A better than our lab isPrime function.

isPrime.c

#include <stdlib.h>
#include <stdio.h>
#include <math.h>
int main(){
	int n;
	printf("Enter a number\n");
	scanf("%d",&n);
	if(isPrime(n)){
		printf("Is prime number\n");
	}else{
		printf("Is not a prime number\n");
	}
	return 0;
}
int isPrime(int a){
	int i;
	if(a==2 | a==3){
		return 1;
		//obviously if the number is 2 or 3, it's prime
	}
	if(a%2==0 | a%3==0){
		return 0;
		//if it is divisible by 2 or 3 they aren't prime.
	}
	else{
		for(i=5;i<=sqrt(a);i=i+2){
		//2 and 3 are already checked, 4 is not prime so we don't need to check, we can start from 5, then we don't need to check with any of even numbers, so skip a number each time.
			if(!isPrime(i)){
				//if I is not prime, that means it can be broken down to prime factors, and that means those factors are less than i, which already checked dividing, so we can skip those non prime numbers.
				continue;
			}else{
				if(a%i==0){
					return 0;
					//if the counter is prime and it divides the number without a 0 reminder, that means number is not prime
				}
			}
		}
		//at this point, every possible check are done and the conclusion is, the number is prime.
		return 1;
	}
}

While compiling on GNU/Linux OS, (I don't care about other operating systems) please use -lm flag for math.h file, only including it is not enough, or you'll get an error saying "undefined reference to sqrt" so while compilation use:
cc isPrime.c -o isPrime -lm
./isPrime