C program that checks whether or not an input number is prime.

ifprimenumber.c

/* To check if a number is prime or not */
#include <stdio.h>
#include <ctype.h>
int main()
{
	int n, i, flag=0;
	printf(“Enter a positive integer: ”);
	scanf(“%d”, &n);
	if (n<1 || isalpha(n))
	{
		printf(“Invalid input.”);
		break;
	}
	for (i=1; i<=n/2; i++)
	{
		if (n%i==0)
		{
			flag=1;
			break;
		}
	}
	if (flag==0)
		printf(“%d is a prime number.”, n);
	else
		printf(“%d is not a prime number.”, n);
	return 0;
}