A prime number (or a prime) is a natural number greater than 1 that has no positive divisors other than 1 and itself. A natural number greater than 1 that is not a prime number is called a composite number. For example, 5 is prime because only 1 and 5 evenly divide it, whereas 6 is composite because it has the divisors 2 and 3 in addition to 1 and 6. [Definition from Wikipedia]

The above algorithm is little bit slower. So make this line:

to

and include the header file math.h.

#### C Program to Find Prime Number:

#include <stdio.h> int main() { int num, i, is_prime; printf("Enter a number: "); scanf("%d",&num); is_prime = 0; if(num<2) is_prime = 0; else if(num == 2) is_prime = 1; else { for(i=2;i <= num/2;i++) { if(num%i==0) { is_prime = 0; break; } else is_prime = 1; } } if(is_prime == 0) printf("The number is not Prime!"); else printf("The number is Prime!"); return 0; }

The above algorithm is little bit slower. So make this line:

for(i=2;i <= num/2;i++)

to

for(i=2;i <= (int)sqrt(num);i++)

and include the header file math.h.