. Now let's look at the syntax to write a race is a variable so we can start it just like any other variable with the type of array that we want. So let's say that we want to have a fixed type array that contains for you. And we will start with the type uint8. Then the square brackets. Square brackets is kind of the universal sign or symbol for arrays or lists in programming languages. And then inside the square brackets we define the fixed size. So in this case we'll put four. If we wanted to have a dynamic size, then we wouldn't put anything in the brackets. So you went for, we're going to declare this one here as public so we can access from it outside the contract, will give it a name. So let's say we wanted to put numbers and let's call it numbers, array numbers. To put some data in it, we can just do they equal square brackets and then we put the data, let's say that we want to put 1234 separated by commas. And so here we have it. Here we have fixed size array that has four unsigned integers in it. Let's say that we want to create a string array. How will we do that? We will type string the square brackets. Then we can put inside. Let's hit it. We want to have another fixed one, just one element in it. There will be public again so we can access it, will give a name string array inside. We can just have one string inside the double-quotes, and let's say is color yellow. This one is again another fixed size array and can only contain one element because we specified that is only one. Let's try to code dynamic size array. We will do that. Let us do a dynamic size array for numbers again. So we'll do U int because it's the type. And this time I was like I spelled it wrong. This time we're gonna leave the brackets empty because this is a dynamic size array, we can change will prove with the public again, and this is dynamic. Dynamic arrays, the name. Let's put the number ten in it. But remember that I forgot the semicolon here. But remember that because this is a dynamic size array, we can add how many elements we want in it. This array, like this. Is what are called a one-dimensional array because they only have, is only one array in there. We can also create something that are called to the arrays, or two-dimensional arrays. And essentially those are simply an array containing other arrays. There are some limits in solidity. We can make arrays with the, we're calling them nesting. We're calling this next thing. So there are nested arrays. We can nest up to 15 arrays, and they all must be in the same type. Let's say that we want to create dynamic array of arrays. So we create a dynamic array. So this will be the first one. And then inside we nest other arrays. And let's say that we only put two misspell this again. This is public. Then we call it an esteem. Now, how does this work? We just declare an array. This one, this part here we declared an array, has two arrays in it. So how can we picture this? Here is the first array, and then it has other two inside one, comma n2. And then these two arrays can have numbers in them because these are all uint8, so 123123. Here we go. Here now we have a 2D array containing two arrays. And how can we access the information inside the right? That's a very important topic because we need to be able to take the information and the data from it. So arrays are what we call index-based means that every element inside of an array correspond to an index. Arrays in Solidity are what they're called zero-based. Index. Zero-based then means that the count of the elements starts from 0. So if we take this first right here with the UN, and it has four, if we want to access to the arrays, we are now going to find the number one. You're looking for the index number one, it starts from 0, so this one will be 0, this one will be one over V2, and this one will be three. We'll see when we deploy the contract that we want to access this array. If we want to take the number one, will have to type the 0. If we want to take number two will have two type one and so on. Now we just saw how we structure an array and how we can, we can syntax one, but how do we add things in it? Let's say that we have these. Now this one here cannot be addled because it's fixed. But let's say we want to add numbers to these dynamic array here, we will have to write a function to do that. How do we do that? We wrote a function just like we were doing the last time. And let's call the function add number. We'd write the function, the function keyword, the name, and then we want to add a number to that array. So we will need to pass a parameter in here. We're going to pass in the function are uint8. We called number. So underscore number because underscore because we use it inside the function even if it doesn't really have a difference on the name. And this function is public so we can access from outside or we can actually add our number. In this case, we are now going to return it because we're not going to put the return because we don't need to pull any number. And if you notice, we're now going to pour any modifier state because this function actually goes and changes something on the blockchain. So the view and the pure wouldn't know make your work because if you remember, pure means that the function doesn't do anything on the blockchain at all. And view it means that we're only read. While in this case we are going to actually write on it. Then the two brackets here. To add elements to arrays, we use the push method. How do we use methods is that we take the name of the array. In this case the dynamic array dot push these the syntax. And what this does is takes the parameter that will pass through the function and pushes it to the array. And it's just going to basically add the number. Next. We're going to pass the parameter that we typed, the function name, the function header. And that's semicolon. Here. This is a very simple function to add a number to the original number here. Now we're ready to test it. Salt, test the contract. Now we're going to have to compile it. Let's go check. Our compiler is the right version first, because we're writing solidity version eight. And we can see we have the compiler for version eight, so we can go ahead and compile it. Here are just press control S That's a little bit quicker. Going to deploy in transactions section. We have our JavaScript virtual machine on account and go ahead and deploy it. Received the transaction was successful, the control was deployed. Now if we explored the contract, you will see we have a whole bunch of stuff. And that's because pretty much every variable and function that we have here is expressed as public. So we can see all of them. Now, let's start from the first array, the number right here. You'll see that the name's gonna be a little bit more mixed in. I think it's just because they display them in alphabetical order, but that doesn't really make a difference. Let's start from the numbers array here, which is going to be this one here. We said that to extract an element, we can use an index since the arrays are indexed place and they are based on zeros starting from 0. So if we want to extract the two here, for example, what index we're going to have to type here will start from 01. So if we type one in numbers array and we call that, then it's gonna show us right here on extract the form. We're going to have to extract 0123. If we call it. Here, we have the form. What happens if we try to call for an industry doesn't exist. For example, this one is fixed. A fixed size array with only four elements. So what if we tried to call for a fifth element? Type the four here, here's gonna give an error. It will say that the call was not successful. Here it's not going to show anything because this one here is from the previous call. The same thing works for the other race. For example, the string array here. It only has one element. So if we go look for the next zeros, is going to show that we have yellow in there and our dynamic array. Same thing as only one element. So if we go for 0, it's gonna have the ten. Now, the nested array is a little bit more interesting because remember these are nesting nested array. So we have arrays into an array. And here you can see that it is calling for two inputs, is the same principle with the zero-based index, where the first input, it's asking basically for which array we are looking for, if it's this one or this one. The second input is asking for the position of the actual element that we want to see. In this case, let's say that we want to extract the three here. We want destructive from the first array. There will be in position 0. Then we want to extract the third element. So 012, b two here. If we call it, here we go. We have extracted the three. And the same thing can be done with the Ottoman. If you want to extract the file. We'll pass one here because it's the second array and 01 call. And we'll have the five. That's how you will explore nesting nested arrays. You can see how it can become pretty complicated issue can have a maximum of 15 arrays in there, and then you have to start calling him for 15 arrays inside other arrays. It can refer to the complicated. But let's explore and look at our function. That's if you work. So we design dysfunction to add an element to the dynamic array. Let's look at here. What is a superstar happen? We're supposed to put a number here, major transaction, and then we should be able to recall for another number out of here, let's say that we want to add 100. We made the transaction, were considered as transaction was successful. Now in theory, we should have two elements in this dynamic array. So how do we call for lemon and number two, we're gonna have to go and call for one here. And here from instead of ten, it should show 100, which thus, we can just keep adding numbers as we please. Let us say 60. And then we'll call to and saw. Now we have an, a dynamic array that has multiple elements in it. But how do we know how many elements? Because for this one only has three. If we throw, if we try to call for another one, not gonna work because it says it doesn't really say anything actually, but it's because we don't have more than three numbers right now. So how do we know the length of the array? How many elements we can write a function for that. So let's go back into the editor and let's write another function. We can run a function called count elements. Will count how many elements are in the array. In this function, for example, we don't have to pass anything because we are not trying to add elements or anything like that. We're just trying to see how many elements are in there. We will have public visibility because we want to access to it. Since we're not writing things, we're just reading from it is going to be viewed. We want to receive number back a result. So in this case it's gonna be returns and it's going to return a uint8. All right. Now, there is another method. We saw the push method to add elements to an array. We can have another method to see how many elements are in there, and that's called the length element. The function is also very simple. We can just make this function that return the name of the array. So the dynamic dot length, remember dot is how you access the methods and the semicolon access methods, you will put the name of the variable that you want to access dot the method. And then we can compile it so we can destroy these contract controllers to compile it. And you'll see that it works as an NADH here I want to show you something about the solidity in the room mics idea. We said that this function only only reads from the blockchain, so we don't need to write anything and we can put it in view. But what happens if we don't? What happens if we don't put View? We tried to compile it. It's going to compile it anyway, you'll see that it doesn't give you a fatal error. The icon is orange. Basically means compiled with a warning, say compile compilation finished successful with warning. Remix gives you basically suggestions about your code. For example, in this case, is able to understand what the function does and how it can be restricted. For example, the warning says Warning function statement stability can be restricted to view because it understands that these function is not writing to the blockchain was reading. It tells you, hey, why aren't you put it as view? And it just makes it a little bit cleaner. Let's continue here with the test. We are ready to deploy now is already compiled. We're ready to deploy as deploy. And here we have these one here now is to reset. The contract is reset to the beginning. In the dynamic array we only have one number. If we do count elements, gonna show one because it says, Hey, there is only one element in there. Let's add, add a bunch of them. Let's say we add 40. Then we add two. And we add 1 thousand. Now, we didn't keep track of how many elements are in there or we simply don't know. We can call the function count elements. And it doesn't look, there are five elements. And let's say you want to know what is the fourth one? The fourth one will be 0123. So now you can go on the dynamic array and call the element in position number three. And this is going to take the data element is 89. If you call position for the element is 1000 and so on. So this is a situation that can be useful when you have an array with many elements and you simply want to understand which value is at a specific index. I don't know. You'll have an array of addresses and you know that address in the index 25 has to do something. And then you can go and take that index and recall the number for the others in that case. Here was a quick summary about arrays and how we can ride them and how we can make them work. Next lesson is going to be about mappings. Mappings are very, very important element or variable in solidity because it allows us to associate values together. It's a very important lesson. See you next time. 5. Lesson 4 - Mappings: Hi and welcome back to the basics. Today we're going to explore mappings. Mapping