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1. Introduction to qsort
原型:void qsort (void* base, size_t num, size_t size, int (*compar)(const void*,const void*));
Explanation in C plusplus:
Sort the elements of an array
Sort the elements pointed to by in the array, each element is byte long, use this function to determine the order.
The sorting algorithm used by this function compares pairs of elements by calling the specified function with pointers to them as arguments.
The function returns no value, but modifies the contents of the array pointed to by reordering the elements of the array (as defined).
The order of equivalent elements is undefined.
Two, qsort detailed explanation
头文件：#include<stdlib.h>
原型:void qsort (void* base, size_t num, size_t size, int (*compar)(const void*,const void*))base: It can be an array or a pointer (note: this pointer should point to an array);
size_t num: array length, according to cplusplus definition, he is unsigned int
size_t size: The bytes occupied by the array elements, according to the definition of cplusplus, it is unsigned int
compare: Pointer to a function that compares two elements.
Call this function repeatedly to compare two elements. It should follow the following prototype: qsort
int compar (const void* p1, const void* p2);
That is, it can also be a function name.
The code is as follows (example):
/* qsort example */#include <stdio.h>
/* printf */#include <stdlib.h> /* qsort */
int values[] = { 40, 10, 100, 90, 20, 25 };
int compare (const void * a, const void * b)
{
return ( *(int*)a - *(int*)b );
}//此处为整形递增排序
int main ()
{
int n;
qsort (values, 6, sizeof(int), compare);
for (n=0; n<6; n++)
printf ("%d ",values[n]);
return 0;
}
focus:
The most important part of this function is int (*compar)(const void*, const void*) ;
Because the main changes are in this part, including the data type and ascending and descending order. In fact, other parts can be filled in according to the rules. Among them, size_t size can be calculated by using sizeof() to calculate the size of one of the array elements.
3. Type
Int
int compare (const void * a, const void * b)
{
return *(double*)a >*(double *)b ? 1 : -1;//升序
return *(double*)a <*(double *)b ? 1 : -1;//降序
}double
int compare (const void * a, const void * b)
{
return *(double*)a >*(double *)b ? 1 : -1;//升序
return *(double*)a <*(double *)b ? 1 : -1;//降序
}character
int compare (const void * a, const void * b)
{
return *(char *)a - *(char *)b;//升序
return *(char *)b - *(char *)a;//降序
}string
int compare (const void * a, const void * b)
{
return strcmp(*(const char**)a, *(const char**)b);//升序
return strcmp(*(const char**)b, *(const char**)a);//降序
}4. Example code
#include <stdio.h>
#include <stdlib.h> /* qsort */
//Int
int compare1(const void* a, const void* b)
{
return (*(int*)a - *(int*)b);//升序
//return ( *(int*)b - *(int*)a );//降序
}
//Double
int compare2(const void* a, const void* b)
{
return *(double*)a > *(double*)b ? 1 : -1;//升序
//return *(double*)a < *(double*)b ? 1 : -1;//降序
}
//字符
int compare3(const void* a, const void* b)
{
return *(char*)a - *(char*)b;//升序
//return *(char*)b - *(char*)a;//降序
}
//字符串
int compare4(const void* a, const void* b)
{
return strcmp(*(const char**)a, *(const char**)b); // 升序
//return strcmp(*(const char**)b, *(const char**)a);//降序
}
int main()
{
int values1[] = { 40, 10, 100, 90, 20, 25 };
double values2[] = {1.2,0.42,0.2,8,3.14,15.2};
char values3[] = { 'e', 's', 'a', 'w', 'k', 'l' };
char* values4[] = { "nobody","everything","anything","nonthing","anybody","something" };
int n;
qsort(values1, 6, sizeof(int), compare1);
qsort(values2, 6, sizeof(double), compare2);
qsort(values3, 6, sizeof(char), compare3);
qsort(values4, 6, sizeof(char*), compare4);
printf("Int:");
for (n = 0; n < 6; n++)
printf("%d ", values1[n]);
printf("\n");
printf("double:");
for (n = 0; n < 6; n++)
printf("%.2lf ", values2[n]);
printf("\n");
printf("字符:");
for (n = 0; n < 6; n++)
printf("%c ", values3[n]);
printf("\n");
printf("字符串:");
for (n = 0; n < 6; n++)
printf("%s ", values4[n]);
printf("\n");
return 0;
}