Int value is changing by itself after a for loop

Question

In my CRA_Account.cpp file my socialInsuranceNumber gets changed after for

(j = 0; j <= max_name_length; j++) {
                given_name[j] = givenName[j];
            }

occurs. example: If i enter 123123123 as my SIN, it gets changed to 123123148. The previous for loop that has

for (j = 0; j <= max_name_length; j++) {
                family_name[j] = familyName[j];
            }

doesnt change my socialInsuranceNumber value.

My code is:

w3_in_lab.cpp

#include <iostream>
#include "CRA_Account.h"

using namespace std;
using namespace sict;

int main() {
    const int size = 1000;
    char familyName[size];
    char givenName[size];
    sict::CRA_Account myCRA;
    int sin;
    bool quit = false;
    if (sict::max_name_length + 1 > size) {
        cerr << "Increase size to value greater than " 
            << sict::max_name_length + 1 << endl;
        return 1;
    }

    cout << "Canada Revenue Agency Account App" << endl
         << "=================================" << endl << endl;
    cout << "Please enter your family name: ";
    cin >> familyName;
    cin.ignore();
    cout << "Please enter your given name: ";
    cin >> givenName;
    cin.ignore();

    do {
        cout << "Please enter your social insurance number (0 to quit): ";
        cin >> sin;
        cin.ignore();
        if (sin != 0)
        {
            myCRA.set(familyName, givenName, sin);
            if (myCRA.isEmpty()) {
                cout << "Invalid input! Try again." << endl;
            }
            else {
                quit = true;
            }
        }
        else {
            quit = true;
        }
    } while (!quit);
    cout << "----------------------------------------" << endl;
    cout << "Testing the display function" << endl;
    cout << "----------------------------------------" << endl;
    myCRA.display();
    cout << "----------------------------------------" << endl;

    return 0;
}

CRA_Account.cpp

#include <iostream>
#include "CRA_Account.h"
#include <cstring>

using namespace std;
using namespace sict;
    void CRA_Account::set(const char* familyName, const char* givenName, int sin) {
        int j;
        if (sin >= min_sin && sin <= max_sin) {
            socialInsuranceNumber = sin;
            for (j = 0; j <= max_name_length; j++) {
                family_name[j] = familyName[j];
            }
            for (j = 0; j <= max_name_length; j++) {
                given_name[j] = givenName[j];
            }
        }
        else {
            socialInsuranceNumber = 0;
        }
    }

    bool CRA_Account::isEmpty() const {
        bool empty = false;
        if (socialInsuranceNumber <= 0) {
            empty = true;
        }
        return empty;
    }
    void CRA_Account::display() const {

        if (isEmpty()) {
            cout << "Account object is empty!" << endl;
        }
        else {

            cout << "Family Name: " << family_name << endl;
            cout << "Given Name: " << given_name << endl;
            cout << "CRA Account: " << socialInsuranceNumber << endl;

        }
    }

CRA_Account.h

#ifndef CRA_ACCOUNT_H
#define CRA_ACCOUNT_H

namespace sict {
    static int max_name_length = 40;
    static int min_sin = 100000000;
    static int max_sin = 999999999;

    class CRA_Account {
    public:
    void set(const char* familyName, const char* givenName, int sin);
    bool isEmpty() const;
    void display() const;

    private:
        char family_name[40];
        char given_name[40];
        int socialInsuranceNumber = 0;

    };

}
#endif

Answer 1

In C++ and C arrays indexes are 0 based, which means index is valid in [0,n-1] range inclusive. Common pattern to iterate is:

for( size_t i = 0; i < array_size; ++i )

some even prefer to write:

for( size_t i = 0; i != array_size; ++i )

so your loop:

for (j = 0; j <= max_name_length; j++) {

is wrong as it iterates over range [0,n] inclusive and leads to UB by accessing array with wrong index. In your case you copy part of memory where socialInsuranceNumber resides. So fix your loop and problem should disappear.

Note: you should use std::string instead of char array, this will not only simplify your code but make it less error prone. If you are required to use char array, then you should use strncpy() function from standard C library, not manual loop.