is a very important datatype in solidity and it's actually similar to a database. In a sense, mappings are made of qi to value. There's so you will have a key with a number associated to a specific value. If you're thinking there will be similar to arrays, is not quite because as we remember, arrays start from 00 index space while mappings or not, they started from one just like it could be a database, the SQL, they start from one. In a way you can see as a database, but it's on the blockchain. Looking at the basic syntax, how you will hire, we'll call them mapping. You will start with a keyword mapping. Then mappings parameters are inside parenthesis. And we said that we have a key associated to value will have to specify what VIA what variables, what type of variables we want to have. So let's say that we want to associate number to a string. So we will have u int that is a number. Then the equal and bigger than. That is. It makes a narrow in this case. And the second data type, sorry, variable type that is associated to. Then we'll need to have the visibility state and the name. So in this case we can just call it example semicolon. So this will be the basic syntax. And now we can start our contract and we will call that a little bit more examples. The contract restarts as usual with the keyword contract. We'd call it mappings. Here we go. Now, let's put them on wrapping up mapping. In this case, we will just use it doesn't make it the same as the example, your Int associated to a string. But what we want to do is that we want to associate a number, data number to a color. Let's say that we want to have colors. You are planning to make an app that describes and lists the player choose different colors. So you'll want to put it through a mapping. We can call it colors. Here we have the mapping. The mapping, obviously it's empty because we didn't put anything in it. There are different ways to code data, data into our mapping. One way is a special function that we haven't seen so far. It's called the constructor function. You can write it just with the keyword constructor, the parenthesis, and the brackets. So what is a constructor function? Call is defined as a special function because it only runs once at the start of the contract when the conduct is diploid. And so we can use it to, for example, associated the address that deploys the contract as the owner. Or we can use it to place some standard data in it. And in this case, we can use it to fill up the map in one when the contract is deployed. From here. How do we fill up a mapping? In this case, the mapping names, colors. And mappings also are indexed, but they just don't start from 0. So how do we define the different key-value pairs will put the name of the mapping, the value of the key equal to the value that is associated with. Right now we want to have a uint8, the one associated with a color, registering that in our case wants to be a color. Let's hit a report. Three of them will do callers. The second one, the second colors, color, sorry, the third color, green. By convention, we want to use double quotes for strings. This is a simple constructor function. Then when we deploy the contract, gonna fill up the mapping automatically. Less try it out and see how it works. We can compile it with Control S. I always forget to change. The compiler virtual visits. So I'll put the last compiler version, recompile it. It doesn't give any error. And let's go into the course successfully. So let's, let's see how this looks. You will see here we have our mapping called colors and we can see it because it's public. Any receives a uint8. So let's say that we want to recall what value is associated to the key number two, then we'll type two would call it. I will have yellow. We call number one, will have read. And if we call three, will have green. These are very, very nice way to associate different datatypes. Right now replaced or you into associated with a string. But we can place string or strings, or we can place addresses were numbers and strings. And we'll see a few of these options later during the, during this lesson. Now, this is one way. This is one way to initialize or fill up a mapping, but we can run a function for it, can run a function for anything. Let's write a function that allows us to add values, key-value pairs to the mapping. We'll start with function. We'll call it at color. In this case, we'll have to pass a few parameters, right? We have two parameters to pass because we want to add a key and a value. The parameters will be a uint8, though we can called index because this will be the index of the mapping a string. Now, this is the first time that we see how to pass a string through a function. Whenever we pass strings, will have to tell the function what type of memory we use. When we type memory, it means that the parameter is going to be stored only during the calling of the function and it's not permanently, permanently stored on the blockchain. And this is the preferred way because it saves gas phase. And then we'll just type the name of the variable color. In dysfunction will pass an index number that is going to go in this part here of the mapping and strained with a color that is gonna go on this part of the mapping. We can make the function public. We want to call it from outside of the contract. The principle is exactly the same that we use here. We will type the name of the mapping, in this case the index. Instead of hard-coding manually a number we'll type, we'll pass in the variable index, and it will be associated with whatever string that we pass through. Now we can, we can combine it, Control S, compiled correctly, that's free. Deploy it. We should have a function available as well. Here it is. Let's see, We call the number three again. It shows up as green. Let's say we want to add a fourth one where we can E14 comma and then we want to have orange instead. Then we call the function at column. It was processed it correctly. So now if we call number four, here we have orange. And well, in this case it's just a string or we can write really wherever we want. If I type Apple is still going to pass the airport because it's just a string. An interesting function is that we can basically override the previous contents. So let's say I don't want to have read the number one anymore. But number one I want to have purple. Number one, purple. That process. Now if I go look up what number one is and how it's purple. So this is a more dynamic way to construct mappings because we can pass functions through them right? Now. These are new way to explain it. Now let's do something a little bit more elaborate with mappings used to have. Another way. The interesting thing and why it makes it so powerful is that we can associate mappings with pretty much any variable. So we can make a mapping with key associated to a struct. And inside the struct we can have multiple variables. So let's say that we are making a game, our game the works on the blockchain where players can have cars to play with race. So let's start writing down a struct that is going to keep, going to keep track of what cars each player has. We can have a struct name. Car will have two string variables inside the struct, we'll have a variable name maker, wherever we can save the brand of the car. Austrian with the model, the car. Though we have here we have our strengths. So now we can, we can create a mapping that associates player with a car. Let's say. Let's create a function. First. We'll create a mapping. Will make a mapping. For now, let's keep it simpler. We just associate number to the structure just like we did before. We'll have a uint8. They'd gauze associated with, in this case, the struct car like this. And then it will be public. And renaming cars. What do we do here? We are just coding that this database that we can have is gonna have a number as an index and the struct as a value. So whenever we call this index, these two variables are gonna show up. Obviously now we have to make a function to this course, to the struct enema. Now, we can call it ADKAR. Will have to pass three parameters this time, because we will have to pass the uint8 for the index and the two parameter from the for the struct itself. So the first one of BUN, and we can call it index again because is a good mnemonics for us. Then we'll pass a two strings. Remember the string is mammary. We can call it underscore maker. Then we'll pass the second string memory. Underscore model. This one is gonna be a public function as well. Same principle of the regular mapping. We're not gonna call it course this time. It's the mapping this time is named cars. Let me scroll down a little bit. Just named cars. We call out the mapping cars where we're going to associate the index. Here. We'll have to build the struct. Struct. The structural member is name car with a capital C. Will pass the two-variable Maker model. Now we have made were completed the function to fill up this mapping. Let's compile it and test it. Deploy it. Here. Now we have the other mapping with the ADKAR. These mapping here is empty. We didn't put anything inside yet. If we call it cars. If recall something in cars, not going to show anything, it just says that there is a string, a maker in a stream name model, but there is nothing associated with it. Let's add, gonna extend this so it makes it a little bit easier. So let's say that the index number one, we have a bin WWW, we'll add Albion W, and the model is three-thirds. We start transaction and it was successful. So now if we try to, we try to read the first position in the mapping cars, going to show that we have our BMW and 330. Then we can add to the Lamborghini. Transaction and number two, and if we check it. So this is an example of how you could use mappings to transfer or keep track of multiple data. Now let's check something interesting here. Obviously, on the blockchain we work with wallet addresses. Why not create a mapping? We can always use it for the car, let's say for the cars game, where we associate instead of just an index that we called manually, we associate an address. So we can make that we associate IP address of whoever calls the function. To fill out warm, make a model of cars they have will have to create another mapping for this, because we are going to create a mapping of another mapping is sort of like an ester mapping. Let's have the mapping where the first type of variable is going to be an address. Instead. The key is going to be an address and the values are gonna be another mapping. The mapping is going to be made of the struct, basically that we created. The struct is going to have the uint8. This second mapping is going to have inside the UN. And the car's going to be a public mapping. We can call it player cars. Here too will have to make a function to this information. It's gonna be very similar to the R1. We can call it a player car. The layer car. In this case, we pass again only three parameters. We can actually just copy from here because we're gonna be the same. Because the address is gonna be taken automatically from the transaction. And I'll show you in a second how to do that. The public as well. Now how do we take the address? There is a simple command in solidity. Will have, we want to fill up the players, player cars mapping. So the index on this side here, the address is going to be m as G. Sender is a new keyword that we are seeing now, MSG center stands for Message Center, and it basically takes the address from the wallet that calls a function and stores it. In this case, what we do is that we take the address and put it here in this first side of the mapping. Then on this other side, because we are creating an atom mapping in it, we'll have the index. The other mapping. On this side we'll have the struct car. Inside the car, we have the maker, the module. Here we go. We can compile it. It worked the point of contract. Now the point here is going to show a few more things. Will have a player cars mapping down here. I'm not play your car. The player car has three variables because we want to add an index here. I just started the spot number one. We're going to add up in W again three-thirds. If we make the transaction, we'll see that it went through correctly. Now, because we call these transaction, these first spot in the mapping with the BMW 330 is associated to our address. If I go back up here and we take these datas that we use, if we copy it, paste it here and the player car, we can the index as well. We call it, shows that we have been W3 30. Now, we could make it just associated with the address. I just wanted to in this case, I just wanted to show how we could put a mapping in sign-on and mapping. But to make it a little bit more functional, we could just use the address without using the other mapping. Remember the message sender is a very important quality because we're gonna use it a lot to associate addresses with data. For example, if we take a matter address and tried to pass a year, when we call it, nothing shows up because there is nothing associated to this address. Here are mappings. I would recommend to go read some of the documentation in solidity about mappings because these are a very powerful, very powerful system that we're going to use a lot. Next lessons, we're going to go through if statement conditionals and loops. And so these are going to wrap up a little bit of the basic. And then after that, we're going to start writing some smart contract that actually do something a little bit more practical. 