primes.c

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

#define TRUE    1
#define FALSE   0

int nthprime(int n) {
    int *primes = (int *)calloc(n, sizeof(int));
    int i, count = 0;
    
    primes[count++] = 2;
    for (i = 3; count < n; ++i) {
        int j, prime = TRUE;

        for (j = 0; primes[j] * primes[j] <= i; ++j) {
            if (i % primes[j] == 0) {
                prime = FALSE;
                break;
            }
        }

        if (prime)
            primes[count++] = i;
    }
    
    int result = primes[n - 1];
    free(primes);
    
    return result;
}

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

    switch (n % 10) {
        case 1:
            printf("The %dst prime is %d.\n", n, nthprime(n));
            break;
        case 2:
            printf("The %dnd prime is %d.\n", n, nthprime(n));
            break;
        case 3:
            printf("The %drd prime is %d.\n", n, nthprime(n));
            break;
        default:
            printf("The %dth prime is %d.\n", n, nthprime(n));
            break;
    }
    
    return 0;
}