i have been trying to compile the code below but keep geting this TypeError: Operator == not compatible with types string storage ref and bytes32

Question

//SPDX-License-Identifier: Unlicense

// contracts Exam
pragma solidity ^0.8.0;


contract Exam {
    // The address of the exam administrator
    address public administrator;

    // The exam questions and answers
    mapping(uint => string) public questions;
    mapping(uint => string) public answers;

    // The exam submissions
    mapping(address => Submission) public submissions;

    // The number of questions in the exam
    uint public numQuestions;

    // The minimum score required to pass the exam
    uint public passingScore;

    // The maximum number of attempts allowed per user
    uint public maxAttempts;

    // The state of the exam (open or closed)
    bool public isOpen;

    // The Submission struct represents a user's exam submission
    struct Submission {
        uint[] answers;
        uint score;
        uint numAttempts;
    }

    constructor(uint _numQuestions, uint _passingScore, uint _maxAttempts) public {
        administrator = msg.sender;
        numQuestions = _numQuestions;
        passingScore = _passingScore;
        maxAttempts = _maxAttempts;
        isOpen = true;
    }

    // The addQuestion function allows the exam administrator to add a question to the exam
    function addQuestion(uint _questionNumber, string memory _question, string memory _answer) public {
        require(msg.sender == administrator, "Only the administrator can add questions to the exam.");
        require(_questionNumber > 0 && _questionNumber <= numQuestions, "Invalid question number.");
        require(questions[_questionNumber] == bytes32(0), "Question has already been added.");

        questions[_questionNumber] = _question;
        answers[_questionNumber] = _answer;
    }

    // The submitAnswers function allows a user to submit their answers to the exam
    function submitAnswers(uint[] memory _answers) public {
        require(isOpen, "Exam is closed.");
        require(_answers.length == numQuestions, "Incorrect number of answers.");

        address user = msg.sender;
        Submission storage submission = submissions[user];
        submission.numAttempts++;

        require(submission.numAttempts <= maxAttempts, "Exceeded maximum number of attempts.");

        // Grade the submission
        submission.score = 0;
        for (uint i = 0; i < numQuestions; i++) {
            if (_answers[i] == uint(answers[i])) {
 {
                submission.score++;
            }
        }

        // Save the submission
        submission.answers = _answers;
    }}

    // The getScore function allows a user to retrieve their current score for the exam
   function getScore() public view returns (uint) {
        address user = msg.sender;
        return submissions[user].score;
    }

    // The closeExam function allows the exam administrator to close the exam
    function closeExam() public {
        require(msg.sender == administrator, "Only the administrator can close the exam.");
        isOpen = false;
    }
}

Answer 1

to check if string value has been changed from default you need to check length of this string. To do this change in your code from:

require(questions[_questionNumber] == bytes32(0), "Question has already been added.");

To:

require(bytes(questions[_questionNumber]).length == 0, "Question has already been added.");

You need to convert string value to bytes, cuz string doesn't have .length functionality.