6. Lesson 5 - Conditionals & Loops: Hi and welcome to the lesson about conditionals and loops. Conditional statements and loops are a very important part and the bulk of making decisions and make automations through code. So what are conditionals? Conditionals handled decisions. So basically a conditional statement will return a true or false. And then based on the value of return, we can trigger some other type of instruction. Let's say that we want to check that an address of another sign a contract is the owner of the contract before we trigger a specific function, or in a more simpler and abstract way. Let's say we have a variable and we want to check that the variable is bigger than another one. For example, when you want to send coins to another address, while we will have to check that the balance is bigger than the requested amount. So in that case then, if you have enough balance and you can trigger the function and send coins. Let's check how we do conditionals. In Solidity, conditionals are simply the if and else statement. So just the regular syntax would be if some conditions are met, then trigger something else. If they're not met, trigger something else. Let's create a contract. Though we can call conditionals. Let's write it the same or tried the same name on the file. Conditionals. We wrote a function in it. So let's say we want to have a function that passes a number and checks if this number is bigger or smaller than a specific value. If he's bigger, returns true, and if it's smaller returns false. It could be a very simple, It's a very simple function just to see how the statement can work. We've make a function, we can call it big number. Big number. And we said we wanted to pass a number in it so we can check it against a preset, another preset value. So it will be a uint8. And we can call it number this function. We want to have it public because we want to access to it. This function is going to do everything inside the function itself so we can write it as pure. And we're going to return something. We'll type returns. Now usually we return up to now we just return a UN, today we're going to return a different value because we said that conditional statements return a true or false value. And if you remember from the, one of the first lessons, true and false values are Booleans. He's going to return a bool. Going to return a bool variable. How do we write the code? We will start with the keyword if. And then in the parenthesis we're going to have the conditions. Now we want to check that the parameter that we pass is the parameter that we pass is smaller or bigger compared to a number. So let's say that we want to check if the number that we pass is bigger than five. This is just an arbitrary number that we said. Now as an example, then we have the brackets. After the if statement. And in the brackets, we're going to type the instructions that are going to be executed if this condition is met, let's say the way pass a number that is ten, the function is going to check, is ten bigger than five? Yes, and then return something. In this case, we said we return a bool variable, we're simply going to return true. Else is a second keyword that indicates what is going to be a return in case the condition is not met brackets and inside the brackets we will return false. So what this function does, again, we pass a parameter in it. We check if the parameter is bigger than five, if it's true, is going to return true and it's false, it's going to return false. And then after this, based on what we get back, we could trigger something else. For example, the number. If this function returns true, then Call another function or add some other data. Let's test it out. We can compile it. Control S is going to give me an error because I didn't change the compiler, knew it. Let's make sure that we put the version eight compiler. We can compile. It. Looks like I forgot something else. Let's read through the air or usually forget. I work a lot in Python. And Python doesn't really use semi-colons at the end of the instruction. So I always forget, let's try again. Now this time compiles correctly. Let's deployed will have on the eyes new address with deployed, deployed successfully. And let's check what's in the contract. Here we have a function name, big number, and accept a uint8. So if we put four, which is smaller than five, we should return false, which indeed does. If we put seven, it will return true. So these are very abstract and simple way to see how these conditionals work. But that's really just the basic conditional statements. Now, let's talk about loops. Loops are a very important part because they allow you to basically iterate through multiple data all at once. And then you can combine it with conditional statements, like if to trigger some instructions. So let's try to do this. We want to create a function that reads an array and count how many elements are in it in that array. So how would we do that? We will have to start typing an array. How do we replace the array? We type the type of array, uint8. It's going to be dynamic array, public. Let's call it values. We can play some values in it. Or you could just write a function that first devalue like we made in the previous lesson, and then maybe counts through them. Let's just write some values and we write ten numbers in it. Now we want to have a function that iterates through these numbers and through these elements and count how many of them are in there. Let us do that. We'll write a function here that we can called numbers, count elements. This function is not gonna take any parameter because it's just a function that does something without the, without us inputted. Gonna be public because we want to, we want to access it. And this one does something on the blockchain. This one reads off the blockchain because this array is on the blockchain, right? This one is going to be a view function. It's gonna return you int. Because we want to loop and iterate through this array, count how many elements in it. And then we want to get the number back, is going to give us back. Now this function has to start with a variable. They keep trucks of the loops. Or rather the keep track of how of the items in the array. So this is called, this is defined usually as a count variable and it's going to be a number. So it's gonna be a uint8. Then we can call count. We can just leave it like this because when he starts is initialized to 0, or you could write count equal to 0, but we can just initialize it like this so we see how this works. Then we have to lie to write the loop. In this case, we're going to use a for-loop. Four-loop basically follows the instructions that we give and loop through how many times we want. How we syntax that we'll put the parentheses here. And we had to tell the, we have to tell the loop where to start. Have a uint8. We can call AI that could stand for index, but remember that these numbers are arbitrary. You can write, you can call them however you want. It will start at I equals 0. This means, Hey, start the count of your loops from 0. And then we want to write some sort of condition on here. Say, Hey, keep going until something happens. In this case, we're going to keep going until the index of the account. The loop matches how many elements are in the array. For this, we use a method on the array. That is the array dot. Length. Length basically returns how many items are in there. If we do less than values, dot length. Then this means keep going until the index of the for loop matches how many items are in the array. Then the point after that, after each loop will do I plus, plus I plus, plus means at the end of each loop adds one, add one to this count, will go through these elements and then we add one, and then we go through these elements and add one until we match the length. These I could also be I equal I plus one. That's what the plus plus means. It basically means add one to the previous count. But we can write it like this. It looks a little bit more efficient. We'll have the parentheses or brackets. And this is the instruction that tells how the Loeb should work inside this structure here. But then inside the brackets, we'll say what it does each time it loops through. What do we want to do? We want to count how many, so we want to do that. Each time we lope throat, we add one to the count here so we can keep track of how many elements are in there. How is that going to work? We're going to do I'm sorry. We're just going to do count. Plus, plus means add one to whatever it is income. So if count now is equal to three, while after the next top is going to be four and so on. And then outside the brackets we will return the value of count. So this means once the loop is done, return the count was forgetting a semicolon to, alright, let's compiler or Control S and see if we can test it, will destroy this contract we were deployed. And now we have a few more elements. Here we have values, which is the array that we, that we created. Remember how we can use these to access the array. So let's say that we want to see one number is in position 5012345 should be six. If we put position five, it gives us six. But now we don't know how many elements are in there. How do we check it? We wrote a function for that we do count element. And the loop is going to tell us, Hey, there are ten elements in there. It's going to loop through each one of them and keep track of it. Now if we add a few of them, redeploy the contract and tested again. Now we have 12. There is a very simple for-loop to keep track of something. Now, let's combine a for-loop with the conditional statement. Let's say that we still have this array, but we want to count how many of these numbers are bigger than five, for example. Then we will run a function that combines the two of them. How do we do that? We're write the function. We can call it. Let's just call it how many. Call it how many? This function doesn't a nozzle neither again, any parameter because it's just going to do everything internally. It's public function, it's view because we are going to access the array. And it's going to return a uint8 as well. Because we want to return how many numbers are bigger than five? You will need to define a couple more variables again. Here. We are trying to count something. We'll have to use another account variable just like we did before your event and we call it count again. And just as before, it's going to start from 0. Then we can write the loop, the for loop. It's gonna be pretty much the same as before. Because we're still going to loop through all the values in there. So we can just copy this previous for loop that we wrote and paste it in here. Now, we can do it in a couple of ways. Toward the conditional statement. We could simply take the value of I, compare it to a hard-coded, hard-coded condition. So we could do if, well, we'll have to extract the content of i. The conjugal view will be what's in the index of the array. So value, I will do so if what's in the array, it's bigger than five. Well then do something. What do we do? We add a number here to count. Else is not gonna do anything in this case because we don't really need to do anything else in case the value is not bigger than five. But we are still going to return a uint8. So we want to return after the loop is done, we want to return the count. Not forget those here to. This function is going to loop through the array. And if the value that reads from the array is bigger than five, is going to add it to count. So in this case, how many numbers are bigger than 51234, 567 numbers are bigger than five. When we call this function, we should get seven. As a result. Let's try to compile it, see if it goes. It doesn't. What did I do here? I misspelled the name of the array C. I typed value. Why the name of the array values? Let's compile it again with controllers. This time it worked. We can apply it. Let's call the function of how many numbers are bigger than five, and here we get seven. Our conditional works. We could do it in another way because here we have already a function, we're already up here. We already wrote a function that checks if a number is bigger than five. So how could we use this function instead of hard-coding it like this? Well, inside the if statement, the values, we can just pass it through the function. The function is called the big number. Then we just pass the value of, we just pass the value of the element in the array through the function big number, which has already gone and check if the number is bigger than five. If recompile it discharges and already deployed. Let's check how many and instead gives us seven. This is a little bit better way to do it because we could then change this function to check however you want. Let's hit every instead of five, we want to check numbers that are bigger than three for recompile it and redeploy it. Then we have a bigger number because now we're checking if numbers are bigger than three instead. Very good. Okay, So let's continue. And I introduced the last loop that I want to mention today. Before we go, I just wanted to mention that these explanations and I'm making are very, I know they feel very slow, very dumbed down, but I just want to make sure that I explained the logic of how these parts work step-by-step. And just in case you're following. Not familiar with this consequent already. Programming some other languages. These concepts are very, very opposite, especially the for-loop structure like this issue code in JavaScript is pretty much the same, but there's one. I'll make sure that I cover everything step-by-step so you'll be able to understand better when you're actually want to use those. Now let's talk about the while loop. So the difference between the for loop and while loop is that on the while loop, we just continue the loop all the way until a specific condition is met. Let's, let's show an example of that. We write a function that loops until a number. Is rich in our variable. We write a function that we called go-to 0. Go to 0 function. This function is not really gonna get anything. It's going to be a public function. Will be pure because we're doing everything internally to the function will return a uint8. How do we syntax of while loop? We'll use the keyword while and then inside the keyword while. We don't have the instructions. Just saying this. Before we continue with that, we will have to add a couple of variables just like we did in the R1. So this time we can set how many times we