DeclarationError: Undeclared identifier - solidity

Question

I am trying to create a token using solidity programming, but I keep getting this undeclared identifier error when I compile on remix browser IDE. I am new to solidity and how can I solve this problem?
I have attached my code here:
pragma solidity >=0.4.16 < 0.6.0;
/*declare an interfaced named tokenReceipent so that any contract that implements receiveApproval function counts as a tokenReceipent*/
interface tokenRecipient
{
function receiveApproval(address _from, uint256 _value, address _token, bytes calldata _extraData) external;
}
contract TokenERC20 //create a contract ERC20 and declare public variables of the token
{
string public name;
string public symbol;
uint8 public decimals = 18;
uint256 public totalSupply;
mapping(address => uint256)public balanceOf; // create mapping with all balances
mapping(address => mapping(address => uint256)) public allowance;
event Transfer(address indexed from, address indexed to, uint256 value); //create an event on blockchain that will notify clients
event Approval(address indexed _owner, address indexed _spender, uint256 _value);
event Burn(address indexed from,uint256 value); //create an event that notifies clients about the amount burn
constructor(uint256 initialSupply, string memory tokenName, string memory tokenSymbol) // create a construct that initialized tokens to the creator of the contract
public
{
totalSupply = initialSupply*10** uint256(decimals); //update total supply with des 1 1 out
balanceOf[msg.sender]=totalSupply;//give creator all initial tokens
name=tokenName; //set the name and symbol for display purposes
symbol=tokenSymbol;
}
//create an internal function and can only be called by this smartContract
function _transfer(address _from, address _to, uint _value) internal
{
//prevent transfer to 0x0 address
//check that the balance of the sender has enough
//add thesame to the recepient
//insert assert to use static analysis to find bugs in your code,they should never fail
require(_to!=address(0x0));
//subtract from the sender
require(balanceOf[_from]>= _value);
//add thesame to the receipent
require(balanceOf[_to] + _value>= balanceOf[_to]);
uint previousBalances = balanceOf[_from] + balanceOf[_to];
balanceOf[_from] -= _value;
balanceOf[_from] += _value;
emit Transfer(_from , _to, _value);
//assert are used to find static analysis in your code,they should never fail
assert(balanceOf[_from] + balanceOf[_to] == previousBalances);
}
//create to transfer function
function transfer(address _to, uint256 _value)
public returns(bool success)
{
_transfer(msg.sender, _to, _value);
return true;
}
//create a from transfer function to transfer tokens from other address
function transferFrom(address _from, address _to, uint256 _value)
public returns(bool success)
{
require(_value <= allowance[_from][msg.sender]);
allowance[_from][msg.sender] -= _value;
_transfer(_from, _to, _value);
return true;
}
//create allowances for other address
//allows spender not spend a certain allowance on your behalf
function approveAndCall(address _spender, uint256 _value, bytes memory _extraData)
public returns(bool success)
{
tokenRecipient spender = tokenRecipient(_spender);
if (approve(_spender, _value)) {
spender.receiveApproval(msg.sender, _value, address(this), _extraData);
return true;
}
}
function burn(uint256 _value)
public returns (bool success)
{
require(balanceOf[msg.sender]>= _value);
balanceOf[msg.sender] -= _value; //subtract from the sender
totalSupply -= _value; //update the total supply of tokens
emit Burn(msg.sender, _value);
return true;
}
// function that destroys token from other(users/subscribers) accounts
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;
emit Burn(_from, _value);
return true;
}
}

Answer 1

The code is failing to compile due to the error:
browser/Token.sol:73:17: DeclarationError: Undeclared identifier.
if (approve(_spender, _value)) {
^-----^
Your code doesn't declare an approve function, hence the error.
If you didn't write the code yourself, I suggest you check the original source of the code for the approve function.
function approveAndCall(address _spender, uint256 _value, bytes memory _extraData)
public returns(bool success)
{
tokenRecipient spender = tokenRecipient(_spender);
if (approve(_spender, _value)) {
spender.receiveApproval(msg.sender, _value, address(this), _extraData);
return true;
}
}
To learn about tokens I suggest you read the OpenZeppelin documentation on tokens:
https://docs.openzeppelin.org/v2.3.0/tokens
You could deploy to a testnet a simple token that uses OpenZeppelin with Remix
pragma solidity ^0.5.0;
import "http://github.com/OpenZeppelin/openzeppelin-solidity/contracts/token/ERC20/ERC20.sol";
import "http://github.com/OpenZeppelin/openzeppelin-solidity/contracts/token/ERC20/ERC20Detailed.sol";
/**
* #title SimpleToken
* #dev Very simple ERC20 Token example, where all tokens are pre-assigned to the creator.
* Note they can later distribute these tokens as they wish using `transfer` and other
* `ERC20` functions.
*/
contract SimpleToken is ERC20, ERC20Detailed {
/**
* #dev Constructor that gives msg.sender all of existing tokens.
*/
constructor () public ERC20Detailed("SimpleToken", "SIM", 18) {
_mint(msg.sender, 10000 * (10 ** uint256(decimals())));
}
}
You can also ask questions at:
Ethereum Stack Exchange: https://ethereum.stackexchange.com/
Zeppelin Community Forum: https://forum.zeppelin.solutions/