Solidity - Programming ERC-20 Token

ERC token standard 20 tokens in a smart contract token that follows the Ethereum Request for Commentproposal 20 guidelines. Our program will have the following mandatory functions: 

• function totalSupply() public view returns (uint256);
• function balanceOf(address tokenOwner) public view returns (uint);
• function allowance(address tokenOwner, address spender)
• function transfer(address to, uint tokens) public returns (bool);
• function approve(address spender, uint tokens)  public returns (bool);
• function transferFrom(address from, address to, uint tokens) public returns (bool);

And two events which emit to the external application: 

• event Approval(address indexed tokenOwner, address indexed spender, uint tokens);
• event Transfer(address indexed from, address indexed to, uint tokens);

Important keywords used:

• Event: An event is Solidity’s way of allowing clients e.g notifying your front end application about the occurrence of a specific event.
• View: View functions ensure that they will not modify the state
• public: A public function can be accessed outside the contract itself
• indexed: Up to three parameters can receive the attribute indexed through which we can search respective arguments.

Step 1: The expression mapping(address => uint256) defines an associative array whose keys are of type address and value of type uint256 used to store the balance. 

1. balances will hold the token balance of each owner's account.
2. allowed will include all the accounts approved to withdraw from a given account together with the withdrawal sum allowed for each.

mapping(address => uint256) balances;
mapping(address => mapping (address => uint256)) allowed;

Step 2: Define the total supply of the token.

Step 3: Declare the events required and add the compulsory functions as per the specified signature.

Output: