Check Prime Number C Program

By | September 19, 2015

C Program To Check Prime Number

Learn How To Check Prime Number in C Programming Language. It is important that we should know How A For Loop Works before getting further with this C Program.

What is a Prime Number?

A Prime Number is a Natural Number Greater than 1 and it should have No Positive Divisors other than 1 and the Number itself. A Natural Number greater than 1 but not a Prime Integer is known as a Composite Number.

Example

2, 3, 5, 7 ,11

These numbers are evenly divided by 1 and the number itself.

Must Read: C Program To Find Prime Numbers using Sieve of Eratosthenes Algorithm

Method 1: C Program To Find Prime Number using For Loop

#include <stdio.h>

int main()
{
      int num, count, temp = 0;
      printf("\nEnter a Number:\t");
      scanf("%d", &num);
      for(count = 2; count <= num/2; count++)
      {
            if(num%count == 0)
            {
                  temp = 1;
                  break;
            }
      }
      if(temp == 0)
      {
            printf("\n%d is a Prime Number\n", num);
      }
      else
      {
            printf("\n%d is Not a Prime Number\n", num);
      }
      return 0;
}

Method 2: C Program To Check Prime Numbers using While Loop

#include <stdio.h>

int main()
{
      int num, count = 2, temp = 0;
      printf("\nEnter a Number:\t");
      scanf("%d", &num);
      while(count <= num - 1)
      {
            if(num%count == 0)
            {
                  temp = 1;
                  break;
            }
            count++;
      }
      if(temp == 0)
      {
            printf("\n%d is a Prime Number\n", num);
      }
      else
      {
            printf("\n%d is Not a Prime Number\n", num);
      }
      return 0;
}

Also Read: Convert Number To Words in C Programming

 

Output

C Program To Check Prime Number using For and While Loop

Alternatively, you can also use this Condition  in the Loop.

 

for(count=2;count<=num/2;count++)

The above Condition count<=num/2 is an efficient Algorithm to Check Prime Integers and helps to reduce the Number of Comparisons and thereby efficient time complexity.

Method 3: Sieve Method To Find Prime Numbers in C Programming

#include <stdio.h>
#include <math.h>

void sieve_method(int limit, int arr[])
{
    	int m, n;
    	for(m = 0; m < limit; m++)
      {
		arr[m] = 1; 
	}
    	arr[0] = 0, arr[1] = 0; 
    	for(m = 2; m < sqrt(limit); m++)
	{ 
        	for (n = m*m; n < limit; n = n + m)
		{
            		arr[n] = 0;	
		}
	}
}

int main()
{
    	int count, limit;
	printf("\nEnter The Limit:\t");
	scanf("%d", &limit);
    	int arr[limit];
    	sieve_method(limit, arr);
	printf("\n");
    	for (count = 0; count < limit; count++)
      {
		if (arr[count] == 1)
        	{
			printf("%d\t", count);
		}
	}
	printf("\n");
	return 0;
}

If you have any compilation errors or doubts in this C Program To Check Prime Numbers, let us know about in the Comment Section below.

5 thoughts on “Check Prime Number C Program

  1. Vishwajeet Kadam

    What is this sieve method for finding prime numbers about? Its quite confusing.

    Reply
  2. Ashok Kandpal

    I used to get lost in the For loop conditions. This is by far the best code for Prime Number checking that I have come across so far.

    Reply
  3. Vijay Lumar

    Amazing. I had never heard that there something called as Sieve method to find Prime Numbers in C programming. Thanks. You introduce so mamy concepts that no has even heard of.

    Reply

Let's Discuss