How to manipulate strings in Solidity?

Manipulating strings in Solidity is notoriously difficult. This is surprising for most developers who are used to manipulating easily in other programming languages like Javascript or Python.

In this article I give a couple of useful code snippets to do some simple string manipulations in Solidity.

Warning: Keep in mind that manipulating strings in Solidity is costly in gas. If you can avoid it and use other types like bytes and bytes32, that’s better.

Get length of a string in Solidity

function length(string calldata str) external pure returns(uint) {
        return bytes(str).length;
    }

Concatenate 2 strings in Solidity

function concatenate(
        string calldata a,
        string calldata b)
        external 
        pure
        returns(string memory) {
            return string(abi.encodePacked(a, b));
        }

Compare 2 strings in Solidity

function compare(
    string calldata a,
    string calldata b)
    external
    pure
    returns(bool) {
        return keccak256(abi.encodePacked(a)) == keccak256(abi.encodePacked(b));
    }

Reverse 2 strings in Solidity

function reverse(string calldata _str) external pure returns(string memory) {
        bytes memory str = bytes(_str);
        string memory tmp = new string(str.length);
        bytes memory _reverse = bytes(tmp);

        for(uint i = 0; i < str.length; i++) {
            _reverse[str.length - i - 1] = str[i];
        }
        return string(_reverse);
    }

Bonus Stringutils libary

If you want to do even more string manipulations you can use the Stringutils library. Warning: This wasn't updated for the last 2 years and is not compatible with Solidity 0.6.