How to use require() with stings

Question

I am trying to run this code on remix IDE:

pragma solidity ^0.4.24;

contract Aa {

    string public result;    

    function feedData(string _result) public {
        require(_result = "e" , "error");
        _result = result;
    }
}

I keep getting an error: No matching declaration found after argument-dependent lookup. require(_result = "e" , "error");

I looked up the error on stackexchange but the answer doesn't solve this issue. (No matching declaration found after argument-dependent lookup)

When I change the line:

require(_result = "e" , "error");

to => :

require(_result == "e" , "error"); // adding an extra equal sign.

I get the error: Operator == not compatible with types string memory and literal_string "e" require(_result == "e" , "error")

So now my question is: How can I use require() with a string variable or is there a better way to achieve what the code is trying to do?

Answer 1

There's no built-in equality test for strings (or other arrays), so a common technique is to compare the hashes of the strings instead:

function feedData(string _result) public {
    // Compare hashes
    require(keccak256(bytes(_result)) == keccak256("e"), "error");

    result = _result; // This line was backwards
}