/* Sieve of Eratothenes
   Adapted from Emily Pedersen's code
   by Sanjiv Bhatia
   Feb 1 07    */

#include<stdio.h>
#include<stdlib.h>
#include<math.h>
#include <strings.h>	// For use with memset

#define N 100

int main()
{
    char	sieve[N];	// Array to hold sieve
    char	* ptr;		// Temporary pointer into the sieve
    char	* last_element = sieve + N;
    int    	i;		// Counter

    // We terminate the loop at square root of N.
    // Add 1 to the termination condition just to make sure of rounding.

    int		sqrt_n = (int)(sqrt ( N )) + 1;

    // Set the value of each element in array to 1.
    // Every integer is considered to be a prime number.
    // The numbers that are not prime will fall through the sieve.

    memset ( sieve, 1, N * sizeof ( char ) );

    // We want to make sure that the first two elements are not prime

    sieve[0] = sieve[1] = 0;

    // Start evaluating array content

    for ( i = 2; i < sqrt_n; i++ )
    {
	// If the pointer is equal to not prime, just go to next iteration

	if ( ! *( ptr = sieve + i ) ) continue;

	// Drip all the multiples through the sieve

        for ( ptr += i; ptr < last_element; ptr += i )
            *ptr = 0;
    }

    // Print out primes

    for ( ptr = sieve + 2; ptr < last_element; ptr++ )
        if ( *ptr )
	    printf ( "%6d\n", ptr - sieve );

    return ( 0 );
}