How to check a number is prime or not in C

This is a C program to check if a number if prime or not

A prime number is a number which is not completely divisible by any number except 1 and itself.

If n is the number to be checked, the program will check if n is completely divisble by any number from 2 to n divided by 2

Strictly speaking the check need to be done only till the square root of n but to keep the program simple we will carry on the division until n/2

check_prime.c

 /* **********************************************
 * Program to check a number is Prime or not	*
 ************************************************/

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

int main() 
{
 int num, i;

 // Read a number
 printf("Enter a number : ");
 scanf("%d",&num);

//Check if the number is divsible
for (i=2; i < num / 2; i++) {
	if (num % i == 0) {
		printf("%d is not a prime number. It is completely divisible by %d",num,i);
		getch();
		return 0;
	}
}

printf("%d is a prime number",num);

 // Wait for key press
 getch();

return 0;
}

Program Output

Enter a number : 1324801
1324801 is not a prime number. It is completely divisible by 1151


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