qsort 中的比较函数是如何工作的?

问题描述

我在网上找到了这个示例代码，它解释了 qsort 函数的工作原理.我无法理解比较函数返回的内容.

I found this sample code online, which explains how the qsort function works. I could not understand what the compare function returns.

#include "stdlib.h"

int values[] = { 88, 56, 100, 2, 25 };

int cmpfunc (const void * a, const void * b) //what is it returning?
{
   return ( *(int*)a - *(int*)b ); //What is a and b?
}


int main(int argc, _TCHAR* argv[])
{

   int n;

   printf("Before sorting the list is: \n");
   for( n = 0 ; n < 5; n++ ) {
      printf("%d ", values[n]);
   }

   qsort(values, 5, sizeof(int), cmpfunc);

   printf("\nAfter sorting the list is: \n");
   for( n = 0 ; n < 5; n++ ) {
      printf("%d ", values[n]);
   }
    return 0;
}

推荐答案

int cmpfunc (const void * a, const void * b) //what is it returning?
{
   return ( *(int*)a - *(int*)b ); //What is a and b?
}

相当于:

int cmpfunc (const void * a, const void * b) //what is it returning?
{
   // qsort() passes in `void*` types because it can't know the actual types being sorted
   // convert those pointers to pointers to int and deref them to get the actual int values

   int val1 = *(int*)a;
   int val2 = *(int*)b;

   // qsort() expects the comparison function to return:
   // 
   //    a negative result if val1 < val2
   //    0 if val1 == val2
   //    a positive result if val1 > val2

   return ( val1 - val2 ); 
}

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