prime.c

/* This program is whether a number is prime number.
 * 
 * usage
 *  - The number of argument of main function is 1.
 *  - The argument is The number you want to check.
 */
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char **argv){
	int num=atoi(argv[1]), i, j, index = 2, primes[1000], flag;
	if(argc != 2){printf("argument is invalid"); exit(1);}
	primes[0] = 2;
	primes[1] = 3;
	for(i = 5; i < sqrt(num); i++){
		flag = 1;
		for(j = 0;j < index;j++){
			if((i % primes[j]) == 0){
				flag = 0;
			}
		}
		if(flag == 1){
			primes[index] = i;
			index++;
		}
	}
	/* for(i = 0;i < index; i++){
		printf("%d\n",primes[i]);
	} */
	flag = 1;
	for(i = 0;i < index;i++){
		if(num%primes[i] == 0){
			flag = 0;
		}
	}
	if(num == 2 || num == 3){
		flag = 1;
	}
	if(flag == 1){
		printf("a prime number");
	}
	if(flag == 0){
		printf("not a prime number");
	}
	return 0;
}

check whether a number is prime.