Digital items with Ethereum contract

Basic

pragma solidity ^0.4.19;

contract HelloWorld {

}

State variable

State variables permanently write on to the Ethereum blockchain.

pragma solidity ^0.4.19;

contract HelloWorld {
 uint dnaDigits = 16;
}

Math operations

Addition: x + y Subtraction: x - y, Multiplication: x * y Division: x / y Modulus / remainder: x % y

pragma solidity ^0.4.19;

contract HelloWorld {
 uint dnaDigits = 16;
 uint dnaModulus = 10 ** dnaDigits;
}

Struct and Array

To store data in a custom data structure, use the combination of Struct and Arrays

    struct Zombie {
        string name;
        uint dna;
    }
    
    // declare a dynamic array of Zombie
    // Zombie[4] is a static fix length array
    Zombie[] public zombies;
    

Function, private, public, view and pure

All functions are public by default

    function createZombie(string _name, uint _dna) {
        zombies.push(Zombie(_name, _dna));
    }
    ```
    
By convention, use `_` as prefix for private functions. Function declaration will also
have `private` keyword follow the parameters.

    ```
        function _createZombie(string _name, uint _dna) private {
        zombies.push(Zombie(_name, _dna));
    }
    ```
    
When accessing an internal state, this function is `view` as to say read-only. 
While `pure` functions do not use any state and only return the calculation.

function _generateRandomDna(string _str) private view returns (uint) {
    uint rand = uint(keccak256(_str));
    return rand % dnaModulus;
}

function createRandomZombie(string _name) public {
    uint randDna = _generateRandomDna(_name);
    _createZombie(_name, randDna);
} ```

Events

Events are notify by the contract to the corresponding watcher.

event NewZombie(uint zombieId, string name, uint dna);

Final program with events

pragma solidity ^0.4.19;

contract ZombieFactory {

    event NewZombie(uint zombieId, string name, uint dna);

    uint dnaDigits = 16;
    uint dnaModulus = 10 ** dnaDigits;

    struct Zombie {
…
}

pragma solidity ^0.4.19;

contract ZombieFactory {

    event NewZombie(uint zombieId, string name, uint dna);

    uint dnaDigits = 16;
    uint dnaModulus = 10 ** dnaDigits;

    struct Zombie {
        string name;
        uint dna;
    }

    Zombie[] public zombies;
    
        function _createZombie(string _name, uint _dna) private {
        uint id = zombies.push(Zombie(_name, _dna));
        NewZombie(id, _name, _dna);
    } 

    function _generateRandomDna(string _str) private view returns (uint) {
        uint rand = uint(keccak256(_str));
        return rand % dnaModulus;
    }
    
        function createRandomZombie(string _name) public {
        uint randDna = _generateRandomDna(_name);
        _createZombie(_name, randDna);
    }

}

Frontend with web3.js

// Here's how we would access our contract:
var abi = /* abi generated by the compiler */
var ZombieFactoryContract = web3.eth.contract(abi)
var contractAddress = /* our contract address on Ethereum after deploying */
var ZombieFactory = ZombieFactoryContract.at(contractAddress)
// `ZombieFactory` has access to our contract's public functions and events

// some sort of event listener to take the text input:
$("#ourButton").click(function(e) {
  var name = $("#nameInput").val()
  // Call our contract's `createRandomZombie` function:
  ZombieFactory.createRandomZombie(name)
})

// Listen for the `NewZombie` event, and update the UI
var event = ZombieFactory.NewZombie(function(error, result) {
  if (error) return
  generateZombie(result.zombieId, result.name, result.dna)
})

// take the Zombie dna, and update our image
function generateZombie(id, name, dna) {
  let dnaStr = String(dna)
  // pad DNA with leading zeroes if it's less than 16 characters
  while (dnaStr.length < 16)
    dnaStr = "0" + dnaStr

  let zombieDetails = {
    // first 2 digits make up the head. We have 7 possible heads, so % 7
    // to get a number 0 - 6, then add 1 to make it 1 - 7. Then we have 7
    // image files named "head1.png" through "head7.png" we load based on
    // this number:
    headChoice: dnaStr.substring(0, 2) % 7 + 1,
    // 2nd 2 digits make up the eyes, 11 variations:
    eyeChoice: dnaStr.substring(2, 4) % 11 + 1,
    // 6 variations of shirts:
    shirtChoice: dnaStr.substring(4, 6) % 6 + 1,
    // last 6 digits control color. Updated using CSS filter: hue-rotate
    // which has 360 degrees:
    skinColorChoice: parseInt(dnaStr.substring(6, 8) / 100 * 360),
    eyeColorChoice: parseInt(dnaStr.substring(8, 10) / 100 * 360),
    clothesColorChoice: parseInt(dnaStr.substring(10, 12) / 100 * 360),
    zombieName: name,
    zombieDescription: "A Level 1 CryptoZombie",
  }
  return zombieDetails
}

Etherum Contract Intro