Write a program to check whether the given number is prime or not

C Program to check whether a number is prime or not

In this example, you will learn to check whether an integer entered by the user is a prime number or not.

A prime number is a positive integer that is divisible only by '1' and itself. For example: 2, 3, 5, 7, 11, 13, 17

Let's see the prime number program in C. In this c program, we will take an input from the user and check whether the number is prime or not.

Prime Number program in C:

  •  Program:

#include<stdio.h>
#include<conio.h>

int main() 
{
  int n, i, flag = 0;
  printf("Enter a positive integer: ");
  scanf("%d", &n);

  for (i = 2; i <= n / 2; ++i) 
  {
    if (n % i == 0) 
    {
      flag = 1;
      break;
    }
  }

  if (n == 1) 
  {
    printf("1 is neither prime nor composite.");
  } 
  else 
   {
    if (flag == 0)
       {
          printf("%d is a prime number.", n);
       }    
    else
       { 
          printf("%d is not a prime number.", n);
       }

   }
   getch();
   return 0;
}


  •  Output:

Enter a positive integer: 56
56 is not a prime number.

  •  Explore:


  • In the program, a for loop is iterated from 'i = 2' to 'i < n/2'
  • In each iteration, whether 'n' is perfectly divisible by 'i' is checked using:
                if(n%i==0)
                {
                    flag = 1;
                    break;
                }
  • If 'n' is perfectly divisible by 'i', 'n' is not a prime number.
  • In this case, 'flag' is set to 1, and the loop is terminated using the 'break' statement.
  • Notice that we have initialized 'flag' as 0 during the start of our program.
  • So, if  'n' is a prime number after the loop, 'flag' will still be 0. However, if  'n' is a non-prime number, 'flag' will be 1. 

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