[C/C++] Quick sort library functions

C language

Primitive

void qsort ( void * base, size_t num, size_t size, int ( * comparator ) ( const void *, const void * ) );

head File

#include<stdlib.h>

Code example

Description

Wherein const void * adenotes a constant pointer declared a, (int*)arefers to the constant force a pointer into a pointer integer a, then add it to one of the preceding *says element position pointer points to take up.

Integer

#include<iostream>
#include<stdlib.h>
using namespace std;

int a[] = { 4, 3, 2, 1 };

int compare(const void* a, const void* b);

int main()
{
	int i;
	qsort(a, 4, sizeof(int), compare);
	for ( i = 0; i < 4; i++)
	{
		cout << a[i] << endl;
	}
	
	return 0;
}

int compare(const void* a, const void* b)//升序
{
	return *(int*)a - *(int*)b;	
}

Output: 1 2 3 4

Character type

#include<iostream>
#include<stdlib.h>
using namespace std;

char a[][4] = { {"bca"}, {"abc"}, {"cab"} };

int compare(const void* a, const void* b);

int main()
{
	int i;
	qsort(a, 3, sizeof(a[0]), compare);
	for ( i = 0; i < 3; i++)
	{
		cout << a[i] << endl;
	}
	return 0;
}

int compare(const void* a, const void* b)//升序
{
	return strcmp((char*)a, (char*)b);
	//return *(int*)a - *(int*)b;	
}

Output: abc bca cab

C++

Generally, when sorting an array, this is simple, because the third parameter "compare" function can be defaulted, and the default is ascending order.

Primitive

There are three parameters:
1. The starting address of the array;
2. The ending address of the array;
3. How to sort (large to small or small to large).

head File

#include<algorithm>

Code example

Comparison function default

#include<iostream>
#include<algorithm>
using namespace std;

int value[] = { 5, 4, 3, 2, 1 };

bool compare(int a, int b);

int main()
{
	int i;
	sort(value, value + 5);
	for ( i = 0; i < 5; i++)
	{
		cout << value[i] << endl;
	}
	return 0;
}

Output: 1 2 3 4 5

Partial array sorting

You can also play like this:

#include<iostream>
#include<algorithm>
using namespace std;

int value[] = { 5, 4, 3, 2, 1 };

bool compare(int a, int b);

int main()
{
	int i;
	sort(value + 1, value + 5);
	for ( i = 0; i < 5; i++)
	{
		cout << value[i] << endl;
	}
	return 0;
}

Output: 5 1 2 3 4

Custom comparison function

#include<iostream>
#include<algorithm>
using namespace std;

int value[] = { 5, 4, 3, 2, 1 };

bool compare(int a, int b);

int main()
{
	int i;
	sort(value, value + 5, compare);
	for ( i = 0; i < 5; i++)
	{
		cout << value[i] << endl;
	}
	return 0;
}

bool compare(int a, int b) //降序
{
	return a > b;
}

Output: 5 4 3 2 1 in
descending order

Library function comparison function (namespace std)

less<数据类型>()
greater<数据类型>()

Character type:

#include<iostream>
#include<algorithm>
using namespace std;

char value[] = { 'a', 'b', 'c', 'd', 'e' };

int main()
{
	int i;
	sort(value, value + 5, greater<char>());
	for ( i = 0; i < 5; i++)
	{
		cout << value[i] << endl;
	}
	return 0;
}

Output: edcba

Sort structure-custom comparison function

#include<iostream>
#include<algorithm>
using namespace std;

struct student
{
	string name;
	int age;
}value[3]; 

bool compare(const student& a, const student& b);

int main()
{
	int i;
	value[0] = { "Kite", 10 };
	value[1] = { "Tom", 15 };
	value[2] = { "Mike", 20 };
	sort(value, value + 3, compare);
	for ( i = 0; i < 3; i++)
	{
		cout << value[i].name << " " << value[i].age << endl;
	}
	return 0;
}

bool compare(const student& a, const student& b)
{
	if (a.name == b.name)
	{
		return a.age < b.age;
	}
	else
	{
		return a.name < b.name;
	}
}

Export:
Kite 10
Mike 20
Tom 15

Sort structure-overload relational operators

#include<iostream>
#include<algorithm>
using namespace std;

struct student
{
	string name;
	int age;
	bool operator < (const student& b) const //符号重载
	{
		if (name == b.name)
		{
			return age < b.age;
		}
		else
		{
			return name < b.name;
		}
	}
	//简单地说，和之前的 compare 函数相比，就是把当前“对象”看作 a 了
}value[3]; 

int main()
{
	int i;
	value[0] = { "Kite", 10 };
	value[1] = { "Tom", 15 };
	value[2] = { "Mike", 20 };
	sort(value, value + 3);
	for ( i = 0; i < 3; i++)
	{
		cout << value[i].name << " " << value[i].age << endl;
	}
	return 0;
}

Export:
Kite 10
Mike 20
Tom 15