Smart contracts are an exciting way to build decentralized applications dapps on a blockchain. This tutorial helps you learn and build your first smart contract using Solidity on Ethereum blockchain. You can say it's like an agreement between two people in the form of computer code.
The transactions in a smart contract are processed by the blockchain and stored as a 42 character hex address with the prefix "0x". All of which means that they can be sent automatically without needing a third party. And once a smart contract is deployed, it cannot be changed. Solidity is one of the most popular languages used for building smart contracts on Ethereum Blockchain. It's also an object-oriented programming language.
Create some modifiers that will help you verify a few things before running a function. Here require The following will check whether the tenant has enough Ether in his wallet to pay the rent. The following will check whether the tenant has enough Ether in his wallet to pay a one-time security deposit and one month's rent in advance. The following will check whether the tenant's address is the same as who has signed the previous rental agreement. Now, create a function to sign the rental agreement for a hostel room between the landlord and a tenant.
Remember those modifiers in point no. Use those modifiers here to execute the following function. Remember that whenever a transaction is getting executed, it stores all the details in a unique hash key. Now the landlord of the room is your 1st Ether Address. The one with Scroll down and click on signAgreement , enter 1 , and press signAgreement. You have to remember that smart contracts store data in a block of the blockchain, and all transactions are stored with a unique hash key. For that, follow these steps:.
You may have noticed that whenever a transaction is executed, a few wei is getting deducted from your ether wallet. It's called gas fee , which is the payment made by users to compensate for the computing energy required to process and validate transactions. As more Ethereum miners come up in near future, the gas fee will decrease in an inverse relation.
After this, if you want to build a fullstack website using React, you can use this smart contract as a backend. Just follow the official documentation of Web3. You've successfully understood what Solidity is and how smart contracts work. And you've successfully built and deployed a perfectly working smart contract where a tenant can pay rent in ether ETH directly to the landlord's wallet without paying a single wei to a middle man.
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It has a lot of nice features like persistent file storage! We'll use Remix so that we don't have to download any developer tools or install anything to get started. Head on over to Remix in a new tab in order to follow along with this tutorial. Let's start by creating a new file to write some Solidity code. You can do this in the file browser on the left hand side of the screen.
Let's create a new file named MyContract. On the first line of this file, we'll declare the version of the solidity programming language we want to use:. Let me explain a few things before we continue. A smart contract is a piece of code that gets executed on the Ethereum blockchain. It functions somewhat like an API microservice on the web that is publicly accessible to everyone. All of the code of the smart contract is visible to the public, and we can allow anyone connected to the network to call functions on the smart contract.
Now let's continue building out this smart contract. First, we'll program a simple "storage" smart contract that will be able to:. We'll start by creating a way to store a string value in the smart contract like this. We'll do that with a variable called value. Solidity is a statically typed language, so we must first specify the data type when declaring the variable like this:.
This variable is called a "state variable" because it actually persists data to the blockchain. Anytime that we set this value, the string will be saved to the blockchain! It will get written to storage, not memory. This variable also has special scope, as it is accessible to the entire smart contract unlike a local variable which is only accessible inside of functions, and will not persist after the function has been called.
We'll see more examples of this as we continue along with this tutorial. Now let's create a function to read this value from storage. We'll start by declaring a function called get with the function keyword like this:. Now we'll return the value from the state variable with the return keyword like this:.
Now we'll set the "visibility" of this function to public so that anyone connected to the blockchain can call it not just from within the smart contract code itself :. Now we have a way to read this value from the smart contract. I'll show you how to do this after we compile it momentarily. Before we get there, let's create a way to set this value from outside the smart contract. We'll create a set function like this:.
This function is also publicly visible so that anyone connected to the blockchain can call it. Now let's actually update the smart contract value like this:. This is a common convention when writing Solidity code, as well as other languages.
Now let's set a default value for the value state variable. We'll do that inside the smart contract constructor function like this:. We first declare the constructor function with the constructor keyword. This function is run only once, whenever the smart contract is deployed. It also must have the public visibility.
