C Program to Find LCM of two Numbers

This tutorial will teach you how to find the LCM (Least Common Multiple) of two numbers in C. To understand this example, you should have knowledge of the following C programming topics.

• Operators
• if statement
• while statement

The LCM (Least Common Multiple) of two integers is the smallest positive integer that is perfectly divisible by both the numbers  (without a remainder). For example, the LCM of 30 and 40 is 120.

---

Find LCM in C using loop

#include <stdio.h>

int main()
{
  int num1, num2, largest;

  printf("Enter 2 positive integers:\n");
  scanf("%d %d", &num1, &num2);
  
  //the largest among these two is stored in largest
  largest = (num1 > num2) ? num1 : num2;

  while (1)  //always true
  {
    if (largest % num1 == 0 && largest % num2 == 0)
    {
      printf("The LCM of %d and %d is: %d", num1, num2, largest);
      break;
    }

    ++largest;
  }

  return 0;
}

Output:

Enter 2 positive integers:
30
40
The LCM of 30 and 40 is: 120

---

C Program to Find LCM using GCD

#include <stdio.h>

int main()
{
  int num1, num2, i, gcd, lcm;

  printf("Enter 2 positive integers:\n");
  scanf("%d %d", &num1, &num2);

  for (i = 1; i <= num1 && i <= num2; ++i)
  {
    // check if i is a factor of both integers
    if (num1 % i == 0 && num2 % i == 0)
      gcd = i;
  }

  lcm = (num1 *num2) / gcd;   //use of gcd

  printf("The LCM of %d and %d: %d", num1, num2, lcm);
  return 0;
}

Output:

Enter 2 positive integers:
30
40
The LCM of 30 and 40: 120

---