Euler 7

euler7.c

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

typedef unsigned long ul;

/*
 * Finds the nth prime
 */
ul euler7(ul n) {
    int cprimes = 10;
    ul *primes = (ul *)malloc(cprimes * sizeof(ul));
    int nprimes = 0;

    int i, j, prime;
    for (i = 2; nprimes < n; ++i) {
        prime = 1;
        ul max = (ul)sqrt((double)i);
        for (j = 0; j < nprimes; ++j) {
            if (max < primes[j])
                break;
            if (i % primes[j] == 0) {
                prime = 0;
                break;
            }

        }
        if (prime) {
            ++nprimes;
            if (nprimes >= cprimes) {
                cprimes *= cprimes;
                primes = (ul *)realloc(primes, cprimes * sizeof(ul));
            }
            primes[nprimes - 1] = i;
        }
    }
    ul nth_prime = primes[nprimes - 1];
    free(primes);
    return nth_prime;
}

int main(int argc, char **argv) {
    if (argc != 2) {
        printf("Usage: %s number\n", argv[0]);
        return 1;
    }

    ul n = strtoul(argv[1], NULL, 0);
    ul prime = euler7(n);

    printf("The %luth prime is %lu.\n", n, prime);
    return 0;
}