You can interact with the smart contract by calling its functions below, with the form that was generated based upon the smart contract's interface. First, let's read the value. Let's click the get function. You should see the default "myValue" that was set in the constructor. Now let's update the value with the set function. Add a new value inside the form field, just make sure that you wrap it in quotation marks like this: "New Value". Now run it! Now read the value again.
It should changed to "New Value"! You might have noticed this transaction window below the text editor. This is a complete list of all the transactions on this virtual blockchain. Remember, the Etherum blockchain is made up of bundles of records called "blocks" which are "chained together" to make up the public ledger. The basic units of all these blocks are transactions.
You're seeing them listed here below. You can click the "down" arrow to see all details of the receipt. They look like this:. Alright that's it! Let's move on to the next section where we'll build off of this code and learn more about the language! Before we move on, let's update the code in our smart contract to be compatible with the latest version of Solidity at the time of writing this article. We can modify the first line in the smart contract code like this:.
Doing this will help us learn best practices for the current version of Solidity moving forward! Now let's address the warnings for the new version of solidity. First, modify the get function to look like this:. Now that we're up to date, let's talk about basic data types and data structures so that you can start implementing them into your own Ethereum smart contracts.
We'll do this by first creating state variables of varying data types. First, let's look at some nice features of state variables provided by Solidity. Currently, we simply declare the value state variable, and the use the get function to set it's value. There is a shortcut in solidity that makes this much easier. We can simply do this:. This sets the visibility of the state variable to public , which means that anyone outside of the smart contract can read its value.
When we do this, Solidity provides a value function for free! Now we no longer need the get function! We can also assign a default value to the state variable like this:. Now we no longer need to set this in the constructor function either! That really reduces the amount of code we have to write.
We can also set this value as a constant that will never change like this:. Now that we've done this, we must remove the set function because Solidity will not allow us to update this value since it is a constant. That's an overview of the shortcuts Solidity provides for state variables.
Now let's create some more state variables to examine some different data types available in Solidity. Just like the string state variable, we always declare the data type, then the visibility before declaring the variable name. First, we can create a boolean state variable like this:. This can be either true or false. Now we can create an unsigned integer like this:. Integers can be positive or negative, i. If you don't want a number to be negative, you can create an unsigned integer like this:.
We can specify the number of bits for the number. The above example defaults to bits. We can be explicit like this:. Now let's go over some basic data structures in solidity. First, let's look at the Enum data structure, which are a way of keeping track of enumerated lists in Solidity. We can check against this list whenever you're writing smart contracts. Let's create an enum in the smart contract like this:.
Just as an example, this will help us keep track of the active state of the smart contract by providing Waiting , Ready , and Active options. We can check the current state of the smart contract like this:. And finally, we can check against the enum list to see if the state is currently active like this:. That's an example of how you can use enums inside your smart contracts to track state!
Just as another example, this is very useful for tracking the state of a crowdsale ICO smart contract that is open or closed. You can check that out in my other tutorial where I show you how to build a real world ICO. Solidity allows you to define your own data types with Structs. You can basically model any kind of data you want with arbitrary attributes of varying data types.
Let's look at how this works by creating a people struct:. Notice that we're able to specify any data type that we want! We've used strings for both attributes here. We could add up to 17 different attributes here with any data type. For now, let's keep it simple and only use two attributes to model the person. Now we need a place to store this person struct. Let's use an array for this! We can declare people array like this:. Notice we declare the type within the array, i.
We also set the visibility to public and assigned it to a state variable called people. This will provide us with a function that will allow us to access the people inside this array. I should mention though, that calling the people function outside the smart contract will not return the entire array. Instead, the people function will accept an index argument that will allow us to reference the person inside the array based upon that index, which is zero-based.
For example, we can access the first person in the people array like this:. Now let's create a way to add a person to this array. We'll create an addPerson function like this:. This function will accept the arguments for the person struct attributes, then instantiate a new person and add it to the people array with the push function. It also increments the peopleCount state variable by 1.
