ERC20 Token Contract

BasicERC20.sol

pragma solidity ^0.5.1;

// SAFE Mathematical Operations
library SafeMath { 
    function sub(uint256 a, uint256 b) internal pure returns (uint256) {
      assert(b <= a); //error handling condition
      return a - b;  //To Avoid Overflows
    }

    function add(uint256 a, uint256 b) internal pure returns (uint256) {
      uint256 c = a + b;
      assert(c >= a);
      return c;
    }
}

contract IntelliContract {

    string public constant name = "CustomEnergyToken"; // solidity automatically creates a getter function for public variables
    string public constant symbol = "CET"; // getter function is a function used to retrive a specific value from ledger
    uint8 public constant decimals = 18;  // Similar to Ether 
       
// Setter functions : Function Which Creates or Updates A Value of a variable on Legder.
// Getter Functions : Function Which Get or Fetches A Value of a variable From Legder.

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


    modifier onlyOwner  {
        require (msg.sender == ownerCon);
        _;
    }

    mapping(address => uint256) balances; 
   // ----------------------------------------------------------------------------
    mapping(address => mapping (address => uint256)) allowed;  // nested mapping

    // Nested Mapping Has 2 Keys & 1 Value

    uint256 totalSupply_;
    address ownerCon;

    using SafeMath for uint256;


   constructor(uint256 total) public {  // special function , only called at time of deployemnet
	totalSupply_ = total ;
	balances[msg.sender] = total ;  // To Deposit all the newly generated tokens in owner's account
	ownerCon = msg.sender;
    }  

    function totalSupply() public view returns (uint256) {
	return totalSupply_;
    }
    // getter function

    function balanceOf(address inputAddress) public view returns (uint) {
        return balances[inputAddress];
    }
    // getter function 
    
    function transfer(address receiver, uint numTokens) public isAddressWhitelisted(receiver) returns (bool) {
        require(numTokens <= balances[msg.sender], "Not Sufficient Balance");
        balances[msg.sender] = balances[msg.sender].sub(numTokens);
        balances[receiver] = balances[receiver].add(numTokens);
        emit Transfer(msg.sender, receiver, numTokens); // logging these values using events
        return true;
    }

    function approve(address approved_addr, uint numTokens) public returns (bool) {
        allowed[msg.sender][approved_addr] = numTokens;
        emit Approval(msg.sender, approved_addr, numTokens);
        return true;
    }

    function allowance(address owner, address token_manger) public view returns (uint) {
        return allowed[owner][token_manger];
    }// what allowance has been provided by token_owner to Token_manager

    function transferFrom(address owner, address buyer, uint numTokens) public returns (bool) {
        require(numTokens <= balances[owner]);    
        require(numTokens <= allowed[owner][msg.sender]);
        balances[owner] = balances[owner].sub(numTokens);
        allowed[owner][msg.sender] = allowed[owner][msg.sender].sub(numTokens);
        balances[buyer] = balances[buyer].add(numTokens);
        emit Transfer(owner, buyer, numTokens);
        return true;
    }

//---------------------------------------------------------------------------------------//
                              // Adding Assignment Functions //
//----------------------------------------------------------------------------------------/
// create a feature to selfdestruct but it should only be called by owner .
// we want to increase the total supply of tokens (only be called by owner : Modifiers)
// we want to burn some of tokens out of  total supply of tokens (only be called by owner)
//--------------------------------------------------------------------------//
// we want to transfer tokens only to whitelisted addresses ,firstly you will have to create a function to whitelist addresses , and then at time of transfer we will have to ensure that its one of whitelisted address


    function mintNewTokens(uint increasedAmount) public onlyOwner returns(bool){
    require(increasedAmount > 0);
    balances[msg.sender] = balances[msg.sender].add(increasedAmount);
    totalSupply_ = totalSupply_.add(increasedAmount);
    emit Transfer(address(0), msg.sender, increasedAmount);
    return true;
}
    function decreaseSupply (uint burnAmount) public onlyOwner returns(bool){
    require(burnAmount <= balances[msg.sender]);  
    require(burnAmount > 0);
    balances[msg.sender] = balances[msg.sender].sub(burnAmount);
    totalSupply_ = totalSupply_.sub(burnAmount);
    emit Transfer(msg.sender,address(0), burnAmount);
    return true;
}
    mapping(address => bool) allwhitelistedAddresses;

    modifier isAddressWhitelisted(address _address) {
    require(allwhitelistedAddresses[_address], "This Address is Not Whitelisted");
    _;
    }
    
    modifier isAlreadyWhitelisted(address _address) {
    require(!allwhitelistedAddresses[_address], "This Address is Not Whitelisted");
    _;
    }

    function addaddress(address _addressToWhitelist) public onlyOwner isAlreadyWhitelisted(_addressToWhitelist) {
         allwhitelistedAddresses[_addressToWhitelist] = true;
    }
}