euler7

By listing the first six prime numbers: 2, 3, 5, 7, 11, and 13, we can see that the 6th prime is 13.

What is the 10001st prime number?

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <limits.h>
#include <math.h>

#define NUM    10001
int prime[NUM];

#if 0
int is_prime(int num)
{
    int i;

    int sqrt_num = 1+sqrt(num);
    for (i = 3; i < sqrt_num; i+=2) {
        if (!(num%i))
            return 0;
    }

    return 1;
}

#else
int is_prime(int num)
{
    int i;
    int sqrt_num = sqrt(num);

    for (i = 0; i < NUM; i++) {
        if (prime[i]) {
            if (!(num % prime[i]))
                return 0;
            else if (prime[i] > sqrt_num)
                break;

        } else
            break;
    }

    return 1;
}
#endif

int main(int argc, const char *argv[])
{
    int nr = 2;
    int i;

    memset(prime, 0, sizeof(prime));
    prime[0] = 2;
    prime[1] = 3;

    for (i = 5; i < (INT_MAX); i+=2) {
        if (is_prime(i)) {
            prime[nr] = i;
            if (++nr == NUM)
                break;

        }
    }

    printf("result: %d\n", i);

    return 0;
}