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C Program to Check Whether a Given Number is Prime or Not

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In this C programming example, we will learn about the Prime Number program in C. We specifically write a program to check the entered number is prime or not.

Prime Number:

A Prime Number is a number that is only divisible by 1 and itself. Example: 2, 3, 5, 7, 11, 13, 17, etc. These numbers are only divisible by 1 and the number itself.

Note:
0 and 1 is not a prime number and 2 is the only even and smallest prime number.

You must know the following topics in C first:

C Program to Check Whether a Given Number is Prime or Not

#include <stdio.h>

int main()
{
  int num, i, flag = 0;

  //user input
  printf("Enter the number: ");
  scanf("%d", &num);

  for (i = 2; i <= num / 2; ++i)
  {
    // checking for non-prime
    if (num % i == 0)
    {
      flag = 1;
      break;
    }
  }

  //checkinh for the entered number is 1 or not
  //because 1 cannot be a prime number
  if (num == 1)
  {
    printf("1 is not a prime number.");
  }
  else
  {
    if (flag == 0)
      printf("%d is a prime number.", num);
    else
      printf("%d is not a prime number.", num);
  }

  return 0;
}

Output:

//Run 1
Enter the number: 13
13 is a prime number.

//Run2
Enter the number: 1
1 is not a prime number.

You may go through the following C program on the prime number.

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