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

Article by: Manish Methani

Last Updated: September 27, 2021 at 8:04am IST
2 min 13 sec

A prime number is a number greater than 1 with only two factors – themselves and 1. 

A prime number cannot be divided by any other numbers without leaving a remainder.

An example of a prime number is 13. It can only be divided by 1 and 13. Dividing a prime number by another number results in numbers left over e.g. 13 ÷ 6 = 2 remainder 1.

15 is not an example of a prime number because it can be divided by 5 and 3 as well as by itself and 1.

C program to check whether a given number is prime or not.

#include 
int main()
{
    int number = 7;
    char flag = 0;
    for(int i = 2; i <= (number/2); i++)
    {
      if(number % i == 0)
      {
         flag = 1;
         break;
      }
    }

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

return 0;
}

Output

7 is a prime number.

In this program, a loop is iterated from 2 to number/2. In each iteration, using the condition (number % i) it is checked whether a given number is perfectly divisible by i. 

If a number is perfectly divisible by i, then the flag is set to 1 and break. After iteration, we will check whether a given number is 1 or not. if it is 1, the number is considered as a non-prime number else, if the flag is 0, then the given number is a prime number else the number is not a prime number.