Question

//Sort character array lexicographically (main code):
char* words[MAX_SIZE];
qsort(words, len, sizeof(char*), cmp);
int cmp(const void* w1, const void* w2) {
    char** str1 = w1;
    char** str2 = w2;
    return strcmp(*str1, *str2);
    //return strcmp(w1, w2);
}

Why does the cmp function have to define a secondary pointer to output normally? Why can t it directly return strcmp(w1, w2);? Please! Help!

My understanding is that w1 and w2 are pointers (words) pointing to the string head pointer (words[i]), is the pointer of the pointer, and needs to be dereferenced to compare. Is it right？

Answer 1

对<代码>qsort的比较功能通过两名成员的地址比较。由于该阵列含有“<代码>char *的内容,这些成员的地址为char **。

www.un.org/chinese/ga/president

Answer 2

这里的直观答案,我可以说:

What you know vs what qsort() knows

You know that your array is an array of char*.

qsort() only knows it has an array of bytes. That is why you also have to tell it how many bytes each element takes (in our example we are pretending a char* is 4 bytes, though on modern hardware it is almost always 8), in addition to the number of elements in the array.

当qsort(>> 然后打电话给你的比较功能时,not知道它有一系列的char*。它有各种各样的 by。

因此,它也不能直接通过该要素(char*)。因此,该要素要有一个点。因此,第12号星号的点子是<条码>第4条要素(指数=3)的点子。

注意特性:char**。

仅凭的简单明细表(<>/code>>)而不知(或谨慎)点码。

因此,比较职能只是指点,在职能范围内,你必须将该点变成正确点。

The comparison function (strings)

如果您的阵列是一系列扼杀(char*s),那么:

int pstrcmp( void* ptr_to_a, void* ptr_to_b )
{
  // Convert the argument to the correct type of pointer
  char** ptr_to_string_a = ptr_to_a;
  char** ptr_to_string_b = ptr_to_b;

  // Access the pointed-to array element
  char* a = *ptr_to_string_a;
  char* b = *ptr_to_string_b;

  return strcmp( a, b );
}

当然,我们可以将这一点简单地缩小到:

int pstrcmp( void* ptr_to_a, void* ptr_to_b )
{
  return strcmp( *(char**)ptr_to_a, *(char**)ptr_to_b );
}

The comparison function (integers)

相比之下,如果我们的阵列是一系列的愤怒:

<代码>int integers [5];

然后,qsort(>,仍然不知道这些要素是什么,并将给你一个要点的协调人。计算单位仅为<代码>int*。

int pcmpint( void* ptr_to_a, void* ptr_to_b )
{
  // Convert the argument to the correct type of pointer
  int* ptr_to_int_a = ptr_to_a;
  int* ptr_to_int_b = ptr_to_b;

  // Access the pointed-to array element
  int a = *ptr_to_int_a;
  int b = *ptr_to_int_b;

  if (a < b) return -1;
  if (b < a) return  1;
  return 0;
}

或更简明扼要:

int pcmpint( void* ptr_to_a, void* ptr_to_b )
{
  if (*(int*)ptr_to_a < *(int*)ptr_to_b) return -1;
  if (*(int*)ptr_to_b < *(int*)ptr_to_a) return  1;
  return 0;
}

简单类型,如ger,通常会看以下,因为更容易阅读。 (汇编者可为所有这些变量制定相同的代码。)

int pcmpint( void* ptr_to_a, void* ptr_to_b )
{
  int a = *(int*)ptr_to_a;
  int b = *(int*)ptr_to_b;

  if (a < b) return -1;
  if (b < a) return  1;
  return 0;
}

What you know vs what qsort() knows

The comparison function (strings)

The comparison function (integers)

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