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Created Jul 31, 2017
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// 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
isTokenAvailable {
uint tokensAmount = msg.value * tokenSwap; // At this point, msg.value is in wei.
balance [msg.sender] += tokensAmount;
supply += tokensAmount;
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