want the loop to go. We can just create a variable, variable U int, though we can call it times that could stand for how many times we want to loop. And let's say that we want the loop to go for ten times. Then we'll have another variable. We can call how many. For example, these variables is going to keep track of how many times the loop goes. I know this is not extra practical in the real world, whereas this to show an example of how this works. So what we want to do is we want to loop ten times. We want this loop to go all the way until these variable times equals 0. We'll do if times is bigger than 0, then keep looping. And then we can write the instructions here. So how do we structured this? We want to say, we want to count how many times will loop through, but we only want to count, we only want to loop all the way until this one which is 0. What we'll do is that each time there is a loop, we're going to add 12. How many, how many variable will be, how many plus, plus. Because we want to count each loop that we go through. And at the same time, we'll have to change the variable times two times minus one. So basically what happens here is each time we loop through, we add one to this variable and retake one from this variable. So basically this is effectively with the counter of how many loops will do. Then outside the loop, we will return the value of how many will basically check how many times will. This could be just a very simple function to print, determine how many times we want to do something. We can do Control S to compile it. We have a warning. Here, says this declaration shadows on these the collision shadows and exist in the question. It's because we are changing it. And but since it's just a warning is still going to work, I just said, Hey, it's not a, it's not optimal, but it's still going to work. Just an example to see how the while loop works. What happens? We have two variables here. Wonder says 101 does is 0. When we call this function, we should simply see that the how many, because this is the only one that we return, that how many variables just equals ten? We can say it ten. Now if we want to loop 20 times, we can do that. Changed the variable times 20. Here shows how we looped 20 times. Now to check that this actually works, let's change the conditions here. We always keep looping and all the way until the time is equal to 0. But if each loop, instead of taking one, Let's take two. This means that technically we should only loop ten times, even though we have the 20 selected here. So let's try that. Redeployed. If we check it, we should just see ten, which we see ten. This was an example of how to use a while loop. Very good. Now I want to show you something more interesting than just this abstract concept. We can delete all this stuff. I want to show you something with the conditional statement. Let's say that we want to associate the address that creates a contract to a variable so that we can recall it later as the owner. And then we want to create a function that checks if. A specific addresses the owner or not. This can be useful in case you are creating a smart contract to Mint coins or a contract for a coin. And you create a main function. Obviously, you don't want to have any person that acts as the contract to be able to meet new coins. You will only have the person that owns the contract or the entity denotes the country doing that. How can we do that? Well, first of all, we can use a constructor function to associate the ulnar variable with the with the address that creates the contract. So as we remembered, the constructor function is called that with the keyword construction constructor. Remember the constructor function is only run once when the contract is created. We would ever instructions in there. How can we pick the address of whoever calls these the creation of the contract? Well, we have the message sender. We can create a variable named owner, will have to create it outside the construction. So we'll create a variable which is an address variable called owner. We can make this one public so we can access and see which address is the owner. Then inside the construction, we simply associate the variable owner with the Message Center. So MSG dot sender. In this case, as soon as the contract is created, we have the variable owner field with the address that is creating the contract. We already know which addresses the owner. Now, let's create a function to check if a specific other's calling this function is the owner, will call the function, and we can call it is owner. We're not passing anything through this because it reads it automatically. And this one is going to be a public function so we can access it. It's gonna be viewed because we can see something from the blockchain. And this one will return, again a ball variable because we want to check if something is true or false. How do we check it? We can use an if statement, will do if message senders. So MSU, those sender, if the others calling this function is the same as the owner of the address that isn't the owner. Here in this. In conditional statements, to check if something equals something else, we use a double equal. As you remember in the first lesson about the variables, we were saying that the equal we use for variables doesn't really mean. Equal means more like an association. This is this. While we want to check if something is equal, then we use the double equal, so many equals. Then what do we want to do if the message sender is the same as this one here, then we return true. Else return false. In this case. Let's not forget the semicolons. In this case, if someone tried to call this function is not the owner is going to give false and then reconstruction of the system to allow these others to do some funding, maybe not something else. Let's test it. Let's try to compile it. Control S. I spelled returns instead of returns. So many mistakes, even when you already know how things work, you're still going to make a lot of mistakes. Now we'll compile correctly. Let's check now here we have all these addresses. This address here ending in C4 is the other. So we'll use to deploy, deploy. It was deployed successfully. Let's check what's in the contract. Now we have these two, these two things that we can access. The first one here, the second one here is the owner variable. Though we specified here. If you click on this one, we should be able to see the address that created a contract. So we are creating CNES on C4. We say, okay, this is address that owns the contract. And if I call the function is owner, because we're using these otters who should return true? Which return stroke? Now let's say that we access this function from a second address, this one here. And then in CB2, the owner is still the previous artists. And if we call the function analysis false because we are not the honor, these gonna be a very useful structure that you'll see often In functions. Two means coins or maintain of t's or do something that you don't want anybody to just be able to do. Deals was an introduction to loops and conditionals. I know I might be a little confusing. If you go in the solidity documentation, you can find probably more information that could explain and even more examples. Next lesson is going to be about enumerated list. And then after that, we'll just focus on cognitive and smart contracts. 7. Lesson 6 - Enumerated Lists: Hi and welcome to lesson six. Today we're gonna talk about enumerator list and we'll see how we can use them to create application that keep track of statuses. An enumerator list simply restricts how many states variable can have. In today's example, we'll plan to create an app. They keep track of doors or all adore. We can keep track of the door to see if he's open or close and associated to the blockchain. The syntax for a numerator list starts with the keyword enum, followed by the name that we want to give to these variables. So we'll have the name, and then we will have a list of the variable conditions or statuses. Between brackets. We'll have one status. We can blend one status and then another status and so on. And this means that the variables that we call associated to these enumerator list can only have these two statuses that we have that we have selected. Let's start with the contract. Will start with the usual contract, and we can call it. Let's start putting down the first variable which we're going to create. As we mentioned, an enumerator list to keep track of a door. We can create an enumerator list called enum. Enum. We can call it dark condition. A convention is to use the capital initial for enumerator list condition. Then between brackets, we'll put the two statuses. So the first time we're going to have a variable is closed, the doors close and open. The door is open. Enumerator list, or at least the declaration doesn't need the semicolon at the end. So if you get use to put semi-colons than you might get an error which is easy to solve. Now after that, we can create variables that we can use associated. It's enumerator list that we made. So we'll start creating a door variable. We can just call it, will start colony the enumerator list, door condition, and then space and the name that we want to use for the variable. In this case, we can call it a door. Let me just change this. We can call it a door. We can do multiple things. We can create another one for a window if we want to keep track of a window as well, for example. Now, we need to have ways to associate this door variable width. One of the conditions that we have stated, we create a function for that. The function will start with the usual keyword function, and we'll call it an open door. We don't have to pass any parameters through the open door. Will make it public so we can access it from outside. Here's how we change the status of the door. The variable, we'll name the variable name equal door condition, which is the name of the in order to list dot one are the statuses. So in this case we can open. This is how this function will modify whatever state is in right now to open. And we can do the same thing for having a function that declares it as closed. So we have function is only can call it close door public as well. And it will be the same thing, door equal dork condition dot closed. Perfect. Now we have these two functions that allow us to change the condition of the door or whatever variable that you are keeping track off. Now how do we know what the current condition is? I guess what we'll have to create a function for that. Solidity is a lot of functions. So we can do a function called Get condition is going to be public. This one we can write it down as view, since it's gonna read something of the blockchain is gonna read what? It's doing, it's gonna return something as well. What is going to return? This one is gonna return the enumerator list is going to return the condition here. We can put that is going to return the door condition type of variable. The texts of the function is simply going to return the door. Which means return whatever condition variable is it. Right now? We don't know, we don't have a way to know which condition is the standard. We could do it as a constructor. So before should ask, alter the variables, we could put a constructor function. Will do constructor. The constructor does. Remember the constructor only runs once when the quantity is created. And we can select in which condition they're, the door is going to be solid. Say that as a standard. When the contrary is created, we want the door to be considered open. I'm missing something out here. So we have to do is just declare that the door will be open. Condition that open. These ensure that when the contract is created, the door is already open. Let's try it out. We'll try to compile it. And let's see if the compiler version is the correct one and is not change it. Control-s to compile it doesn't give any error. So that's a good sign. And we come here and Deploy, deploy their contract successfully. And let's go check it. We have all these things that we can access. Now, let's say we want to check and make sure that the door is actually open. So we, we assume that it is open because the constructor is, let's use the Get condition function. We can check. So we'll do Get condition gives us one. So one is the second element inside the different statuses. So these are works a little bit like the arrays 0 index base. So the first element is going to be 0, the second element is going to be one. Sorry, It tells us one. So we know that the door is open here. Now. We want to close it. So we call it the fashion closed door went through. So in theory now the door should be closed and if we call the condition is closed again. We can use the Open Door condition function to reopen the door and so on. This is a good way to keep track of something on the blockchain. In the next few lessons where we actually going to code some contracts. We're gonna see how we can, how we can tell the blockchain and external applications. There's something changed in this status here. These were sort of a short lesson since there was only to cover the enumerator list. In the next lesson, we're actually going to code as my contract. I mean, we've been caught in smart contracts so far, but we're going to code as smart contract. They could have a potential real utility in practice. And it's gonna be very simple. You're CA, will be only a few lines are called. I'll see you in the next lesson. 