Solidity 0.4.26 check if element already exists in array

Question

How do I make it so this only adds an identifier to the dataArray if it doesn't exist already?

pragma experimental ABIEncoderV2;

contract structWithMapping{

    struct Data{
        string[] user;
        string[] catagory;
        string[]  data;
    }

    mapping(string => Data) mappedData;
    string[] public dataArray;

    function setUserData(string _user, string _catagory, string _data)public{
        var addData = mappedData[_user];

        addData.user.push(_user);
        addData.catagory.push(_catagory);
        addData.data.push(_data);

        dataArray.push(_user) -1;
    }

    function getUsers() view public returns(string[]){
        return dataArray;
    }

    function getUserData(string _user) view public returns(string[] memory, string[] memory, string[] memory){
        return(mappedData[_user].user, mappedData[_user].catagory, mappedData[_user].data);
    }
}

Answer 1

In order to avoid For loops, You can add another mapping to check if a user exists or not. You can add mapping(string => bool) userExists; so your whole code will look like this:

pragma experimental ABIEncoderV2;


contract structWithMapping{

    struct Data{
        string[] user;
        string[] catagory;
        string[] data;
    }


    mapping(string => Data) mappedData;
    mapping(string => bool) userExists;

    string[] public dataArray;

    function setUserData(string _user, string _catagory, string _data)public{
        var addData = mappedData[_user];


        addData.user.push(_user);
        addData.catagory.push(_catagory);
        addData.data.push(_data);

        require(!userExists[_user]);
        dataArray.push(_user) -1;
        //dont forget to set the mapping value to true for userExists
        userExists[_user] = true;
    }

    function getUsers() view public returns(string[]){
        return dataArray;
    }

    function getUserData(string _user) view public returns(string[] memory, string[] memory, string[] memory){
        return(mappedData[_user].user, mappedData[_user].catagory, mappedData[_user].data);
    }
}

Answer 2

Reading your code I think I understood what you want to achieve, but I suggest to completely change the approach. Please note that using a smart contract costs money, so as a developer you should aim to create business logic that is both clean - so it cannot be misinterpreted or misleading - and light - so every transaction costs as few as possible.

To achieve that you need also to deep dive into the EVM and Solidity or Vyper.

That said, I think your entire smart contract should be transformed to this:

contract 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;
    }

}

Note also that Solidity adds getter functions at compile time for every public variable, so in this case a users function will be created. You can use that function passing the username as parameter to retrieve user data in the exact same way you would have called getUserData function.

I created a gist so you can directly experiment through the Remix IDE via this link.