contract token {
    string public standard = '';
    string public name; 
    string public symbol; 
    uint8 public decimals = 18;  
    uint256 public totalSupply; 

    mapping (address => uint256) public balanceOf;
    mapping (address => mapping (address => uint256)) public allowance;

    event Transfer(address indexed from, address indexed to, uint256 value);  //转帐通知事件
    event Burn(address indexed from, uint256 value);  //减去用户余额事件

    function token(uint256 initialSupply, string tokenName, string tokenSymbol) public {

        totalSupply = initialSupply * 10 ** uint256(decimals);    //以太币是10^18，后面18个0，所以默认decimals是18

        balanceOf[this] = totalSupply;

        name = tokenName;
        symbol = tokenSymbol;

    }

    function _transfer(address _from, address _to, uint256 _value) internal {

      require(_to != 0x0);

      require(balanceOf[_from] >= _value);

      require(balanceOf[_to] + _value > balanceOf[_to]);

      uint previousBalances = balanceOf[_from] + balanceOf[_to];

      balanceOf[_from] -= _value;

      balanceOf[_to] += _value;

      Transfer(_from, _to, _value);

      assert(balanceOf[_from] + balanceOf[_to] == previousBalances);

    }
    
    function transfer(address _to, uint256 _value)  public {
        _transfer(msg.sender, _to, _value);
    }

    function transferFrom(address _from, address _to, uint256 _value) public returns (bool success) {
        require(_value <= allowance[_from][msg.sender]);   // Check allowance
        
        allowance[_from][msg.sender] -= _value;

        _transfer(_from, _to, _value);

        return true;
    }

    function approve(address _spender, uint256 _value) public returns (bool success) {
        allowance[msg.sender][_spender] = _value;
        return true;
    }

    function burn(uint256 _value) public returns (bool success) {
        require(balanceOf[msg.sender] >= _value);   // Check if the sender has enough

        balanceOf[msg.sender] -= _value;

        totalSupply -= _value;

        Burn(msg.sender, _value);
        return true;
    }

    function burnFrom(address _from, uint256 _value) public returns (bool success) {

        require(balanceOf[_from] >= _value);

        require(_value <= allowance[_from][msg.sender]);

        balanceOf[_from] -= _value;
        allowance[_from][msg.sender] -= _value;

        totalSupply -= _value;
        Burn(_from, _value);
        return true;
    }
}
function token(uint256 initialSupply, string tokenName, string tokenSymbol) public {

    totalSupply = initialSupply * 10 ** uint256(decimals);    //以太币是10^18，后面18个0，所以默认decimals是18

    balanceOf[this] = totalSupply;

    name = tokenName;
    symbol = tokenSymbol;

}

//减去用户余额事件