8. Simple Storage: Hi, Welcome to the single storage lesson. Today, we'll type a contract, an actual contract that we could use for something and it's actually very common contract. Then you can also find in the solidity documentation, It's referred normally as simple storage and we'll use it to store and retrieve data. Let's start digging a little bit more into the professional side and see how we can plan and design contracts. Usually contracts are designed following some sort of steps. You will always have the pregnant directive on top, then the contracts name. And then we will specify all the data and the variables. And then we'll have the functions. It's always this type of structure. And so in the next videos, we're going to just work on coordinate smart contracts. And we always going to use this type of structure and layout. The first version that we're going to create a simple storage contract is simply gonna be storing a number. We're going to create a contract that has a number variable, then a function to store the number and a function to retrieve it, That's it. Then we're going to work on the second version where we can plan to maybe make something that it could be useful for a contact app where you will store and being able to retrieve name and phone numbers. Let's start unless they're following the structure that we mentioned. We said we always gonna start with this solidity with the primary directive. So that's what we have here ready. And then we'll type the contract name, the contract keyword, and then we call this simple storage. Then the first part of the contract, we said that is always data in variables. Today we only have one variable and it's gonna be a uint8. We can call, went too far. There. We can call user number, for example, because it's the number that we want to store. Here. We don't associate it to anything, so it's always initialized to 0 at first when the contract is created. Then we need two functions, one to store it and one to retrieve it. By now you can probably even typed those functions by yourself. So we'll do the function keyword will call it store, will have to pass a number through it. I was going to be a UN we can call number. Then this function has to be public because we want to access it. And that's it because this function is going to make changes to the blockchain, because this is going to change this variable here. Then the next step is should be very simple. We just take our variable user number and associated to whatever variable we pass Through the function number. And that's it. While the second function is going to be the function to retrieve it. We can create a function, we can call it retrieve. In this function we're not passing any parameter because we're only looking to take a number that is already there. It's public and view because we're going to read something from the blockchain and it also has to return stuff. Because we want to return our uint8. And now we only have to make a return instruction and we want to return what? The user number that is stored in the contract. And that's it. In just these few lines, we created a functional contract that actually follows a structure that you see in an actual real smart contract. Something else we could see in an actual smart contract will be comments. We will see something like this. If you go on the ether scan and pull up a contract from a cryptocurrency or some other protocol, then you will see that It will see comments like this to a close of functions obviously. In this case here is a little bit superfluous because these functions are very obvious. But you will see something like this in a real contract. Function to store a number. Here, just a simple comment. And then another comment here says function to retrieve the number. Now we can start adding comments. A little bit of explanations. Obviously this is all, this is pretty common and obvious to you. But let's say that you called this contract and then someone else has to work on it. It's very good practice to leave some comments so they can understand what's going on. Obviously don't start commenting absolutely everything like put here. A u in variable. Here, associate the number. There will be a little bit overdoing it. And you can actually be a little bit confusing for people that read it from outside, but some, some basic comments to understand the logic behind the contract is very good practice and I recommend to do it when you can't. Now, the files that you look there, you find them. The GitHub will have a lot of comments and that's just because they are trying to explain something that let's test this contract and we can compile it. Control S is always. This time. I thought it was the compiler know this time is one of the usual mistakes that I make. Compiled correctly. We can kill them In previous contribute we have from here, put deploy back. Here's our contract. We can store a number. Let's say we want to store 50. Then we retrieved the number 50. Very good. 100. We retrieve it on a 100. Very good. I also misspelled retrieve. This is the logic behind these very simple contracts. Now, let's make this contract gone a step further. Let's create a contract that we could use to store information and maybe related the users of an app. And we can maybe store name and phone numbers. Let's try to do that. Trend to think before I go ahead and try to think for a second, how could we do this? Based on what we've learned before? We could use structs, can put different variables together. We could use arrays, we could use mappings. In this contract, we're going to use all of those. And so we can do things in different ways. Now, let's, we're not going to need those anymore and those functions here anymore. I'm gonna delete those and the variable as well. Let's start over with the second version of the contract. Remember, variables go on top. We'll start with the first one. We can have a struct. And thus destruct can be called people because we want to have, I'm not sure why it's highlighting that is kind of annoying, but we want to have multiple variables in the related to customers or users. We want to store names and phone number. The first variable will be a string that we can call name. The second variable would be a uint8. We can call number. Here's our struct of people already made. The second variable that we are going to need is an array. Because in this case, we can have an array where we can store all this trucks. So we can recall all the informations relates to everybody. We can have an array, though we can call people or dynamics, so we can add things in. It will be public so we can access it from outside. We can just call it, actually, I made a mistake here. This one we can just call IT people and then the actual array we call it People array. Just do that. Because we want to create an array of structs, solve these variable here. This variable type has to be a struct. It's people's front array. Then the next and last function that I'm sorry, variable that we need is a mapping. We can make a mapping. So we can associate directly name two numbers. So the array and the strike will be used to have a general overview of the users that we can have. And the mapping is going to be more like a database search type of thing that where we can input a name and retrieve the number. We'll start with the mapping. These mapping inside is going to have a string because we want to be able to input a name that is associated to the number you went. We can make it public so we can access it. Let's call it name to number. Like this. Very well. Now we have all of the variables in the data setup. We have a struct where we're going to have a name and a number inside. Then we'll have an array. If we're going to have all the struct inside, then a mapping where we can, we can associate name and number and then we can look up for them. Now let's create the function. This function is going to be used to do two things. One to add the information into the strap and then to add the struct into the array. We do all of it in the same, in the same function. So we'll start with function, we'll call it add person. This function is going to take two parameters. First one is going to be the name. So we'll have a string. Remember when we put input strings, we have to declare the memory, so it will be strained memory and we can call it name. The second one is going to be a uint8. Though we can call number. This will be public. We don't need to return anything and we're not making a pure review because we're actually going to write stuff on the blockchain. These things. Anyway. Remember how do we add things in the array? We use the push method, will take the array name, people, dot push. That's the method. What are we trying to do in the people array will push a struct. Struct is also called people. People read the name and number associated in it, like this. In the same function, we're also going to add the same name and number two, the struct, I'm sorry, I took the mapping and there will be take the name of the mapping, name to number. The first side will be the name associated to the number. So this is actually a contract that we could use for developing part of an app to keep track of contexts. Let's see if it works. We can we can compile it with Control S doesn't work or did I miss this time? Portion of this happened because I wrote it like this. I tried to pull the type of variable instead of name. Here. I actually have to put people at the name here. Sorry about that. Composite the show work. And indeed it does say the ID is very useful because it helps a lot with mistakes. Now with applied, here we have our contract. So right now we don't have any name and number. And it could be me start the contract with the constructor function if we wanted to have some default inputting there. But right now we have to add it. So I'm going to expand it here so we can see it. Sorry, We can see it a little bit better. We had to add the name and a number. We'll add. And let's make an hypothesis. These are phone number. We'll start a transaction which is complete. Then let's add another name entirely. Charlie has another phone number. I don't know. Don. Donor has another phone number. Now we should have three inputs or three different variables inside the array. If we go look for the spot 0, we see that we have Dave with the phone number, Spot Run. We have Charlie with the phone numbers but three sorry, spot to three, non-existent yet we have done. Now let's say that we had the name and we want to have the phone number. Well then we will just type it here. Top Charlie. And it's going to retrieve the phone number. And we retrieve the phone number. Just like this, we create a simple storage contract. Very nice. These conclude this part where we were able to actually structure and make a smart contract. In the next video, we'll do something even a little bit more interesting. Agreed to create a contract. Where are we going to keep track and allow to rent a car? In the video, we're going to see few extra uses for functions where we're going to also talk about events AND function modifiers. So I'll see you in that video. 9. Rent a Car: Hi, Welcome to the second project video that we'll have in this hearing today. We're going to call it another contract that has a little bit more of a practical purpose to show everything that we learned so far. We'll make a contradiction, for example, be used and linked to a nap. They will verify the availability of something. In this case. Today we're going to pretend that we have a car. Let's pretend we have Rambo and we want to rent it out. This contract is going to allow us to check if the car is available and if it is available to receive a payment for rent in it. And at that point, if the parent is received, the status of the Gameboy is gonna be switched to a non available. This is going to be very good contract because we can introduce a couple of new things. First of all, we introduce the payable status or variable. That means that we can, we can specify if a variable is a variable of a function or a function is payable. Let's go through that right now. Let's start with the usual contract and we would call a rental amble. Just render lumbar. Now if you remember from the previous video where we discuss the structure of a smart contract, we want to have all the variables, the data, the top, solve for right now at this moment, in this version, on the contrary, we are going to make, we just have two variables. We will need to have an address that receives the payment. Then we will need to have an enumerated list so we can keep track what the status of the car is. Let's start creating the address for the owner. Will create an address variable. In this case, before making it public or putting a name, we'll have to define that this variable is payable with the payable keyword. This means that this variable can receive payments, will make it public so we can call it from outside and we can call it contract owner. Contract. Now we have a variable that defines specifically the address that we received, the payment, the enumerator list. And we can call the lumbar conditions. The idea of this contributes to rent a car, right? So we need to know if he's already rented or if he's available. The two conditions will be available. Rented. That's it. We're going to call these enumerator list just lambda real have to recall the number of conditions and then we'll give it a name. Now, how do we know who the contract coronaries? Well, we can initialize it with a constructor just like we did in the previous one. So we'll start with the constructor function, that contract constructor. Constructor function that if you remember, it just runs the first place. The first time that the contract is random. We want to do two things in this constructor. We want to associate the constructor, sorry, we want to associate the address of the address of the wallet creates the contract to the owner variable. We want to also initialized a standard situation for the enumerator list. We will take the contract on our variable. We will associated to the payable address. So now I just want to talk for a second about one thing here before we proceed. We're using version of solidity. In this case issue can remember in the previous lesson, we will take the address of the contract. Construction from the message sender will be something like this. The sender. Now these in, in Solidity, 78 will not work because the message sender instruction is not payable anymore as it was before, we will have to cast the payable functioning here for the