get_prime

gistfile1.c

#include <stdio.h>
#include <assert.h>
#include <stdlib.h> 
#include <errno.h>
#include <stdbool.h>
#include <math.h>

void die(const char *message)
{
  if (errno) {
    perror(message);
  } else {
    printf("ERROR: %s\n", message);
  }
  exit(1);
}

unsigned int get_prime(unsigned int n)
{
  assert(n > 0);
  unsigned int *const primes = malloc(sizeof(unsigned int) * n);
  if (!primes) {
    die("Memory error");
  }
  
  unsigned int num = *primes = 2;
  --n;
  unsigned int *last = primes;

  while (n) {
    ++num;
    int s = sqrt(num);
    bool is_prime = true;
    unsigned int *i;
    for (i = primes; *i <= s && i <= last; ++i) {
      if (0 == num % *i) {
        is_prime = false;
        break;
      }
    }
    if (is_prime) {
      *(++last) = num;
      --n;
    }
  }

  free(primes);
  return num;
}

int main(int argc, char *argv[])
{
  assert(2 == argc);
  printf("%d\n", get_prime(atoi(argv[1])));
  return 0;
}