C pointer advanced 2-- > function pointer array callback function simplifies calculator code, and implements qsort function based on callback function simulation

Function pointer array

The function pointer array is an array , Array elements are function pointers .
The form of function pointer array :int (*parr[10])();
parr The first and [] combination , explain parr It's an array , What is the content of the array ？
yes int(*)() Function pointer of type .

Purpose of function pointer array ： Transfer table

Example write a calculator ：
requirement ： There is addition , Subtraction , Multiplication , division .
Our previous code implementation ：

#include<stdio.h>
void menu()
{
    
	printf("***************************\n");
	printf("****** 1.add 2.sub ******\n");
	printf("****** 3.mul 4.div ******\n");
	printf("****** 0.exit ******\n");
	printf("***************************\n");
}
int Add(int x, int y)
{
    
	return x + y;
}
int Sub(int x, int y)
{
    
	return x - y;
}
int Mul(int x, int y)
{
    
	return x * y;
}
int Div(int x, int y)
{
    
	return x / y;
}

int main()
{
    
	int input = 0;
	do
	{
    
		menu();
		printf(" Please select :>");
		scanf("%d", &input);
		int x = 0;
		int y = 0;
		int ret = 0;
		switch (input)
		{
    
		case 1:
			printf(" Please enter two operands :>");
			scanf("%d %d", &x, &y);
			ret = Add(x, y);
			printf("%d\n", ret);
			break;
		case 2:
			printf(" Please enter two operands :>");
			scanf("%d %d", &x, &y);
			ret = Sub(x, y);
			printf("%d\n", ret);
			break;
		case 3:
			printf(" Please enter two operands :>");
			scanf("%d %d", &x, &y);
			ret = Mul(x, y);
			printf("%d\n", ret);
			break;
		case 4:
			printf(" Please enter two operands :>");
			scanf("%d %d", &x, &y);
			ret = Div(x, y);
			printf("%d\n", ret);
			break;
		case 0:
			printf(" Exit calculator \n");
			break;
		default:
			printf(" Wrong choice \n");
			break;
		}
	} while (input != 0);
	return 0;
}

It is found that the main function of the above code is too cumbersome , There's a lot of repetition .

Every case The blue boxes in the statement are all repeated parts , So how do we use the function pointer array to solve these repeated parts ？

#include<stdio.h>
void menu()
{
    
	printf("***************************\n");
	printf("****** 1.add 2.sub ******\n");
	printf("****** 3.mul 4.div ******\n");
	printf("****** 0.exit ******\n");
	printf("***************************\n");
}
int Add(int x, int y)
{
    
	return x + y;
}
int Sub(int x, int y)
{
    
	return x - y;
}
int Mul(int x, int y)
{
    
	return x * y;
}
int Div(int x, int y)
{
    
	return x / y;
}


int main()
{
    
	int input = 0;
	do
	{
    
		menu();
		printf(" Please select :>");
		scanf("%d", &input);
		int x = 0;
		int y = 0;
		int ret = 0;
		int (*arr[5])(int, int) = {
     NULL, Add, Sub, Mul, Div };
		if (input == 0)
		{
    
			printf(" Exit calculator \n");

		}
		else if(input>0&&input<5)
		{
    
			printf(" Please enter two operands :>");
			scanf("%d %d", &x, &y);

			ret = arr[input](x, y);
			printf("%d\n", ret);
		}
		else
		{
    
			printf(" Wrong choice \n");
		}
			
	} while (input != 0);
	return 0;
}

Callback function

A callback function is a function called through a function pointer . If you put a pointer to a function （ Address ） Pass as a parameter 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 controlled by this function
The implementer of directly calls , It's called by another party when a particular event or condition occurs , Used to enter the event or condition
Row response .

