C program to find prime factors of a number

Write a C program to input a number from user and find Prime factors of the given number using loop. C program to list all prime factors of a given number. Logic to find prime factors of a number in C programming.

Input any number: 10

Prime factors of 10: 2, 5

Required knowledge

Basic C programming, If statement, For loop, Nested loop

What is Prime factor?

Factors of a number that are prime numbers are called as Prime factors of that number. For example: 2 and 5 are the prime factors of 10.

Logic to check prime factors of a number

Step by step descriptive logic to find prime factors.

1. Input a number from user. Store it in some variable say num.
2. Run a loop from 2 to num/2, increment 1 in each iteration. The loop structure should look like for(i=2; i<=num/2; i++).

   You may think why loop from 2 to num/2? Because prime number starts from 2 and any factor of a number n is always less than n/2.

3. Inside the loop, first check if i is a factor of num or not. If it is a factor then check it is prime or not.

   Print the value of i if it is prime and a factor of num.

Program to find prime factors of a number

/**
 * C program to find all prime factors of a given number
 */

#include <stdio.h>

int main()
{
    int i, j, num, isPrime;

    /* Input a number from user */
    printf("Enter any number to print Prime factors: ");
    scanf("%d", &num);

    printf("All Prime Factors of %d are: \n", num);

    /* Find all Prime factors */
    for(i=2; i<=num; i++)
    {
        /* Check 'i' for factor of num */
        if(num%i==0)
        {
            /* Check 'i' for Prime */
            isPrime = 1;
            for(j=2; j<=i/2; j++)
            {
                if(i%j==0)
                {
                    isPrime = 0;
                    break;
                }
            }

            /* If 'i' is Prime number and factor of num */
            if(isPrime==1)
            {
                printf("%d, ", i);
            }
        }
    }

    return 0;
}

Enter any number to print Prime factors: 15
All Prime Factors of 15 are: 
3, 5,

Happy coding 😉