We can implement that state variable in the smart contract like this:. We'll use this value as a counter cache. Remember how I said that you can't return the entire people array with the people function? Knowing how many people are inside this array will help us know how many times we must call the people function to get each person.
Solidity provides a data structure called a mapping which allows us to store key-value pairs. This structure acts much like an associative array or a hash table in other functions. We can declare a mapping in the smart contact like this:. This will be a mapping where we store person structs. It will replace the people array we used in the previous example.
The key will be an unsigned integer, and the value will be a person struct. We'll treat the key like a database id. We can update the person struct to keep track of that id like this:. Now we can modify the addPerson function to update the people mapping like this:.
This uses the peopleCount counter cache to create an id for the person. Then it instantiates a new person struct with the id and the passed in attributes. It then adds this to the people mapping. I would hate to see us reduced to essentially one contract language well, two with the other seldom-used language, Serpent. I know there's talk of switching to a Web Assembly-based VM which would open it up to multiple languages with some work to add gas metering, but that's likely years off.
Jacob Stenum Czepluch. If a library emits an event what is the associated contract address? Andy Loughran. Is it possible to prove that a contract matches a deployed contract and hasn't been modified? Can I check subsequently created contracts against the one I know to be valid to check that it's the same code? Would that work if the constructor was different? Another option is to deploy a "contract factory", which can be called to create a new contract, and maintains a mapping of valid contract addresses.
Technically, you don't even know that - if you know the factory's address and the nonce which, for a contract, is the number of other contracts it's created , you can compute the child contract's address, which proves it was created by the parent contract. So a contract can be set up to contain its own proof-of-ancestry. Guys, If I want to access a public variable inside another contract I just call a function with same name. I want to read variable A in external contract and I just do externalContract.
A and this is working. How can I call a public mapping and pass a variable to it? I can't understand.. Not sure why it wasn't working sorry. How can I then access the inner variables? I declare the struct in both contracts Roman Mandeleil. Dave Hoover. Alex Beregszaszi. I don't see such a commit. Is it possible to delete an entry from a mapping or just put the key to a zero value or empty string depending what type of variable the value is?
When i try to use return like return bytes32 using mix ide, The debugging shows that there is only one value in the return array , where there should be many. Is it prob with solidity or would it work fine with web3? Lucas Ansei. Is there a way to store md5 hashes in a array inside a contract? And how do I get that md5 hashe as output? Harry Pottash. Yeah, sorry about that, I read your question as a much simpler one.
Ok, so I'm still getting my bearings on a lot of this stuff, so I'm not entirely sure how much I can help, but I'm happy mess with the problem with you if you want. The array returns empty. I had some trouble for a few hours before I figured out that.
Gonna try to use. Yep, someone had to tell me about that one. It should probably be more prominent in the docs. I take it it didn't work What is this line doing? Right, I guess I'm a little unsure on that line, it could be tripping something up if you ended up somehow not having enough eth to send back.
I would drop it for the moment. So what do you get when you type eth. Junior Moraes. Ops, I miss the 'constant' keyword at getMessage method. But I tried and no success too. I'm going to try pushing this onto testnet using the web compiler which I have had more luck with and see if it works for me. Mauro Caffaratto. Sirs, it works with a simple contract! This is good! Now i will debug my contract parts. Thank u!
So you can load the contract I put on testnet with the following. So I'm having trouble understanding something Here is a contract I just wrote that minimally demonstrates my issue. When I push this onto the block chain testnet, using the web-compiler. The function "works" works as expected.
Can anyone help me out with this? If you want to load it it on testnet here's the address n' such. Anyone here able to help a little? Have you tried tracing an excecution on, eg, morden. Nope, I'm unfamiliar with morden. You can then go to 'VM trace'. It requires some knowledge of the EVM, so it's a bit clunky, but it works. Thanks so much!
Hmm, I don't see anything at all under "VM trace" for it. I'm guessing the trace isn't loading because it hasn't processed yet. It's like blocks back in the chain now I think ether. I only say that because if you click on 'x storage entries modified', it says it's not done processing yet.