Use callback function to simplify calculator

#include<stdio.h>
void menu()
{
    
	printf("***************************\n");
	printf("****** 1.add 2.sub ******\n");
	printf("****** 3.mul 4.div ******\n");
	printf("****** 0.exit ******\n");
	printf("***************************\n");
}
int Add(int x, int y)
{
    
	return x + y;
}
int Sub(int x, int y)
{
    
	return x - y;
}
int Mul(int x, int y)
{
    
	return x * y;
}
int Div(int x, int y)
{
    
	return x / y;
}
void calc(int(*pf)(int, int))
{
    
	int x = 0;
	int y = 0;
	int ret = 0;
	printf(" Please enter two operands :>");
	scanf("%d %d", &x, &y);

	ret = pf(x, y);
	printf("%d\n", ret);
}


int main()
{
    
	int input = 0;
	do
	{
    
		menu();
		printf(" Please select :>");
		scanf("%d", &input);
		switch (input)
		{
    
		case 0:
			printf(" Exit calculator \n");
			break;
		case 1:
			calc(Add);
			break;
		case 2:
			calc(Sub);
			break;
		case 3:
			calc(Mul);
			break;
		case 4:
			calc(Div);
			break;
		default:
			printf(" Wrong choice \n");
			break;
		}
	} while (input != 0);
	return 0;
}

Simulation Implementation Based on callback function qsort function

demonstration qsort Use of functions

qsort - This function can sort any type of data

• void qsort(void* base,// The starting position of the data you want to sort
  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(* cmp)(const void* e1, const void* e2)// A function pointer - Comparison function
  );—>MSDN Search for .
  The header file -- <stdlib.h>

Sort a set of integer arrays in ascending order

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

int cmp_int(const void* e1, const void* e2)
{
    
	return (*(int*)e1 - *(int*)e2);
}
int main()
{
    
	int arr[] = {
     10,9,8,7,6,5,4,3,2,1 };
	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]);
	}
	return 0;
}

Use qsort To arrange ascending structure data

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

int cmp_int(const void* e1, const void* e2)
{
    
	return (*(int*)e1 - *(int*)e2);
}
struct Stu
{
    
	char name[20];
	int age;
};
int cmp_stu_by_name(const void* e1, const void* e2)// Compare name
{
    
	return strcmp(((struct Stu*)e1)->name, ((struct Stu*)e2)->name);
}
int cmp_stu_by_age(const void* e1, const void* e2)// Compare age
{
    
	return ((struct Stu*)e1)->age - ((struct Stu*)e2)->age;
}

int main()
{
    
	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);// Yes name Ascending 
	int i = 0;
	for (i = 0; i < sz; i++)
	{
    
		printf("%s %d\n", s[i].name, s[i].age);
	}
	printf("\n");
	qsort(s, sz, sizeof(s[0]), cmp_stu_by_age);// Yes age Ascending 
	for (i = 0; i < sz; i++)
	{
    
		printf("%s %d\n", s[i].name, s[i].age);
	}
	printf("\n");
	return 0;
}

With bubble sort simulation qsort function

qsort Function was originally used to sort by quick sort , But quick sorting is learned from data structure , So in C When the language has not learned the data structure, first use the learned bubble sort to simulate .

void bubble_sort(void* base, int sz, int width, int(*cmp)(const void* e1, const void* e2))
{
    
	int i = 0;
	for (i = 0; i < sz - 1; i++)
	{
    
		int flag = 1;// Suppose the array is ordered 
		// A bubble sorting process 
		int j = 0;
		for (j = 0; j < sz - 1 - i; j++)
		{
    
			if (cmp((char*)base + j * width, (char*)base + (j+1) * width))
			{
    
				// In exchange for 
				Swap((char*)base + j * width, (char*)base + (j+1) * width, width);
				flag = 0;
			}
		}
		if (flag == 1)// Judge whether to exchange after a bubble sort , Order without exchange , Exit loop 
		{
    
			break;
		}
	}
}

This simulation sqort The difference between the bubble sort function and the previous bubble sort function is (char*)base + j * width, and (char*)base + j * width What does that mean ？ Why must I cast to (char*) Well ？

Realization Swap function

void Swap(char* buf1, char* buf2, int sz)
{
    
	int i = 0;
	for (i = 0; i < sz; i++)
	{
    
		char tmp = *buf1;
		*buf1 = *buf2;
		*buf2 = tmp;
		++buf1;
		++buf2;
	}
}

The Swap The first and second parameters of the function are the first addresses of the two data to be exchanged .sz Is the size of the two data to be exchanged .
The data of each byte of the two data are exchanged in turn , Finally, the exchange of the two data is completed .

Right up there qsort Several cases of function use ,qsort Function use bubble_sort Function substitution can verify the accuracy of the simulated implementation function in turn