8、 Callback function

A callback function is a function called through a function pointer . If we put the pointer of the function （ Address ） Pass as argument to another function , When this pointer is used to call the function it points to , Let's just say this is a callback function . The callback function is not called directly by the function's implementer , It's called by another party when a particular event or condition occurs , Used to respond to the event or condition .

First, let's review bubble sorting ：

// Bubble sort （ The first edition ）
#include <stdio.h>
void bubble_sort(int arr[], int sz)
{
    
	// Number of bubbling sortings 
	int i = 0;
	for (i = 0; i < sz - 1; i++)
	{
    
		// A bubble sorting process 
		int j = 0;
		for (j = 0; j < sz - 1 - i; j++)
		{
    
			if (arr[j] > arr[j + 1])
			{
    
				int ret = arr[j];
				arr[j] = arr[j + 1];
				arr[j+1]=ret;
			}
		}
	}
}
int main()
{
    
	int arr[] = {
     9,8,7,6,5,4,3,2,1,0 };
	int sz = sizeof(arr) / sizeof(arr[0]);
	bubble_sort(arr, sz);
	int i = 0;
	for (i = 0; i < sz; i++)
	{
    
		printf("%d ", arr[i]);
	}
	return 0;
}

Bubble sort defects of the first edition ： If it's an ordered array , He will still compare one by one , A waste of time , Therefore, the following modifications are made ：

// Bubble sort （ The second edition ）
#include <stdio.h>
void bubble_sort(int arr[], int sz)
{
    
	int i = 0;
    // Number of bubbling sortings 
	for (i = 0; i < sz - 1; i++)
	{
    
		int flag = 1;// Suppose the array is well ordered 
		// A bubble sorting process 
		int j = 0;
		for (j = 0; j < sz - 1 - i; j++)
		{
    
			if (arr[j] > arr[j + 1])
			{
    
				int ret = arr[j];
				arr[j] = arr[j + 1];
				arr[j+1]=ret;
				flag = 0;
			}
		}
		if (flag == 1)
		{
    
			break;
		}
	}
}
int main()
{
    
	int arr[] = {
     9,8,7,6,5,4,3,2,1,0 };
	int sz = sizeof(arr) / sizeof(arr[0]);
	bubble_sort(arr, sz);
	int i = 0;
	for (i = 0; i < sz; i++)
	{
    
		printf("%d ", arr[i]);
	}
	return 0;
}

The disadvantage of bubble sorting is that it can only sort integers .
stay C There is a library function in the language library qsort, It's a quick sort , It can sort any type of data .
qsort The data type of the function ：

void qsort(void* base,// The starting position of the data to be sorted 
		   size_t num,// The number of data elements to be sorted 
		   size_t width,// The size of the data elements to be sorted （ Unit is byte ）
		   int(__cdecl* compare)(const void* elem1, const void* elem2)// A function pointer - Comparison function 
          );
//__cdecl -  Function calling convention ;compare -  Compare the address of the function ;elem1、elem2 -  They are the addresses of the two elements to be compared 

Here we also need to know void * The pointer

int main()
{
    
	int a = 10;
	char* pa = &a;// here &a The type of int*, Report warning 
	void* pv = &a;//void*  Is a pointer without a specific type , Can receive any type of address 
	//void* Is a pointer without a specific type , So you can't dereference , Also can not + - Integers 
	return 0;
}

qsort Use of functions ：

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

int cmp_int(const void* e1, const void* e2)
{
    
	return (*(int*)e1 - *(int*)e2);
}

void test1()
{
    
	// Test use qsort To sort integer data 
	int arr[] = {
     9,8,7,6,5,4,3,2,1,0 };
	int sz = sizeof(arr) / sizeof(arr[0]);
	qsort(arr, sz, sizeof(arr[0]), cmp_int);

	int i = 0;
	for (i = 0; i < sz; i++)
	{
    
		printf("%d ", arr[i]);
	}
}

struct Stu
{
    
	char name[20];
	int age;
};

int cmp_stu_by_name(const void* e1, const void* e2)
{
    
	// Sort structure data by name 
	return strcmp(((struct Stu*)e1)->name , ((struct Stu*)e2)->name);
}
int cmp_stu_by_age(const void* e1, const void* e2)
{
    
	// Sort structure data according to age 
	return (((struct Stu*)e1)->age - ((struct Stu*)e2)->age);
}

void test2()
{
    
	// Test use qsort To sort structure data 
	struct Stu s[] = {
     {
    "zhangsan",15},{
    "lisi",30},{
    "wangwu",25} };
	int sz = sizeof(s) / sizeof(s[0]);
	qsort(s, sz, sizeof(s[0]), cmp_stu_by_name);
	qsort(s, sz, sizeof(s[0]), cmp_stu_by_age);
}

void Swap(char* buf1, char* buf2, int width)// Exchange function 
{
    
	int i = 0;
	for (i = 0; i < width; i++)// Exchange according to the actual width of the element , For example, two integer data , Exchange its eight bytes in turn 
	{
    
		char tmp = *buf1;
		*buf1 = *buf2;
		*buf2 = tmp;
		buf1++;
		buf2++;
	}
}

// according to qsort Function to transform the bubble sort function 
//void bubble_sort(int arr[], int sz)// original edition 
void bubble_sort(void* base, int sz, int width, int(*cmp)(const void* e1, const void* e2))// After reform 
{
    
	int i = 0;
	// Number of bubbling sortings 
	for (i = 0; i < sz - 1; i++)
	{
    
		int flag = 1;// Suppose the array is well ordered 
		// A bubble sorting process 
		int j = 0;
		for (j = 0; j < sz - 1 - i; j++)
		{
    
			//if (arr[j] > arr[j + 1])
			if (cmp((char*)base + j * width, (char*)base + (j + 1) * width) > 0)
				// Here we need to pay attention to , First cast the type to char* type , Multiply by width 
			{
    
				/*int ret = arr[j]; arr[j] = arr[j + 1]; arr[j + 1] = ret;*/
				// In exchange for 
				Swap((char*)base + j * width, (char*)base + (j + 1) * width, width);
				flag = 0;
			}
		}
		if (flag == 1)
		{
    
			break;
		}
	}
}

void test3()
{
    
	// Test and use the modified bubble_sort To sort integer data 
	int arr[] = {
     9,8,7,6,5,4,3,2,1,0 };
	int sz = sizeof(arr) / sizeof(arr[0]);
	bubble_sort(arr, sz, sizeof(arr[0]), cmp_int);

	int i = 0;
	for (i = 0; i < sz; i++)
	{
    
		printf("%d ",arr[i]);
	}
}

void test4()
{
    
	// Test and use the modified bubble_sort To sort structure data 
	struct Stu s[] = {
     {
    "zhangsan",15},{
    "lisi",30},{
    "wangwu",25} };
	int sz = sizeof(s) / sizeof(s[0]);
	//bubble_sort(s, sz, sizeof(s[0]), cmp_stu_by_name);
	bubble_sort(s, sz, sizeof(s[0]), cmp_stu_by_age);
}

int main()
{
    
	//test1();// Test use qsort To sort integer data 
	//test2();// Test use qsort To sort structure data 
	//test3();// Test and use the modified bubble_sort To sort integer data 
	test4();// Test and use the modified bubble_sort To sort structure data 
	return 0;
}