TokenSale.sol

// To specify what version of compiler this code will compile with
pragma solidity ^0.4.11;

contract tokensale {

  // Declaring the maximum number of tokens available for sale.
  uint public maxTokens = 10000;

  // The number of tokens you can buy for one wei (Solidity converts all unitless values from ethers to wei).
  // We will keep the default currency to be wei. It will get clearer.
  uint public tokenSwap = 10;

  // This variable, initially set to zero,  stores the number of tokens that have been bought, i.e it will
  // get incremented each time someone buys tokens.
  uint public supply = 0;

  // mapping is an equivalent to associative array. We store token balances corresponding
  // to each address that buys tokens in this mapping.
  mapping (address => uint) balance;


  // A modifier checks if a condition is met before executing a function.
  // This  modifer checks if there are tokens available before the buyTokens function is called.
  modifier isTokenAvailable () {
    require (msg.value*tokenSwap + supply <= maxTokens);     // require checks the condition and the function that
    _;                                                    	 // follows, denoted by the '_', is only run if the condition
  }							                                             // is satisfied.


  // A function to check the ether balance of any address.
  function checkBalance (address addr) external returns (uint) {
    return addr.balance;       // The address.balance call returns the ether balance of the given address.
  }


  // To check the token balance of any address. The external keyword allows the function
  // to be called from outside the contract. If no keyword is specified, all functions are external
  // by default..
  function balanceOf (address tokenHolder) external constant returns (uint) {
    return balance[tokenHolder];
  }

  // A function to buy tokens accesible by any address
  // The payable keyword allows the contract to accept ethers
  // from the transactor. The ethers to be deposited is entered as msg.value
  // (which will get clearer when we will call the functions in browser-solidity)
  // and the corresponding tokens are stored in balance[msg.sender] mapping.
  // underflows and overflows are security consideration which are
  // not checked in the process. But lets not worry about them for now.

  function buyTokens () external
  payable
  isTokenAvailable {
    uint tokensAmount = msg.value * tokenSwap;    // At this point, msg.value is in wei.
    balance [msg.sender] += tokensAmount;
    supply += tokensAmount;
  }
}