message sender. If we cast it like this now is going to work. If you're using a solidity version before seven, you don't need to put the payable in here. We can also initialize. In order to list. Whereas lambda will equal to lambda condition dot available because when we create the contract, the car will be available to be rendered. Here is pretty standard so far. Now as usual, we're going to need a few making something wrong here. We will need a few functions to keep track of everything that is going on. First, let's create a function that returns a status of the car so we can understand what it started as n if he's available or not. We'll create the function. We'll call it Get condition. Condition. We are now going to pass any parameters through it. And this is going to be a public function view because it's gonna read something and we want to return something. So returns. What is a variable that is returning is the enumerator list that will be called from lumbar condition. In turn, the lumbar variable that we created turn. Just like that. Now we have a function that can tells us, that can tell us if the car is available or not. Now, how do we book the car? We first need to receive a payment. We're going to have to create a function that can receive the payment function. And we can call the rent lambda2, because the function that will actually gonna issue the rent just as the address it will do. We create a here. This function has to be payable. It means that it can receive the, in this case, it's gonna be public so we can call it from outside as well. Just as simple as that. Now, ideally, we want to do a couple of things before we continue here. Because you don't want to just create a function that receives a payment without checking a couple of things. First of all, we are going to need to check that the car is available, right? Because if the carriers already oriented, we can now call this function again. Otherwise someone else will have to book the car. Then obviously you could create a schedule system, but this is not the point of this video. How can we check if a condition is met? We have a keyword that we could use. The keyword is require. Basically what this does is that it requires a certain condition to be met. So in this case, we want to make sure that the car is not oriented yet, that the carriers available. So how can we do that? We can check the status of the variable lambda2 is equal to available. We can do that like this. We didn't put the name of the enumerator list dot available. Now, before this function triggers, we're going to check first that this condition is met. The cool thing about the requires that we can even give them a error message. For example, here we could put the lambda is already rendered. If someone tries to call this function while the car is unavailable, is gonna receive, the function is not going to complete. And he's also going to receive these error message that says, hey, you cannot do that because it's already rent. That's very nice. And then the second check that we want to make is that we want to make sure that we receive the right amount there we want to run the car for. We're going to use another require statement to check if the value that is the person is sending is at least equal to what we want. So we'll create another require. Now we are introducing a new thing that you remember how we can capture the address or host calling the function with this message sender, we can do the same with the value that they're saying they were message value. So you will just type it like this message value. And we want to make sure that the message value is at least equal, is bigger than or equal to, let's say two weeks. We can specify there like that to ether, just like this. And then if it's not the case, it looks like I spelled it wrong. Many things wrong. Ether. And if that's the case, we're going to give a message that says not enough either. Not enough. So right now like this is pretty good. Because before launching the function, we are making sure that the conditions that we want are met. Now how does the function actually send money to the owner? We're going to use the transfer method will basically take the contract owner variable. Why does it do that contract variable? We're going to use a transfer method. We'll do dot transfer, and then we'll transfer the message value that is associated now with this function, like this, what this instruction does is that takes the value, that is the address descending and ascending n is transfer name to these contract owner variable here. Then after that's done, we can change the status of the car to none available. We'll do landlord equals to the lumbar conditions. Conditions dot granted. Now this function is going to handle all the payments stuff. N is going to switch the car the car status to rent it. So if this happens and then we tried to call this function again, try to run the code again, is not going to go through. The last piece of logic that we will need is a way to return the status back to 12 able. And four there we can just do checkout or check-in rather function where, where we are calling it when we return the car, we will do function. We can call it return lambda2. Also, this function is not really going to get any parameter through it. We have it public because we want to be able to call it. The function does is that takes lambda, turns it back to available. Just say that this point, we basically have a completed smart contract. We can, we can use as a prototype for our app. Let's try to compile it. Test it out. We're using in eight, so let's make sure that we use for your name, compile it Control S is compiled correctly. While we're here, I can maybe show you what I was telling you about the difference the different versions. I mentioned that this payable here is necessary now that we use version eight for it, there is another similar situation that you might encounter a few years older or newer compared to a newer version of solidity. And he's on the constructor. Right now is you can see we are stating every visibility state in the functions, but not in the constructor. That's because version eight doesn't require that anymore. So for example, we could put public here. We could say that the constructor function is public, and if we try to compile it, it's still going to work, but it's going to give us a warning. Here says the visibility constructor in the visibility for cost structure is ignored. And it gives us some other information that we could use, still going to work, but it gives us a warning. And this was different. For example, in version six, Let's say that we use solidity version six. So we'll move the compiler vector version 66 by six. If we remove public is actually going to give an error because version six requires the visibilities to be specified. And hearing, it tells me, Hey, no visibility is specified. Did you mean to add one extra thing that I wanted to mention just to give an example of the difference between the versions, let's turn it back to version control as to recompile it is correct. Let's deploy the contract and see what happens. We have our address virtual machine with deployed here, the contract is deployed. Here we have all our variables and functions. So let's first check who the owner of the contract is and check if the constructor worked. If we call the contract owner, going to give us an address, which here you can see your hands for DC for that is the address that we used. This tells us that we are the owner of the contract and that also means that we can receive the payment. Now in the constructor, we also specified that the car has to be available when it's launched. If we do get condition is going to give us 0 that equals to the available side. Work so far. Now, let's check if the function to rent the car and make the payment works. And if you notice this one here that is payable is red compared to the Ottomans. Now let's, let's try to make a transfer. These is a section of the page that you used to actually specify how much you want to send. And you can see here that we have different measures, make sure that IT or ether. And let's try to send one ether. Our function specifies that we want to have to. In theory, if everything works fine, we tried to send one ether. It shouldn't work, it should give us an error. Let's see what happens. We'll go back down here. We have one Ethan, let's run lumber. Guess what? We have an error here. It provides us with a message, not enough. It looks like our system works so far. Now let's try to send to see if that works. We put two Rambo. Now I worked. If I did something dumb here because I didn't switch their counsel, we basically sent the Ethernet to ourselves. Let's let's change the account. We can use another account and let's do it again. Well, actually we can't. This is gonna be, there's gonna be another arrow is great to test the next, the next part that I wanted to test. If we do run lumbar now that we already rented it, it's gonna give us another error is why? Well, the lumbar is already rented. How do we do that? Well, we can call the function to return the Lambda. Now the is back. So let's try to use these second address to render Lambda. Again. We'll send the to read lambda2. Worked. So now I frequent check-up here we can see how now the second adder has 1970 and the owner of the contract has 101. So that's exactly how this works. Again, if we now take another address and we say, Hey, I'm a third person, I want to run the Rambo. Oh wait, I can someone already has it. In this case, we don't really check if the person who rented the land was the same. That is, doing the return function. That's something that we could implement to make it a little bit more functional. With any free return, the Lanval, then someone else can rent the car. This is a pretty good overview of how payable functions work. Payable function, payable variables. But let's step this up a notch. Let's introduce a couple of more mechanics that will make this contract a little bit better and a little bit more real. So first of all, The require statement here. Okay, way to make sure that we meet some requirements, but is not the preferred way because let's say that we have an app that does this many times. Let's say that we have up there has ten lambda, we have a current of the has ten number. Then you'll want to have a specific function for each of them or something that we want to render Greenland where we want to run the red lumber and so on. This means that you will have to write these code. For each function, which is doable. It will work, but it will be very repetitive. And it's gonna actually, the gas p is quite a bit because we are writing more and more code to use. To overcome this situation, we use something that is called the modifier. Modifier or let's place it here just before the constructor function modifier or is the called function modifier starts with the keyword modifier. And basically what it does, it allows us to place these require inside this modifier and then we can just The modifier to each function that we want to have that checkmate for. Then now, I'll show you here now, let's say that the first one is make sure that the status is available, will create a modifier, though we can call we call it subtle Sarah. Status error. What we will do here, we just take, let me make this a little bit smaller. Just take the require that we wrote before. We cut it and paste it in here in the modifier code. And that's it. That's how we transfer it. When we use a modifier, we have to also just make sure that this is correct. When we use a modifier, we also had two other special character in it. There will be an underscore after the checks. We want. These underscore. Basically it says after this has been checked, then run the function. And if it's not met, then you will get the error. Then we create a second modifier for the payment. Will do modifier. We'll call it payment error. It's basically to not be the same thing. We'll take the payment required. We've cut it out and paste it in here. And then we'll add the underscore to make sure that the function runs out to the checklist complete. We can clean up this on here. Alright, so the function to rent the only thing left is the actual transfer n change in condition. How do we tell this function that we want to access these modifiers before? Well, we simply add them in. We simply add them inside the head. Here. We will just add started to say payment error. Actually, I just realized that we have to make a slide modification here in the permanent or because now the function is passing something through, right? Because it's the message value. Instead of specifying here the payment that one will specify here. In there we'll put two ether here in the require will have to something. The modifier here will need to add a parameter because the payment error passes the value that we want to add, we can add a parameter called value. And then here, the require will check that the message value is at least equal or bigger than that. Now that's going to work. This was the next step that we wanted to show. Let's check if it works. We'll kill this contract. Essentially these contents should work just the same way as it did before. Control S to compile. We made a mistake or they didn't mistake, we make identifier not found or no unique or the dy du. Let me check this. Here, I get it. Here. We didn't specify the type of variable. Now let's try to compile it again. Sorry about that. We can deploy the contract. This time we deployed it for these are from these other address here, so that's gonna be the owner. We can check that we are the owner. The lumbar is 0 is available to be rented. Let's see if we can use these other first daughters to rent it. This time. We switched the address, will try to transfer to rent the car and you worked. The payment is being done and we have the situation like it was before. Let's try to return the Landow and let's test if this works. I started to pass on your 1 eighth this time. When we