First Contract - skanakakorn/eth-nonode GitHub Wiki

Your first contract

Writing your first contract

pragma solidity ^0.4.10;
contract MessageStorage {
    string storedData;

    event Set(
        address indexed _from,
        string s
    );

    function set(string s) {
        storedData = s;
        Set(msg.sender, s);
    }

    function get() constant returns (string retVal) {
        return storedData;
    }
}

This is stored in a file called contract_1st.sol.

Compiling your first contract.

You can compile contract via various means. For now, use Remix IDE. https://remix.ethereum.org/#optimize=false&version=soljson-v0.4.10+commit.f0d539ae.js

Copy the contract code to the window of compiler.

Notice the contract address ? You’ll need it. Save it somewhere.

Verify that contract has been created. On test net, goto https://ropsten.etherscan.io/

Lab 1

• Change message using set method.
• Check if your message is correct using get method.
• Use this contract address with injected web3 0x165bba1094f37203ab46b0107f9050ac84b5cf30. Use get method. What's the string do you get?
• Why is it different from the string you set in other contract?

Interacting with smart contract via mycrypto

Use this link https://mycrypto.com. Access your wallet via MetaMask

Exercise

• What is ABI ?
• Why does a contract have an address ?
• When does a contract generate transaction ?
• What is your transaction ID or number ?
• How do you figure out which block has your transaction ?

Programming Exercise

• Write a Contract called "Math1" that has function multiply. The function would look like this

    function multiply(int a, int b) constant returns(int retVal) {
        return a * b;
    }

• Write a contract called "Math2". Add a function called setBase(int baseVal). This would set multiplication base. Add another function called mulbase(int param). This would multiply param with baseVal and return int. I.E, setBase(3) follow by mulbase(7) would return 21.