How do you compare strings in Solidity?

Question

I would assume comparing strings would be as easy as doing:

function withStrs(string memory a, string memory b) internal {
  if (a == b) {
    // do something
  }
}

But doing so gives me an error Operator == not compatible with types string memory and string memory.

What's the right way?

Cluff

Answer 1

You can compare strings by hashing the packed encoding values of the string:

if (keccak256(abi.encodePacked(a)) == keccak256(abi.encodePacked(b))) {
  // do something
}

keccak256 is a hashing function supported by Solidity, and abi.encodePacked() encodes values via the Application Binary Interface.

Cluff

Probably simple conversion to bytes (from another answer) is cheaper in terms of gas than abi.encodePacked. We have to check. – Hydropic