# C Program to Check Whether a Number is Prime or Not

**To understand this example, you should have knowledge of following C programming topics:**

A positive integer which is only divisible by 1 and iself is known as prime number. For example: 13 is a prime number because it is only divisible by 1 and 13 but, 15 is not prime number because it is divisible by 1, 3, 5 and 15.

## C program to Check Prime Number

```
/* C program to check whether a number is prime or not. */
#include <stdio.h>
int main()
{
int n, i, flag=0;
printf("Enter a positive integer: ");
scanf("%d",&n);
for(i=2;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;
}
```

**Output**

Enter a positive integer: 29 29 is a prime number.

This program takes a positive integer from user and stores it in variable `n`. Then, `for`

loop is executed which checks whether the number entered by user is perfectly divisible by `i` or not starting with initial value of `i` equals to 2 and increasing the value of `i` in each iteration. If the number entered by user is perfectly divisible by `i` then, `flag` is set to 1 and that number will not be a prime number but, if the number is not perfectly divisible by `i` until test condition `i<=n/2`

is true means, it is only divisible by 1 and that number itself and that number is a prime number.

Visit this page to learn, how you can display all prime numbers between two intervals entered by user.