tried to rent it. We can't. And the modifier is going to also give you the there is not enough, eat enough. Same as before. If we try to rent it again, this gives another mistake, color another error because we're passing, we're not passing enough so it triggers the value first. But if we pulled it to eat and we try again, well, not supposed to happen. Okay. I did something wrong there to the alarm is already ranked and yep, that works. I didn't check to make sure that he went through these exactly the same thing as before. Just has a little bit better structures, a little bit more flexible because then if we have our 10th function that do the same thing, then we will have a little bit cleaner and code. Now, let's add one last thing to make it a little bit more usable. And we are going to explain the event. An event that does is that allows an external application to listen to the blockchain for specific changes. In this case, we could have a safe software connected to a piece of hardware that listens to the app on the blockchain and whenever the car is rented. And so whenever the payment went through and this function here, the rank lambda function, is completed correctly. Then maybe you can activate some automatic mechanism to give you the key of the car. So how can we do that? We use an event. So up here in the variables, we'll put on an event variable. We can call it a rented. And rented is going to have a couple of things in it. The event is going to have information. We're going to have to need an address and an amount will have an address variable uint8, that I didn't name. The address can be associated to a variable called customer, the uint8 to a variable called amount. Because we want to make sure that the amount that when the software that list and the amount is correct. So we had the advent here. And how can we trigger this? We can do it in this red lambda function with Emmett event. The meat is the keyword to trigger the event. Once the thread, once these instructions went through, we can emit. We basically meet the event which we rented. The two parameters that we pass through, the message sender. Because we pass the address and the message value. This way. The app that is listening from outside the contract can see who, who said what. So then you can authorize the release of the key or something like that. Now, we also going to require this function here, Rambo. Be external. If we want, if we make it external, then we don't have to actually called the function by itself. So let's step back for a second here. I'm going, I'm getting ahead of myself. In this way we are simply a meeting the event. Now. This function right now has to be called manually. So someone will have to make a payment and manually call this function. What if we want to have a function that is triggered automatically? If a parent is made, we can do it using another special function. It's the same principle constructor because it's a special function and is called the receive. When we use the receive function, visitor will say the retail, the country, hey, whenever our payment is received, you don't need to have someone clicking this button, just do the thing automatically. Now this receive function here has to be external. External is another condition, another visibility state that is only a variable for functions. And so it basically says in an app from outside, the contract can call the function. We have to make this payable as well. We're going to have to add the two modifiers because we still want to check to make sure that everything is good. So we will use the status error here modifier. These are good example of how, uh, why it's better to use a modifier, because otherwise we will have to write the entire code from the modifier again inside it. Status error and payment error. We also want to receive two ether for this. Now, the code inside here, it's gonna be the same that we have in this function. Copy this, paste it in here. If we tried to compile it, Control S. What did I do? I guess I forgot something. Expected a semicolon burger modifier because I created the event, but I didn't close it with a semicolon, save control S. Now it's compiled if we deploy. If you look at this, you can see that this receive function is not, doesn't appear here. Because this receive function, it's only available from outside the contract. We cannot call it from within the contract is from outside. And so whenever we are sending something to it, then it's going to automatically be triggered. Because of the emit. If there is an app, listen to it, it can then giving instruction to something else. This contract here was a little bit longer, a little bit more complex, but it illustrates many, many things available to us in solid. Next contract that we're going to show, it's probably the most, a little bit more exciting because we're gonna show how to make a contract to actually means the transfer coin between four down. 10. Coin Minter: Hi and welcome to the last video this course. Today we're going to put together everything we've learned so far and we even going to add a new element in the mix. We're going to check how we can reverse transaction in case I'll condition is has been failed to match. What we're gonna do today is going to be we're going to create a smart contract where we can meant and send coins to water addresses. Whenever you were trying to make a contract. It's good. Or it's pretty much required that you make a general plan. So you want to have a clear idea of what the contract makes. So for example, here we can see how we can make the structure. We want to make a contract that can mean coins. The coins can be maintained and sent to our addresses. Now we want to add a little bit of functionality. So we're going to make at least have the introduced in the first place. The main function can only be called by the owner. That's pretty essential. Otherwise, every person that acts as the contracts can just create new coins and we don't want to have that. Then we want to have a function that can send the coins. And before the function gets validated and completed, we want to make sure that the balance in the account rent to send coins is enough. So we don't have accounts with 1000 coins trying to send 5 thousand, otherwise I will not work. And then we want to have an event. We want to be able to emit events when the coins are transferred or when a transaction to send coins is called. Because then we can have apps that can listen to that and do some filling when that happens, I don't know. Maybe you can have a confirmation message. You're sending coins for someone else and there's someone who can receive a confirmation message based that. Then at the end, we also want to have an external function that can check the balance of the addresses from other external apps so that we can have, we can access the contract from another app and check the balances of the addresses that you could see that it almost as like ether scan type of application. Let's start by dollar for this. We'll start, I'm sorry, making the usual contract will have the contract keywords and we'll call it a coin Mentor. Mentor. Here's our contract. Now, in the first, the contract we establish all the variables and we're going to have a few. The first one that we want to have is the address for the owner will have an address variable that is public. So that can be accessed and we'll call it owner. This one for now is initializers 0 or nothing. So we're going to place, we're going to associate that variable to the address of the owner when the contract is deployed. Then the second variable that we need is a mapping. We can associate addresses with amount of coins that they have and we can check their balances. So we'll call this a mapping. Inside this mapping we are going to have two variables. We're gonna have a variable of address. This address is going to be associated with a uint8 because we just want to have an address associated with the amount of coins in these coins are checked in numbers. The mapping is public. We can call it balances, because this is the mapping we use to check the balance of the address. Then the next two are going to be the event. Because we remember that we wanted to have a way to emit an event. Solder apps can check if a transaction has been centered, not the event. We can call it sent. Let's call it sent with capital S. We're gonna, we're gonna send, or we're going to meet three different variables. So we're going to send three different information through this. We're going to send the address where the scent, the coins there is a received them and the amount. We will have two variables, address in their address. And these are going to be called from because this is going to associate the arteries ascend the coins. Then the second address variable is gonna be two, which is who receives the address, received, the coins, and the second one, Sorry, I'm, the last one, will be a hint that we can call amount. Because this is gonna be the amount that is sent. Then the last variable that we need is going to be an error variables. So we are introducing this system. In this lesson, in the previous lessons, we checked how to have function, makes sure that every condition was met using a modifier. In this case, we use an error because the error allows us to revert the transaction. When you reverse the transaction, it means that it is not is not transferred, is not validated by the blockchain. That means that no gas is wasted. So this function will fail if in this case, the balance of the account is not enough, but it's not going to waste the gas. So this is a good feature that we cannot do our contract. I'm gonna be an error variable like this. And we can call it insufficient balance. Sufficient. We're gonna check for two things. The amount requested, an amount of variables. So we'll have to uint8 inside this. We can call it requested. The second uint8 would be called the available. We're basically check the balance of the address c, how much is requested. And if there is enough, we can send transaction. Very well. Now, we have all the variables that we need. So the second part of the contracts are the functions. The first function that we want to add if we need one, is a constructor. In this case, we want to have a constructor function because we want to have the address that creates the contract associated to be the owner so they can create the coins. Will start with the constructor function. This should be pretty simple by now. We'll just set the owner variable to be equal to the message sender. So now whenever the contract is deployed, that arteries are deployed, it is going to be the owner of the contract. And so these addresses is going to be able to Mint coins. Otherwise, if there are no coincide domain, then you can't really have them. The next function that we're going to make as a main function because we want to have the coins. We will create a function that we can call Mint. One local minimum. This function is gonna get two parameters, one address and one uint8. One address, because we want to send these coins that were meant to and address. It can be, let's say the owner wants to maintain a bunch of coins for themselves. They can start doing something, then they will place their own areas. Let's say that the owner wants to mean coins to send to other users of the platform. Then this function allows to make the coins and automatically send them right away. The first variable that we will have is gonna be an address variable, and we can call it receiver. The second one is gonna be a UN, where we have the amount that we call the women. Then this function is going to be public, so it can be called from outside the contract. But remember that only the, only the owner can, can mean this herb non-mental can call this function to employment that will use a require will require will require that the message senders. So we required the address that causes function. Is the owner. Said sander equal, equal owner. That part is done. Now, only the owner of the contract can call this function. And then what we want to do here is that we are maintaining coins. What we're gonna do is we're going to take the address that is passed through the function and add the amount to their balance here. So we're basically going to add into the mapping. So we'll do that. Calling the mapping balance, balances. In the first, on the first side of the mapping, we're going to associate the receiver. So that's the address and receives the coins. And we're going to do, is we're going to add the amount of coins to their balance. We'll do plus equal amount. Remember that the plus equal means that we take whatever is in already there and add the new amount. Technically, if we want, we could also write it like this. Amount plus amount. But we want to do it just like this because it's a little bit more efficient, is a little bit more elegant. Now, we have the main function. This means that we can actually meant and send coins to an address. These are indices. We'll have a balance of something in this case, right now, just with this, we cannot transfer the coins, we can only meet them, so create them and send them. I realized that I could say meant without explaining what it is in case you don't know what it means. Means. It means to create like you meant an NFT, you create an NFT, you mean coins, you create coins. You're basically create coins from thin air in this case. So that's why we want to have a restriction Hutu who can do that? Because now if you have a contract where these coins are associated to an actual real monetary value, then you will change the value by maintaining these extra coins. Now, we have the function to create and send the coins on others, but we want to have the ability to call this continues this contract to send coins between addresses. So let's create a function for that. We'll create the function that we can call send. And again, we're going to take two parameters in here. Basically the same as the mean function because we want to take a certain