如何通过空格“”分隔符拆分或标记字符串数组以查找每个字符串中的第二个单词

Question

我有一个包含一堆字符串的数组。 我想在这个数组中包含的每个字符串中找到第二个单词，这样我就可以按句子中的第二个单词的字母顺序对句子进行排序。 我尝试使用strtok但它不适用于标记字符串数组。 分隔符只是每个单词之间的空格。 我必须在 C 中做到这一点。

Answer 1

C 中的排序通常使用qsort完成。 接口是通用的，只是比较功能是特定于排序要求的。

假设数组的元素是 C 字符串，那么这种情况下的比较函数可能如下所示：

#include <string.h>

int cmp(const void * pv1, const void * pv2)
{
  const char * const * pp1 = pv1;
  const char * const * pp2 = pv2;

  const char * p1 = *pp1 ?*pp1 :""; /* Let's do it the ORxCLx way and tolerate NULLs, by treating them as empty-string. */
  const char * p2 = *pp2 ?*pp2 :""; /* Same. */

  /* Try to get space between 1st and 2nd word: */
  const char * pb1 = strchr(p1, ' ');
  const char * pb2 = strchr(p2, ' ');

  /* In case there was no 1st space
     make begin point to right after the 1st word's end: */
  pb1 = pb1 ?pb1+1 :p1 + strlen(p1);
  pb2 = pb2 ?pb2+1 :p2 + strlen(p2);

  /* Try to get space between 2nd and 3rd word: */
  const char * pe1 = strchr(pb1, ' ');
  const char * pe2 = strchr(pb2, ' ');

  /* In case there was no 2nd space
     make end point right after the 2nd's word's end: */
  pe1 = pe1 ?pe1 :pb1 + strlen(pb1);
  pe2 = pe2 ?pe2 :pb2 + strlen(pb2);

  /* Calculate 2nd word's length: */
  size_t s1 = (size_t)(pe1 - pb1);
  size_t s2 = (size_t)(pe2 - pb2);

  return strncmp(pb1, pb2, s1 > s2 ?s2 :s1);
} 

像这样使用它：

#include <stdio.h>
#include <stdlib.h>

int cmp(const void *, const void *);

int main(void)
{
  const char * as[] = {
    "e 5 y",
    "b 2",
    "c 3 z",
    "d 4 x",
    "c 3 ",
    "a 1",
    "c 3"
  };

  qsort(as, sizeof as / sizeof *as, sizeof *as, cmp);

  for (size_t i = 0; i < sizeof as / sizeof *as; ++i)
  {
    printf("'%s'\n", as[i]);
  }
}

并得到：

'a 1'
'b 2'
'c 3 z'
'c 3'
'c 3 '
'd 4 x'
'e 5 y'