*"A prime number is one, which is divisible only by 1 or itself."*

To determine whether a number is prime or not, we have to divide the number successively by all numbers from 2 to one less than itself.

If remainder of any of these divisions is zero, the number is not a prime.

If no division yields a zero then the number is a prime number.

### Program in C to Determine Whether a Number is Prime or Not

**Steps:**

- Declare integers num and i inside main()
- Prompt the message allow the input using printf() and scanf()
- Test whether a Number is prime or not using while() function.

**Code:**

#include <stdio.h> int main() { int num, i; printf("Enter a number"); scanf ("%d",&num); i=2; while (i<=num-1) { if (num%i==0) { printf("Not a prime number\n"); break; } i++; } if (i==num) printf("Prime number\n"); return 0; }