C language, usage of qsort in library function, and explanation

C语言库函数qsort的使用

qsort简介

• Everyone knows bubble sort,But sorting has certain limitations,For larger arrays,函数qsort就派上用场了,它“快速排序”方法是最有效的排序算法之一.

原理：

它把数组不断分成更小的数组,直到变成单元素数组.First divide the array into two parts,一部分的值都小于另一部分的值.This process continues until the array is completely sorted.

Function prototype and step-by-step analysis

一：格式

函数原形：void qsort(void * base ,size_t nmemb, size_t size , int ( * compar)(const void *,const void *））;

二：分段解析

See the format above,There are four parameters in this function：
第一个参数：

是一个指针,Points to the first element of the array to be sorted,easy to seebaseis cast to pointervoid的指针,所以,qsort（）The first argument of can refer to any type of array.

———————————————————————————————————————

第二个参数：

The second parameter is the number of items to be sorted.

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第三个参数：

because of the first parameter,Convert the first argument to void指针,qsortIt doesn't know the size of each element in the array,So the third parameter makes up for this deficiency,The third parameter is the size of an element in the data to be sorted.

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第四个参数：

The fourth parameter is a pointer to a pointer function,The comparison function pointed to by this pointer is used to determine the sort order.

In layman's terms a function used to compare two elements in the data to be sorted.

如果第一项的值大于第二项,则返回正数;如果相等就返回0;若小于,It returns the negative number.

代码示例

Let's review the classic bubble sort first

输出：0, 1, 2, 3, 4, 5, 6, 7, 8, 9

#include<stdio.h>
#include<string.h>
void bublle_sort(int arr[], int sz)//形参arr本质是指针
{
    
	//确认趟数
	int i = 0;
	for (i = 0; i < sz - 1; i++)
	{
    
		//一趟冒泡排序的过程
		int j = 0;
		for (j = 0; j < sz - 1 - i; j++)
		{
    
			if (arr[j] > arr[j + 1])
			{
    
				//交换
				int tmp = arr[j];
				arr[j] = arr[j + 1];
				arr[j + 1] = tmp;

			}

		}
	}

}
int main()
{
    
	int arr[] = {
     9,8,7,6,5,4,3,2,1,0 };
	int sz = sizeof(arr) / sizeof(arr[0]);//计算数组元素个数
	bublle_sort(arr, sz);//数组传参的时候,传递的其实是数组首元素的地址
	int i = 0;
	for (i = 0; i < sz; i++)
	{
    
		printf("%d ", arr[i]);
	}
	return 0;
}

Use it like thatqsortto implement a bubble sort

输出：0, 1, 2, 3, 4, 5, 6, 7, 8, 9

#include<stdlib.h>
#include<stdio.h>
void print_arr(int arr[], int sz)
{
    
	int i = 0;
	for (i = 0; i < sz; i++)
	{
    
		printf("%d ", arr[i]);
	}
	printf("\n");
}
int cmp_int(const void* e1, const void* e2)
{
    
	return *(int*)e1 - *(int*)e2;
}
int main()
{
    
	int arr[10] = {
    9,8,7,6,5,4,3,2,1,0};
	int sz = sizeof(arr) / sizeof(arr[0]);
	qsort(arr, sz, sizeof(arr[0]), cmp_int);
	print_arr(arr, sz);
	return 0;
}

总结

mentioned in detail aboveqsort函数的格式,原理解析,以及代码示例,看似很复杂,But it is very practical,可以实现字符串,Structure content sorting, etc.