amount of coins and send it to an address. The parameter that are gonna be the same actually. There's gonna be another is that we can call receiver. And this is okay to have the same names because there are different functions and they're done in these parameters on interact with each other. The first one is going to be again the receiver and the second is going to be again, the amount. I forgot a few things here. We want to have it public as well. Then the brackets. Now, this is a pretty fairly simple function, but we have to add the functionality of making sure that the balance of the address is enough to be able to send it. So let's say that we have and others that have only has only a 1000 coins but tries to send 2 thousand of them, then that's not allowed, that's not going to be possible. So we're going to make, we're gonna use a conditional statement here. We're going to use it if we check if the amount that we tried to send is bigger than the balance that we have in the address, then is going to revert the transaction. And it's going to use this as a way to do that. It's going to reverse the transaction using the sufficient balance. Will do if the amount that we're trying to send is bigger than balance of the message sender. What are we doing here with this instruction? This instruction, we are taken the amount they were trying to pass through the function and match it with the message sender inside this mapping. And obviously if there is no account in the mapping, then it will show 0. So it still, it will still work because it's still going to show that it has no, it has not coined to same. But let's say that the addresses in the mapping and has a 1000 coins, then it's gonna check, it's going to take the outers, places here measured against it and see how many coins is in there. Here's how we can check that. Then we'll do that. If that's the case. Then we reverse the transaction, will use a keyword revert. We'll use the insufficient balance error. To do that like this. Then we want to do inside the Sufi sufficient balance error. Then we can also add some information here. These error has these parameters in there. What we can do is that we can match the requests that amount, the amount that we're trying to send like this. Here, we just You can just write it on one line, but this makes it a little bit nicer. Then. There are variable amount that we have will match it to the balance of the center, just like we check here. M as G center. Just like that. Then don't forget here to put the semicolon on otherwise now going to work, what this does is that it checks that we have enough coins. If we don't have, it's going to reverse the transaction and call this error and so on the constellation shoes show that we are trying to pick what we're trying to send these many coins. But we only have these many and so that's why it can't work. And then when that condition is not met instead, then we can make the transfer. How do we do that? Is that we take, we basically take the amount that is in the mapping associated to the others, remove it, will associate it to, we'll take that number m basic gauges credited in associated to another address in the mapping. How we do that, we'll call the balances mapping. First, we're going to take the amount of weight from the sender, will do MSG sender. In this case, we're gonna use the minus equal. Oops, sorry about this. I press on to run the minus equal operator. And this means that we are taking whatever is in there and removing the amount from it. So we'll do minus c equal amount because that's the amount that we are trying to send and we're going to credit it to the receiver. Will all again, we're gonna call the balances. But instead of having the sender will have the receiver instead. We'll do the opposite here. We'll do plus equal amount because we credit it to them. So now at this point, the address that is sending the coins, remove the amount and the address that is receiving the coin is going to get the amount. Struggled in there because I realized that this is the wrong one here is not required, is received. Now it's going to work better. I mean, it's got to work at all. Received. We also said that we wanted to emit an event whenever the transaction is made. So we can have an app maybe say, Hey, you'll receive these many coins. So let me do event like this and will emit Sent. Then we're going to take the message sender, the address of the receiver, receiver. And the amount. These are gonna be the information that are transferred through the advent. The address sends it. Who is receiving them, and how many n is pretty much what we have here from to an amount. Very good. So now we have the function made to transfer the coins. And it might seem a little bit complicated, but once you go through it, once you think about it a couple of times and once we actually tested is going to be very understandable. This is very simple code overall. Now the last function that we wanted to have is a function that can call the balances of the different addresses from the outside. So if we have the address of an account that we want to check them we can use we can use this function to see how many coins are in there, similar to chain Explorer ether scan type of thing will call this function. I'll create this function and we can call it good balance. Good balance, and we'll just have one variable, one parameter here we only need the others. Will have the address and we can call it account. The account that we call. These function is going to be external. This means that we can call it from an external application or at it only from an external application is going to be view because this doesn't really change anything on the blockchain and just reads it from the blockchain and returns, it will return a UI in because we want to return an amount of violence. How many coins are in a very simple is just going to return without the ears, the balances because we want to call them mapping, right? So we'll call the mapping, will just look for the account address, for the address called dot count on. Keep doing that. For the headdress. Pass through these parameter. That's it. So now our contract is complete. We didn't test it yet, so there might be a few syntax error. The logic should all be right. If we have some mistakes, is most likely just some syntax errors. So I want to just want you to take a look at the structure for a second. Let's remove all these comments just so we can have a little bit better formatting. Look how we made this country. We have the contract name, the solidity directed identifier name, and then we have all the variables on top. Then we had the constructor function, and then all the other different functions. So this is the general layout that you will see when you're looking up different smart contracts. Now if we put, let's say that we want to put to make it more clear for ourselves. Because sometimes I find out that it is a little bit more clear. Let's say that I want to pull the variables associated with this function just above the function and so on you could do it is still working. But then if someone else goes and looks through the content contract, let's say that you need to have another developer helping you doing something or working with you, then it's gonna be more confusing for them because they are most likely used to have a format like this. Okay, so let's try to test it. Let's see if we can compile it as make sure that we had the right compiler. We have version 8.103 Control S. See if it works and we have a few mistakes. Of course, I would say. Now let's see what those are. Okay, so first of all, I made the first syntax error here. We are trying to call a mapping or use a mountain called balance, but we don't have a mapping. Cold violence, we have a bumping called balances. Will just add the S there and that's going to work. Let's see what these arrows are. These ones here. Again, only mistakes for syntaxes. This one here should be receiver now received, I don't know how I missed that because the parameter that we pass code receiver, we call this receiver. The last one. Did you mean sand, mint or sand? So now the advent, it's sent with a capital S. So be careful with the 0 mistakes. We are still going to make them. It's, for me is rare that our write up a whole bunch of code all at once. And it works without this more mistakes. But the remix ID and this saluted compilers are very good if you notice, it took a second to the bug, the code because it tells you it tells you what did, what did you try to do all is wrong. Do you probably meant to do this? And it does it for yourself. Now if you do Control S, not a word, we compiled it and let's try to deploy it and test it. Let's take that as his first. We're going to deploy from this first others, EDD C4 in 108th. So this is gonna be the owner of the contract. And I'm going to extend this a little bit so we have a bit more. We can see a little better. Let us apply. The control was deployed and we have it down here. I'm going to extend it up a little bit. And here we have all dysfunctions. Now these two functions are orange and they do something, and these are just other functions that you just read stuff. For example, the balances, we can read the balance, the mapping and so on. And the gate balances the external one that is going to read from an external source. So let's first check that the owner one works. Let's call the owner. And here we have the outer side is the owner and we have EDD C4, which is our address. We are the owner of the contract. That means that we can also meant coins. So let's try to create, I mean, I don't know, 10 thousand coins, then we can use how we want. We go here, we copy our address here, and this is going to extend it. So it's a little bit more, a little bit more clear. So the main function, it needs a receiver and an amount the receiver is going to be over self will paste the address in here. And we don't mean to 10 thousand coins. If we called the transaction is going to work because we are the owner. And if now we are going to take our address again, we'll check our balance. So we paste the arteries in here. We should have 10 thousand coins, and indeed we do. Now let's try to saying 2 thousand coins to water addresses because they are user of our platforms and we can, we want to send them so they can do stuff on the platform, will take this second address. We copy it. We want to send it. The sender is going to, this transaction is going to need two variables, is going to need whoever receives the coins. How many services that we want to center. Now we're going to have to call it from the first one. Because otherwise we're not going to have enough coins right? Now if we make the transaction was successful. And if we take this address and check the balance of it, then we have 2 thousand coins. Very nice. Now let's try to see if the In section, if the error works. So we now have a 1000 coins left in this account. Let us say that we tried to send a 10 thousand of them to this same, to the same areas. When we call the function, it's going to give an error. If we scroll up a little bit, we can see what's going on. Here. We are trying to call it and then it reverted. The transaction has been reverted to the initial state. So this is now going to use gas. The error is gonna give these parameters requested 10 thousand, but available is only 10 thousand. That's why we coded it that way. And so we have an information that oh, wait a second, I'm trying to send too many. That's not gonna work. Now. We can try. We can use these arteries. They just received 2 thousand coins to send it to another one. We could switch their address and it said we want to send it to this guy. Let's copy this. Put it in here. Then it was the second address. So now when we tried to send 500 coins, which we should have make the transaction worked, and now others should have 500 coins. Let's use the external function to check this one here. These get buttons. It's an external function I should do basically the same of this, of looking into the mapping. Will the only difference that we can call it from an external application. Now replay, we pasted the address in here. We'll call the balance. And we have 500 coins. Our contract to mint and send coins work just as intended. So these are fairly simple contract, but I think it's very good to put all these different information together. Obviously, contracts for if T's and coins that work may NADH and they have an unnatural value associated and do things are much more complicated. These are just what, 40 ions or code compared to a hundreds that you will find in a regular contract. Now, you should have enough knowledge to be able to at least understand what's going on inside his contract. So what I will recommend to do is go find contract for tokens and coins you're familiar with. You can find them on ether scan, on that scan or whatever chain Explorer you'll want to explore. And try to read through it and see if you understand what's going on. Obviously, you're not gonna understand everything, but you should be able to have a good idea or at least what these functions do and what they're trying to do. Good. This is concluding our fundamentals course. I really hope you enjoyed it and it was fun for me to create it. It's been, I've been thinking about this for a while because when I've started learning solidity, I could only find fairly complicated things or any way courses and tutorials, they don't really explain the basics, how I needed. And so I really hope this course is going to help someone did as, as they're trying to learn to become a developer. Now this was the first one that I created here on this theory. I'm working on another one about Web three interactions. So stay tuned for that one. That one is going to be working with Python and we're going to use the Web three library in Python to be able to access the blockchain and do stuff with Python, which is a greater risk for gray language. Well, thank you very much for following and I'll see you next time.