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contracts...Memory.sol

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

contract Memory {
    struct LuckyNumber {
        uint256 number;
        string reason;
    }

    mapping(address => LuckyNumber) luckyNumbers;

    function addLuckyNumber(LuckyNumber memory luckyNumber) external {
        require(luckyNumber.number != 0, "Lucky number can't be 0!");
        require(luckyNumbers[msg.sender].number == 0, "You already have set lucky number. Edit it if you have another one.");

        luckyNumbers[msg.sender] = luckyNumber;
    }

    function editLuckyNumber(uint256 luckyNumber) external {
        require(luckyNumber != 0, "Lucky number can't be 0!");
        require(luckyNumbers[msg.sender].number != 0, "You don't have a lucky number set yet.");

        LuckyNumber storage _luckyNumber = luckyNumbers[msg.sender];
        // LuckyNumber memory _luckyNumber = luckyNumbers[msg.sender]; -- This won't work, because of "memory" keyword
        _luckyNumber.number = luckyNumber;
    }

    function getMyLuckyNumber() external view returns(uint256) {
        require(luckyNumbers[msg.sender].number != 0, "You don't have a lucky number set yet.");

        LuckyNumber memory luckyNumber = luckyNumbers[msg.sender];

        return luckyNumber.number;
    }

    function multiplyByItself(uint256 number) external pure returns(uint256) {
        uint256 result = number * number;

        return result;
    }
}