Why does my program print 0 instead of the calculated GCD?

Question

Hey I'm trying to calculate GCD (greatest common denominator) with cpp but my program is printing 0 instead of the GCD and there is no error!!

#include <iostream>

using namespace std;

int main()
{
    int emptyi;
    int number1;
    int number2;
    int counter1 = 0;
    int baghimandeh1 = 0;
    int liste1[100];
    int baghimandeh2 = 0;
    int counter2 = 0;
    int liste2[100];

    cout << "add your number: " << endl;
    cin >> number1;

    cout << "add your second number: " << endl;
    cin >> number2;

    for (int i = 1; i <= number1; i++) {
        if (number1%i == 0) {
            baghimandeh1 = number1;
            liste1[counter1] = i;
        }
        counter1++;
        // baghimandeh1 = number1 %i ;
    }

    for (int i = 1; i <= number2; i++) {
        if (number1%i == 0)
            baghimandeh2 = number2;
        liste2[counter2] = i;
    }
    counter2++;

    // baghimandeh2 = number2 %i ;

    for (int i = 0; i <= counter1; i++) {
        for (int j = 0; j <= counter2; j++) {
            if (liste1[i] == liste2[j]) {
                emptyi = liste1[i];
            }
        }
    }

    cout << "b,m,m  shoma hast: " << emptyi;
}

I wanna calculate Denominator of each user's input and then print the biggest among them and it will be GCD but it print's 0 no matter what number I enter.

Answer 1

I'll point out some errors I've seen:

• counter1 and counter2 should be updated only when the if clauses are true.
• The block below always executes the last line; I understand this is a bug you introduced when debugging.
• Also in that block, you use number1 instead of number2 .

if (number1%i == 0)
    baghimandeh2 = number2;
liste2[counter2] = i;

Apart from those errors:

• The algorithm only works with input numbers with no more than 100 divisors.
• Some variables are not initialized.
• The last double for loop is very inefficient.
• It would be preferable not to using namespace std;

Here's a demo of your code with some fixes and some debug info added.

---

The algorithm could be much simpler though; a bit more efficient and without needing to use any array. For example, given two positive numbers, get the mininum of them, and walk an index from that minimum towards 1 , until both numbers are divisible by the index.

[Demo]

#include <algorithm>  // min
#include <iostream>  // cin, cout

int main() {
    int number1{};
    std::cout << "add your number:\n";
    std::cin >> number1;

    int number2{};
    std::cout << "add your second number:\n";
    std::cin >> number2;

    auto min_counters{std::min(number1, number2)};

    int emptyi{};
    for (int i = min_counters; i >= 1; --i) {
        if (number1 % i == 0 and number2 % i == 0) {
            emptyi = i;
            break;
        }
    }

    std::cout << "b,m,m shoma hast: " << emptyi;
}

Answer 2

#include <iostream>
#include <vector>

int main()
{
    int number1;
    int number2;
    std::vector<int> liste1, liste2;

    std::cout << "add your number: " << std::endl;
    std::cin >> number1;

    std::cout << "add your second number: " << std::endl;
    std::cin >> number2;

    for (int i = 1; i <= number1; i++) {
        if (number1 % i == 0) liste1.push_back(i);
    }

    for (int i = 1; i <= number2; i++) {
        if (number2 % i == 0) liste2.push_back(i);
    }

    auto d1 = liste1.rbegin();
    auto d2 = liste2.rbegin();
    while(true) {
        if (*d1 == *d2) {
            std::cout << "GDC(" << number1 << ", " << number2 << ") = " << *d1 << std::endl;
            break;
        }
        if (*d1 > *d2) {
            ++d1;
        } else {
            ++d2;
        }
    }
}

Note: testing all integers for number2 is inefficient. You only need to test those numbers that are a denominator for number1, ie everything in liste1. So just iterate backwards over liste1 till you find a number that is a denominator for number2.