Web3 and Crypto Full Stack - Solidity Code

pragma solidity >=0.7.0 <0.9.0;

/**
 * @title Ballot
 * @dev Implements voting process along with vote delegation
 */
contract Student {
        string public facNum;
        string public  firstName;
        string public lastName;
        string public age;
        constructor( string memory _facNum, string memory _firstName, string memory _lastName, string memory  _age){
            facNum = _facNum;
            firstName = _firstName;
            lastName = _lastName;
            age = _age;


        }

        function getFirstName() public view returns( string memory _firstName){
            _firstName = firstName;
        }
        function getLastName() public view returns(string memory _lastName){
            _lastName = lastName;
        }
        function getAge() public view returns(string memory  _age){
            _age = age;

        }
        function getFacNum() public view returns(string memory _facNum){
            _facNum = facNum;
        }

}

contract Studenti {
    mapping (string => Student) public  students;
    string[]  public facNums;
    uint public num ;

    constructor(){
        facNums = new string[](100);
        num = 0;

    }

    function addStudent(string memory _facNum, string memory _firstName, string memory _lastName, string memory _age) public {

            Student  s = new Student(_facNum, _firstName, _lastName, _age);
            students[_facNum] = s;
            //facNums.push(_facNum);
            facNums[num] = _facNum;
            num = num + 1;
    }
    function concat(string memory s1, string memory s2) public pure returns (string memory) {
        return string(abi.encodePacked(s1, s2));
    }

    function concatenate(string memory s1, string memory s2, string memory s3, string memory s4, string memory s5, string memory s6, string memory s7, string memory s8, string memory s9) public pure returns (string memory res) {
        res = concat(s1, s2);
        res = concat(res, s3);
         res =concat(res, s4);
         res =concat(res, s5);
        res = concat(res, s6);
        res = concat(res, s7);
        res = concat(res, s8);
        res = concat(res, s9);
        return res;
    }




    function getStudentsJSON() public view  returns (string memory jsonArray){
        jsonArray = concat(jsonArray , "[");
        for(uint i=0; i< num; i++){
            //jsonArray = concat(jsonArray, (students[facNums[i]].getFirstName()));

            string memory jsonArray1 = concatenate( "{\"firstName\" : \"", students[facNums[i]].getFirstName(), "\", \"lastName\": \"", students[facNums[i]].getLastName(), "\", \"age\": \"", string(students[facNums[i]].getAge()), "\", \"facNum\": \"", students[facNums[i]].getFacNum(), "\"} ") ;
            jsonArray = concat(jsonArray, jsonArray1);
            if(i<num - 1){
                 jsonArray = concat(jsonArray , ", ");

            }
        }
         jsonArray = concat(jsonArray , "]");
        return jsonArray;
    }

    function getStudentByFacNum(string memory facNum) public view  returns (string memory  firstName, string memory  lastName){

            return (students[facNum].getFirstName(), students[facNum].getLastName());

    }

    function getStudents() public view  returns (string memory studenti){
        studenti = "";

        for(uint i=0; i< num; i++){
            studenti=concat(studenti,
            concat(
                concat(students[facNums[i]].getFirstName(), " "), students[facNums[i]].getLastName()
                )
                );

        }
        return studenti;

    }

    function getFacNums() public view returns(string[] memory _facNums, uint _num){
        _facNums = new string[](num);
        for(uint i=0; i<num; i++){
            _facNums[i] = facNums[i];
        }
        _num = num;
        return (_facNums, _num);
    }





}