Learn Solidity: Events & Logging in Solidity

In this post, we will learn how to declare & use events for logging in Solidity Language.

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Notice: This is one of the multi-post series of Learn Solidity - Build Decentralized Application in Ethereum. This is an attempt to teach you all about Solidity - A Programming Language for Ethereum Based Smart Contracts. If you want to take this as a video course please signup using below button.
pragma solidity 0.4.8;

// Events are the way to inform external monitoring entities about the 
// cenrtain activities in the smart contracts
// they can also be used as a log in the system.

// lets checkout out this coin example where we will jump deeper
// in the next lecture.

contract Coin {
    * @title A Simple Subcurrency Example
    * @author Toshendra Sharma
    * @notice Example for the Solidity Course
    * @dev This is only for demo the simple Coin example
    address public minter;
    uint public totalCoins;

    event LogCoinsMinted(address deliveredTo, uint amount);
    event LogCoinsSent(address sentTo, uint amount);

    // Please note the caps lock. Your mist walled will you capital letters 
    // to seperate th event name into a meaningful name. 

    // As I said Events allow the convenient usage of the EVM logging facilities. 
    // Events are inheritable members of contracts. When they are called, 
    // they cause the arguments to be stored in the transaction's log. 
    // Up to three parameters can receive the attribute indexed which 
    // will cause the respective arguments to be treated as log topics 
    // instead of data. The hash of the signature of the event is one of 
    // the topics except you declared the event with anonymous specifier. 
    // All non-indexed arguments will be stored in the data part of the log. 

    mapping (address => uint) balances;
    function Coin(uint initialCoins) {
        minter = msg.sender;
        totalCoins = initialCoins;
        balances[minter] = initialCoins;

    /// @notice Mint the coins
    /// @dev This does not return any value
    /// @param owner address of the coin owner, amount amount of coins to be delivered to owner
    /// @return Nothing
    function mint(address owner, uint amount) {
        if (msg.sender != minter) return;
        balances[owner] += amount;
        totalCoins += amount;
        LogCoinsMinted(owner, amount);

    function send(address receiver, uint amount) {
        if (balances[msg.sender] < amount) return;
        balances[msg.sender] -= amount;
        balances[receiver] += amount;
        LogCoinsSent(receiver, amount);

    function queryBalance(address addr) constant returns (uint balance) {
        return balances[addr];

    function killCoin() returns (bool status) {
        if (msg.sender != minter) throw;
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