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Reference the video walk through if you get stuck! The dependency is optional, but recommended. I recommend installing syntax highlighting for the Solidity programming language. You can download the code for this portion of the tutorial here. Feel free to use these as a reference point if you get stuck! First, find where you downloaded Ganache , and open it.
Now that Ganache has booted, you have a local blockchain running. Each account has a unique address and a private key. Each account address will serve as a unique identifier for each voter in our election. From within your project directory, install the pet shop box from the command line like this:. This smart contract will contain all the business logic of our dApp. It will be in charge reading from and write to the Ethereum blockchain.
It will allow us to list the candidates that will run in the election, and keep track of all the votes and voters. It will also govern all of the rules of the election, like enforcing accounts to only vote once. From the root of your project, go ahead and create a new contract file in the contracts directory like this:. Open the file and start with the following code:. Let me explain this code. We start by declaring the solidity version with the pragma solidity statement.
Next, we declare the smart contract with the "contract" keyword, followed by the contract name. Next, we declare a state variable that will store the value of the candidate name. State variables allow us to write data to the blockchain. We have declared that this variable will be a string, and we have set its visibility to public.
Because it is public, solidity will give us a getter function for free that will allow us to access this value outside of our contract. Then, we create a constructor function that will get called whenever we deploy the smart contract to the blockchain.
Notice that the constructor function has the same name as the smart contract. This is how Solidity knows that the function is a constructor. From your project root, create a new file from the command line like this:. Notice that we number all of our files inside the migrations directory with numbers so that Truffle knows which order to execute them in.
Next, we add it to the manifest of deployed contracts to ensure that it gets deployed when we run the migrations. You can open the truffle console from the command line like this:. From the console, run this code:. Here Election is the name of the variable that we created in the migration file.
This might look a little confusing at first, but you can reference the console demonstration in the video at for further explanation. We need a way to store multiple candidates, and store multiple attributes about each candidate. Here is how we will model the candidate:.
We have modeled the candidate with a Solidity Struct. We specified that this struct has an id of unsigned integer type, name of string type, and voteCount of unsigned integer type. We need to instantiate it and assign it to a variable before we can write it to storage.
The next thing we need is a place to store the candidates. We can do this with a Solidity mapping. A mapping in Solidity is like an associative array or a hash, that associates key-value pairs. We can create this mapping like this:. In this case, the key to the mapping is an unsigned integer , and the value is a Candidate structure type that we just defined.
This essentially gives us an id-based look up for each candidate. Since this mapping is assigned to a state variable, we will write data to the blockchain anytime we assign new key-value pairs to it. Next, we keep track of how many candidates exist in the election with a counter cache state variable like this:.
In Solidity, there is no way to determine the size of a mapping, and no way to iterate over it, either. For example, if we only had 2 candidates in this election, and we try to look up candidate 99, then the mapping will return an empty Candidate structure.
This behavior makes it impossible to know how many candidates exist, and therefore we must use a counter cache. Inside the function, we increment the candidate counter cache to denote that a new candidate has been added. Then we update the mapping with a new Candidate struct, using the current candidate count as the key. This Candidate struct is initialized with the candidate id from the current candidate count, the name from the function argument, and the initial vote count to 0. Now we can add two candidates to our election by calling the "addCandidate" function twice inside the constructor function like this:.
This migration will execute when we deploy the contract to the blockchain, and populate our election with two candidates. At this point, your complete contract code should look like this:. Now try to interact with the candidates inside the console. You can follow along with me as I demonstrate this in the video at We want to ensure that the contracts are bug free for a few reasons:.
All of the code on the Ethereum blockchain is immutable; it cannot change. If the contract contains any bugs, we must disable it and deploy a new copy. This new copy will not have the same state as the old contract, and it will have a different address.
Deploying contracts costs gas because it creates a transaction and writes data to the blockchain. This costs Ether, and we want to minimize the amount of Ether we ever have to pay. If any of our contract functions that write to the blockchain contain bugs, the account who is calling this function could potentially waste Ether, and it might not behave the way they expect. Make sure you have Ganache running first.
Then, create a new test file in the command line from the root of your project like this:. These come bundled with the Truffle framework. Here is all the code for the tests:. First, we require the require the contract and assign it to a variable, like we did in the migration file.
Next, we call the "contract" function, and write all our tests within the callback function. This callback function provides an "accounts" variable that represents all the accounts on our blockchain, provided by Ganache. The first test checks that the contract was initialized with the correct number of candidates by checking the candidates count is equal to 2.
You can watch me explain this code more in depth in the video at Notice that your application says "Loading In order to connect to the blockchain, we need to import one of the accounts from Ganache into Metamask.
You can watch me set up Metamask in the video at Any revenues earned by the organization will be distributed according the "reputations" of the users, the website states. As a dapp, Ampliative Art will be a transparent cooperative in which users would collaborate, receive rewards and take part in the decision making process.
The project says that since it will be one of many crowdfunding platforms on Web 3. All critical aspects of the platform are completely decentralized. From there they can then start, contribute to, browse and manage crowdfunding campaigns. Unlike conventional crowdfunding services like Kickstarter, though, WeiFund uses smart contracts, which means donations can actually be turned into complex agreements. This gives campaign operators a "broader range of possibilities "when raising funds, the project says.
Further, Web 3. Picture frame and cellphone images via Shutterstock; Feature image via Ethereum. The leader in news and information on cryptocurrency, digital assets and the future of money, CoinDesk is a media outlet that strives for the highest journalistic standards and abides by a strict set of editorial policies.
Empowering artists. Ampliative Art. DISCLOSURE The leader in news and information on cryptocurrency, digital assets and the future of money, CoinDesk is a media outlet that strives for the highest journalistic standards and abides by a strict set of editorial policies. Subscribe to First Mover, our daily newsletter about markets.
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