author ： Knock on the code の Rukawa Kaede

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List of articles

1.qsort Function introduction

2.qsort Realize sorting of different types of data

3.qsort Simulation Implementation of

1.qsort Function introduction

  Function definition ：

Function parameter ：

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 , In bytes

int (*compar)(const void*,const void*)

Is a function pointer , The function function is to compare

Because the type of data to be sorted by the sorting algorithm is different , The comparison method is also different , Therefore, it is necessary to provide users with a customized comparison function

2.qsort Realize sorting of different types of data

#define _CRT_SECURE_NO_WARNINGS
#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()
{

	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* e2)
{
	return strcmp(((struct Stu*)e1)->name, ((struct Stu*)e2)->name);
}

void test2()
{
	struct Stu s[] = { {"zhangsan",20},{"lisi",21},"wangwu",19};
	int sz = sizeof(s) / sizeof(s[0]);
	qsort(s, sz, sizeof(s[0]), cmp_stu_by_name);
	int i = 0;
	for (i = 0; i < 3; i++)
	{
		printf("%s %d ", s[i].name, s[i].age);
	}
}


int main()
{
	test1();
    prinf("\n");
	test2();
	return 0;
}

 test1 and test2 Two functions for each int Sort type data and structure type data , here void* It is worth mentioning

void* The function of the pointer :

int main()
{
	int a = 10;
	char* pa = &a;
	return 0;
}

As the warning says , Their types are incompatible , But if use void* Define a pointer variable to receive int* The data will not have warnings , This explanation void* A pointer is a pointer without a specific type , Can accept any type of address , Is generic , Just because it has no specific type , It cannot be dereferenced , You can't add or subtract , Want to dereference it , You only need to cast the address ,qsort Function parameters of use void* Type is also because I don't know the address of what type of data the user will send , use void* Can receive any type of address , Improved qsort Utility of function

3.qsort Simulation Implementation of

analysis ：

Reference resources qsort We know the parameters of the function ： When designing functions, we don't know what type of data the user wants to transmit , So we will use the void* Type pointer to record the starting position of sorting data , Finally, a comparison function provided will also be used void*; Know the number and width of elements , You can find these data one by one

As shown in the figure above , base Cast to char* Type pointer can access one byte at a time , If it is converted to int * Skip the next time 4 A byte is too much , Convert to char* Posterior Union width It is equivalent to comparing by adding the offset to the starting position , Pass after comparison Swap Function , To achieve the purpose of sorting

  Simulation Implementation ：

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

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++)
	{
        // A sequence 
		int flag = 1;
		int j = 0;
		for (j = 0; j < sz - 1; j++)
		{
			if (cmp((char*)base + j * width, (char*)base + (j + 1) * width)>0)
			{
				Swap((char*)base + j * width, (char*)base + (j + 1) * width,width);
				flag = 0;
			}
		}
		if (flag == 1)
			break;
	}
}

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

void test1()
{

	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* e2)
{
	return strcmp(((struct Stu*)e1)->name, ((struct Stu*)e2)->name);
}

void test2()
{
	struct Stu s[] = { {"zhangsan",20},{"lisi",21},"wangwu",19};
	int sz = sizeof(s) / sizeof(s[0]);
	 bubble_sort(s, sz, sizeof(s[0]), cmp_stu_by_name);
	int i = 0;
	for (i = 0; i < 3; i++)
	{
		printf("%s %d ", s[i].name, s[i].age);
	}
}
int main()
{

	test1();
	printf("\n");
	test2();

	return 0;
}

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