Prime Number Program in C

In this tutorial, We will check a given positive integer N. The task is to write a C program to check if the number is prime or not.

What is Prime number in C?

Prime number in C: A Prime number is a positive integer that is greater than one and divided only by one and itself. For example, {2, 3, 5, 7, 11, 13, 17, 19, 23} are the prime numbers.

To check whether the number is prime or not iterate through each of the numbers starting from 2 to sqrt(N) with a for loop and check if it divides N. When we find any number that divides, then we return false. If we didn’t see any number between 2 and sqrt(N) which divides N, then it implies that N is prime and we’ll return True. For example

3 = 1 * 3
Some prime number are − 1, 2, 3, 5 , 7, 11 etc.

Algorithm for Prime Number

START
 Step 1 → Enter variable V
 Step 2 → Divide the Number by V-1.
 Step 3 → If V is divisible by (V-1 to 2) it is not prime
 Step 4 → Else it is prime
STOP

Pseudocode for Prime Number

procedure prime_number : number
 FOR loop = 2 to number - 1
   check if number is divisible by loop
  IF divisible
     RETURN "NOT PRIME"
  END IF
 END FOR
 RETURN "PRIME"
end procedure

C Program to Check Prime Number

//prime number program in c
#include<stdio.h>
int main() {
    int number, loop, flag = 0;
    number = 19;
    for (loop = 2; loop <= number / 2; ++loop) {
       // condition for non-prime
       if (number % loop == 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;
}