Project Euler, Problem #7, C

/* 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 10 001st prime
 * number? */

/* Solution #1: a basic one */

#include <stdio.h>

int main()
{
     short int number_of_primes = 3;
     char is_prime;
     long int current_number = 4;
     long int i, current_prime;

     while(number_of_primes <= 10001)
     {
          is_prime = 1;
          for(i = 2; i <= current_number / 2; i++)
               if(current_number % i == 0)
               {
                    is_prime = 0;
                    break;
               }

          if(is_prime == 1)
          {
               current_prime = current_number;
               number_of_primes++;
          }

          current_number++;
     }
     printf("%ld\n", current_prime);
     return(0);
}

/* 104743 */

/* real 0m5,833s
 * user 0m5,829s
 * sys  0m0,000s */

/* ########################################################### */
/* Solution #2: a more efficient one: numbers are checked against
 * items of an array with previously found primes. */

#include <stdio.h>
# define TN 10001

int main()

{
     long int primes[TN] = {2};
     short int primes_number = 1;
     short int i;
     long int checked_number = 3;
     char is_prime;

     while(primes_number < TN)
     {
          is_prime = 1;
          for(i = 0; i < primes_number; i++)
               if(checked_number % primes[i] == 0)
               {
                    is_prime = 0;
                    break;
               }

          if(is_prime == 1)
          {
               primes[primes_number] = checked_number;
               primes_number++;
          }

          checked_number++;
     }
     printf("%ld\n", primes[primes_number - 1]);
     return(0);
}

/* 104743 */

/* real 0m1,189s
 * user 0m1,187s
 * sys  0m0,000s */