This is a C Program to find GCD of two numbers using Recursive Euclid Algorithm. In mathematics, the Euclidean algorithm, or Euclid’s algorithm, is a method for computing the greatest common divisor (GCD) of two (usually positive) integers, also known as the greatest common factor (GCF) or highest common factor (HCF). It is named after the Greek mathematician Euclid, who described it in Books VII and X of his Elements.

The GCD of two positive integers is the largest integer that divides both of them without leaving a remainder (the GCD of two integers in general is defined in a more subtle way).

In its simplest form, Euclid’s algorithm starts with a pair of positive integers, and forms a new pair that consists of the smaller number and the difference between the larger and smaller numbers. The process repeats until the numbers in the pair are equal. That number then is the greatest common divisor of the original pair of integers.

The main principle is that the GCD does not change if the smaller number is subtracted from the larger number. For example, the GCD of 252 and 105 is exactly the GCD of 147 (= 252 – 105) and 105. Since the larger of the two numbers is reduced, repeating this process gives successively smaller numbers, so this repetition will necessarily stop sooner or later — when the numbers are equal (if the process is attempted once more, one of the numbers will become 0).

The GCD of two positive integers is the largest integer that divides both of them without leaving a remainder (the GCD of two integers in general is defined in a more subtle way).

In its simplest form, Euclid’s algorithm starts with a pair of positive integers, and forms a new pair that consists of the smaller number and the difference between the larger and smaller numbers. The process repeats until the numbers in the pair are equal. That number then is the greatest common divisor of the original pair of integers.

The main principle is that the GCD does not change if the smaller number is subtracted from the larger number. For example, the GCD of 252 and 105 is exactly the GCD of 147 (= 252 – 105) and 105. Since the larger of the two numbers is reduced, repeating this process gives successively smaller numbers, so this repetition will necessarily stop sooner or later — when the numbers are equal (if the process is attempted once more, one of the numbers will become 0).

Here is the source code of the C program to compute greatest common divisor of two numbers. The C program is successfully compiled and run on a Linux system. The program output is also shown below.

`#include <stdio.h>`

int gcd_algorithm(int x, int y)

`{`

if (y == 0) {

return x;

} else if (x >= y && y > 0) {

return gcd_algorithm(y, (x % y));

`}`

`}`

int main(void)

`{`

int num1, num2, gcd;

printf("\nEnter two numbers to find gcd using Euclidean algorithm: ");

scanf("%d%d", &num1, &num2);

gcd = gcd_algorithm(num1, num2);

if (gcd)

printf("\nThe GCD of %d and %d is %d\n", num1, num2, gcd);

`else`

printf("\nInvalid input!!!\n");

return 0;

`}`

advertisement

$ gcc gcd.c -o gcd $ ./gcd Enter two numbers to find GCD using Euclidean algorithm: 20 12 The GCD of 20 and 12 is 4

**Sanfoundry Global Education & Learning Series – 1000 C Programs.**

advertisement

Here’s the list of Best Reference Books in C Programming, Data Structures and Algorithms.