[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.

Datatype	Default Value
uint, int	0
bool	false
address	address(0)（0x0000000000000000000000000000000000000000）
bytes	0x
string	"" (empty string)

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";
    }
    
}