Prime numbers are those natural numbers which has exactly two factors : 1 and itself. If a number has factors more than two, they are composite. 1 and 0 are neither prime nor composite. C program to determine if a given number is prime or not is shown below:

## Example 1: Program to Check Prime Number

#include<stdio.h> #include<math.h> int main() { int n,i,f=0; printf("Enter a number : "); scanf("%d",&n); if(n < 2) printf("%d is neither prime nor composite",n); else { for(i=2; i<=(int)pow(n,0.5); i++) { if (n % i == 0) f=f+1; } if (f > 0) printf("%d is composite",n); else printf("%d is prime",n); } return 0; }

Here, the number entered by user is stored in integer variable *n*. Variable *f* is used to hold number of factors and* i *is used while looping. To determine whether a number is prime or not, we need to find at least one factor of that number which is neither 1 nor itself. We need to check for that factor starting from 2 to integer part of square root of that number.

Consider 10, the square root of 10 is 3.162. Now, just take the integer part i.e. 3. If we could find a factor for 10 from 2 to 3 then 10 is not a prime number. Here 2 is a factor of 10, so it is not prime. In this way the algorithm works for all possible number.

**Output**

Enter a number : 21 21 is composite Enter a number : 29 29 is prime