[Solidity] How to check for existence in mapping?

This section describes how to check for the existence of a mapping in Solidity.

Conclusion: Mapping has no method to check for existence

First of all, to conclude, Solidity's mapping does not provide a method to check for the existence of a value, such as exists method.

Therefore, it is necessary to take alternative measures, such as checking for the existence of a value by comparing it with an initial value.

Alternative: Compare with the initial value and check for existence

In Solidity mapping, if a key does not exist, it will not be empty (null) but will return an initial value according to its type.

This property is used to check for the existence of a mapping.

Since the table above is the initial value for each data type, it is compared with this to check for existence.

The following is an actual sample code.

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.17;
 
contract TestContract {

    mapping (address => uint) private uintMapping; // uint
    mapping (address => bool) private boolMapping; // bool
    mapping (address => address) private addressMapping; // address
    mapping (address => bytes) private bytesMapping; // bytes
    mapping (address => string) private stringMapping; // string


    function isExistUint() public view returns(string memory) {
        if (uintMapping[msg.sender] == 0) {
            return "Not Exitst";
        }
        return "Exist";
    }


    function isExistBool() public view returns(string memory) {
        if (boolMapping[msg.sender] == false) {
            return "Not Exitst";
        }
        return "Exist";
    }


    function isExistAddress() public view returns(string memory) {
        if (addressMapping[msg.sender] == address(0)) {
            return "Not Exitst";
        }
        return "Exist";
    }


    function isExistBytes() public view returns(string memory) {
        if (keccak256(bytesMapping[msg.sender]) == keccak256(hex"")) {
            return "Not Exitst";
        }
        return "Exist";
    }


    function isExistString() public view returns(string memory) {
        if (keccak256(abi.encodePacked(uintMapping[msg.sender])) == keccak256(abi.encodePacked(""))) {
            return "Not Exitst";
        }
        return "Exist";
    }
    
}

This is not a strict method, but we have introduced it as an alternative. If Solidity's built-in methods (standard methods) are not available, you should create your own or use an external library.