program7_11a.c

C语言入门经典一书的所有代码。书上面的所有代码均在此。希望大家喜欢

/* Program 7.11A  Allocating and freeing memory         */
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>

int main(void)
{
  unsigned long *primes = NULL;       /* Pointer to primes storage area    */
  unsigned long trial = 0;            /* Integer to be tested              */

  bool found = false;                 /* Indicates when we find a prime    */
  size_t total = 0;                   /* Number of primes required         */
  size_t count = 0;                   /* Number of primes found            */

  printf("How many primes would you like - you'll get at least 4?  ");
  scanf("%u", &total);                /* Total is how many we need to find */
  total = total<4U ? 4U:total;        /* Make sure it is at least 4        */

  /* Allocate sufficient memory to store the number of primes required */
  primes = (unsigned long *)calloc(total, sizeof(unsigned long));
  if (primes == NULL)
  {
     printf("\nNot enough memory. Hasta la Vista, baby.\n");
     return 1;
  }

  /* We know the first three primes     */
  /* so let's give the program a start. */
  *primes = 2UL;                      /* First prime                   */
  *(primes+1) = 3UL;                  /* Second prime                  */
  *(primes+2) = 5UL;                  /* Third prime                   */
  count = 3U;                         /* Number of primes stored       */
  trial = 5U;                         /* Set to the last prime we have */

  /* Find all the primes required */
  while(count<total)
  {
    trial += 2UL;                     /* Next value for checking       */

    /* Try dividing by each of the primes we have       */
    /* If any divide exactly - the number is not prime  */
    for(size_t i = 0 ; i < count ; i++)
      if(!(found = (trial % *(primes+i))))
        break;                        /* Exit if no remainder          */

    if(found)                        /* we got one - if found is true */
      *(primes+count++) = trial;     /* Store it and increment count  */
  }

  /* Display primes 5-up */
  for(int i = 0 ; i < total ; i ++)
  {
    if(!(i%5U))
      printf("\n");                   /* Newline after every 5         */
    printf ("%12lu", *(primes+i));
  }
  printf("\n");                       /* Newline for any stragglers    */

  free(primes);                       /* Release the memory         */
  return 0;
}