C++ Program to Find GCD
18/04/2024Finding the Greatest Common Divisor (GCD) of two numbers is a fundamental problem in mathematics and computer science, often introduced early in programming courses to illustrate algorithms, loops, and decisionmaking processes. The GCD of two numbers is the largest positive integer that divides both numbers without leaving a remainder. For example, the GCD of 8 and 12 is 4.
In programming, there are several ways to calculate the GCD, with the Euclidean algorithm being one of the most efficient and commonly used methods. This algorithm is based on the principle that the GCD of two numbers also divides their difference. In C++, implementing the Euclidean algorithm to find the GCD of two numbers can be both a straightforward and insightful exercise.
Introduction to the Euclidean Algorithm
The Euclidean algorithm iteratively reduces the problem of finding the GCD of two numbers until it reaches a case where the GCD is apparent. The algorithm is based on two key observations:
 GCD(a, b) = GCD(b, a % b): The GCD of two numbers a and b is the same as the GCD of b and the remainder of a divided by b.
 GCD(a, 0) = a: The GCD of any number a and 0 is a.
These properties allow us to repeatedly apply the operation a % b (find the remainder of a divided by b) and swap the numbers until one of them becomes zero. The other number, at that point, is the GCD.
Implementing the GCD Algorithm in C++
Here is a simple implementation of the Euclidean algorithm in C++ to find the GCD of two numbers:
#include <iostream>
using namespace std;
// Function to find the GCD of two integers using the Euclidean algorithm
int gcd(int a, int b) {
while(b != 0) {
int remainder = a % b;
a = b;
b = remainder;
}
return a; // When b is 0, a is the GCD
}
int main() {
int num1, num2;
// Prompt the user to enter two numbers
cout << “Enter two integers: “;
cin >> num1 >> num2;
// Ensure the numbers are positive
if(num1 < 0  num2 < 0) {
cout << “Please enter positive integers.” << endl;
return 1;
}
// Calculate and display the GCD
cout << “The GCD of ” << num1 << ” and ” << num2 << ” is ” << gcd(num1, num2) << “.” << endl;
return 0;
}
Explanation of the Program
 Input:
The program begins by asking the user to input two integers. These numbers are the ones for which we want to find the GCD.
 Validation:
It’s a good practice to validate user input. Here, we ensure that the inputs are positive integers, as the GCD concept applies to positive numbers.

GCD Calculation:
The gcd function implements the core logic of the Euclidean algorithm. It repeatedly applies the modulo operation to reduce the problem size until one of the numbers becomes zero. The other number, at this point, is the GCD.
 Output:
Finally, the program outputs the GCD of the two input numbers.