String Operations in Solidity

This example demonstrates different ways to work with strings in Solidity, including concatenation, comparison, and encoding.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract StringOperations {
    // State variables
    string public storedString = "Hello";

    // String comparison using keccak256 hash
    function compareStrings(string memory str1, string memory str2) public pure returns (bool) {
        return keccak256(abi.encodePacked(str1)) == keccak256(abi.encodePacked(str2));
    }

    // String concatenation using string.concat (Solidity 0.8.12+)
    function concatenateStrings(string memory str1, string memory str2) public pure returns (string memory) {
        return string.concat(str1, str2);
    }

    // Multiple string concatenation with a number using string.concat
    function concatenateWithNumber(string memory str1, uint256 number, string memory str2) public pure returns (string memory) {
        return string.concat(str1, " ", toString(number), " ", str2);
    }

    // Legacy concatenation using abi.encodePacked
    function concatenateStringsLegacy(string memory str1, string memory str2) public pure returns (string memory) {
        return string(abi.encodePacked(str1, str2));
    }

    // Convert uint to string using OpenZeppelin's approach
    function toString(uint256 value) internal pure returns (string memory) {
        // Special case for 0
        if (value == 0) {
            return "0";
        }

        // Get number of digits
        uint256 temp = value;
        uint256 digits;
        while (temp != 0) {
            digits++;
            temp /= 10;
        }

        // Create bytes array
        bytes memory buffer = new bytes(digits);

        // Fill buffer from right to left
        while (value != 0) {
            digits -= 1;
            buffer[digits] = bytes1(uint8(48 + uint256(value % 10)));
            value /= 10;
        }

        return string(buffer);
    }

    // Demonstrate different encoding methods
    function demonstrateEncoding(string memory str1, string memory str2) public pure returns (bytes memory, bytes memory, bytes memory) {
        // abi.encode: Pads data to 32 bytes
        bytes memory encoded = abi.encode(str1, str2);

        // abi.encodePacked: Minimal encoding without padding
        bytes memory encodedPacked = abi.encodePacked(str1, str2);

        // keccak256: Creates a hash of the data
        bytes memory hashed = abi.encodePacked(keccak256(abi.encodePacked(str1, str2)));

        return (encoded, encodedPacked, hashed);
    }

    // Get string length (UTF-8 bytes length)
    function getStringLength(string memory str) public pure returns (uint256) {
        return bytes(str).length;
    }
}

Key Concepts

1. String Comparison

   • Strings can't be directly compared
   • Must use keccak256 hash comparison
   • Case-sensitive comparison

2. String Concatenation

   • Use abi.encodePacked() for efficient concatenation
   • Convert result back to string
   • Can combine strings with other types

3. Encoding Methods

   • abi.encode(): Pads each element to 32 bytes
   • abi.encodePacked(): Minimal encoding without padding
   • keccak256(): Creates a 32-byte hash

4. String Length

   • Returns bytes length (UTF-8 encoded)
   • Not character count for non-ASCII strings
   • Gas efficient operation

Best Practices

1. Use abi.encodePacked() for efficient string operations
2. Cache string operations to save gas
3. Be aware of UTF-8 encoding when working with strings
4. Consider gas costs when manipulating strings
5. Use events to log string changes

Common Use Cases

1. String Comparison

   • User input validation
   • State verification
   • Access control

2. String Concatenation

   • Building messages
   • Creating URIs
   • Formatting output

3. Encoding

   • Creating unique identifiers
   • Hashing data
   • Signature verification

4. String Length

   • Input validation
   • Data size verification
   • Gas estimation