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stackExchange_84109_structWithMapping.sol

// https://ethereum.stackexchange.com/questions/84109/solidity-0-4-26-check-if-element-already-exists-in-array

// SPDX-License-Identifier: MIT
pragma experimental ABIEncoderV2;
pragma solidity >=0.6.9 <0.7.0;

contract stackExchange_84109_structWithMapping {

    struct User {
        string  username;
        string  category;
        string  data;
        bool    initialized;
    }
    mapping(string => User) public users;

    function setUserData(string memory _username, string memory _category, string memory _data) public{
        require(!users[_username].initialized);
        users[_username].username = _username;
        users[_username].category = _category;
        users[_username].data = _data;
        users[_username].initialized = true;
    }

}