# C Program To Determine Whether Or Not A Number Is Prime

This tutorial will teach you how to write a C program to determine whether a number is prime or not.

If a positive number is divisible by 1 and itself, it is called a prime number. For example, the numbers 13, 19, and 23 are prime because they are divisible by one and themselves.

## A program that determines whether a number is prime or not

In this example, we are determining whether or not the number entered by the user is a prime number.

The first check is to compare the input number to 0 and 1; if it is either 0 or 1, it is not a prime number.

The second check we perform here is to see if the input number is divisible by any number between 2 and num/2; if it is, it is not a prime number; otherwise, it is.

```
#include <stdio.h>
int main() {
//Here flag value 0 means prime number and
//1 means non-prime number
int num, i, flag = 0;
printf("Enter a number to check whether prime or not: ");
scanf("%d", &num);
// 0 and 1 are not the prime numbers so setting
// the flag to 1 if entered number is 0 or 1
if (num == 0 || num == 1)
flag = 1;
for (i = 2; i <= num / 2; ++i) {
// if num is divisible by i, then num is not prime
// change the flag value to 1
if (num % i == 0) {
flag = 1;
break;
}
}
// If flag is 0 it means the number is prime
if (flag == 0)
printf("Entered number %d is a prime number.", num);
else
printf("Entered number %d is not a prime number.", num);
return 0;
}
```

**Output:** Because the user entered the number 13, which is a prime number, the program displays the message “Entered number 13 is a prime number.”