/* ------------------------------------------------------------------------- * * This is the classic prime sieve of Eratosthenes - a simple algorithm      * * that finds all the prime numbers between 2 and the constant 'N'.          *  *                                                                           * * Name            : sieve.c  (Sieve of Eratosthenes)                        * * Author          : Steve Park & Dave Geyer                                 * * Language        : ANSI C                                                  * * Latest Revision : 9-15-96                                                 * * ------------------------------------------------------------------------- */#include <stdio.h>#include <math.h>                      #define N     46341L                 /* limited only by memory size   */#define SQRTN ((long) sqrt(N)) /* ============== */   int main(void)                                    /* ============== */{  int  prime[N + 1];                  /* n is prime <=> prime[n] == 1  */  long n;                             /* index of possible primes      */  long s;                             /* step index                    */  long t = 0;                         /* index used for tabular output */  prime[0] = 0;                       /* initialize the sieve */  prime[1] = 0;  for (n = 2; n <= N; n++)           prime[n] = 1;  for (n = 2; n <= SQRTN; n++)         /* search all possibilities       */    if (prime[n])                      /* if n is prime,                 */      for (s = 2; s <= (N / n); s++)   /* 2n, 3n, 4n ...  can't be prime */        prime[s * n] = 0;  printf("\tThe primes between 2 and %ld are\n\n", N);  for (n = 2; n <= N; n++)    if (prime[n])  {      if (t == 0)        printf("\t");      printf("%7ld", n);      t = (t + 1) % 10;      if (t == 0)        printf("\n");    }  printf("\n");  return (0);}