Map sorting in C++

Table of contents

1: Map sorting for key (key)

2: Map sorting for value (value)

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1: Map sorting for key (key)

In fact, there is a default sorting in the map. Its structure uses a red-black tree, so its default sorting is sorted according to the keys, and sorted according to the ascending order of the keys.

If we want to customize this kind of sorting, we can solve it by writing a functor. As for what a functor is, this article will not explain it. This article only introduces how to use it (actually it’s my dish, ha Ha ha)!

(1) The default sorting of keys in the map

#include<bits/stdc++.h>
using namespace std;
//对于map中的key的默认排序

map<int,string>m;

int main()
{
	m[1]="iui";
	m[87]="sjddd";
	m[2]="jsd";
	m[67]="yuuuu";
	for(auto it=m.begin();it!=m.end();it++){
		cout<<it->first<<" "<<it->second<<endl;
	}
	return 0;
}

 (2) Custom sorting for keys

#include<bits/stdc++.h>
using namespace std;
//对于map中的key进行paixu

struct rule{
	bool operator()(string a,string b){
		return a>b;//对于键是string型按照从大到小排
	}
};

int main()
{
	//map中的第三个参数其实就是排序规则，之前不写就会默认成默认排序
	map<string,int,rule>m;
	m["asas"]=199;
	m["zx"]=99;
	m["gsgus"]=878;
	m["yuy"]=1515;
	map<string,int,rule>::iterator it;
	for(it=m.begin();it!=m.end();it++){
		cout<<it->first<<" "<<it->second<<endl;
	}
	return 0;
}

 

 (3) key is the sorting of the structure

#include<bits/stdc++.h>
using namespace std;
//按照键是结构体的排序
typedef struct{
	string name;
	int score;
}node;
struct rule{
	bool operator()(node a,node b){
		if(a.score==b.score){
			return a.name>b.name;
		}
		return a.score>b.score;
	}
//排序规则是按照成绩大的在前面，相同按照名字降序
};

int main()
{
	node stu;
	map<node,int,rule>m;
	stu.name="abc";
	stu.score=88;
	m[stu]=1212;
	stu.name="acd";
	stu.score=88;
	m[stu]=1213;
	stu.name="bcbc";
	stu.score=100;
	m[stu]=1214;
	stu.name="zzzzzz";
	stu.score=1000;
	m[stu]=8989;
	map<node,int,rule>::iterator it;
	for(it=m.begin();it!=m.end();it++){
		cout<<"名字="<<it->first.name<<" 成绩="<<it->first.score<<" 学号="<<it->second<<endl;
	}
	return 0;
}

Of course, there is a second sorting method for keys that are structures

 

#include<bits/stdc++.h>
using namespace std;
//对于键是结构体的自定义排序
typedef struct node{
	int score;
	string name;
	bool operator <(const node &s)const{
		if(score!=s.score)return score>s.score;
        return name>s.name;
	}
}node;

int main()
{
	map<node,int>m;
	node u;
	u.name="abc";
	u.score=99;
	m[u]=12;
	u.name="bcd";
	u.score=99;
	m[u]=13;
	u.name="bbb";
	u.score=100;
	m[u]=14;
	u.name="yuy";
	u.score=15;
	m[u]=6666;
	map<node,int>::iterator it;
	for(it=m.begin();it!=m.end();it++){
		cout<<"成绩="<<it->first.score<<" 名字="<<it->first.name<<" 学号="<<it->second<<endl;
	}
	return 0;
}

 

 

2: map for value (value) sorting

The sorting in the map is based on sorting by key, so the value cannot be sorted directly. If you want to sort the value, you need to use the vector container and the sort function. Of course, there are also custom sorting rules (called imitation Function pull down), in fact, it is just a set of templates .

#include<bits/stdc++.h>
using namespace std;
//map中对于value排序
//之前说的map是个键值对，所以需要vector
//来接收的话，那么就需要一对一，就需要用到pair了

bool cmp(const pair<string,int> a,pair<string,int>b){
	return a.second>b.second;
}

int main()
{
	map<string,int>m;
	m["asas"]=18;
	m["ioio"]=90;
	m["cj"]=89;
	vector<pair<string,int>>v(m.begin(),m.end());
	sort(v.begin(),v.end(),cmp);
	map<string,int>::iterator it;
	for(int i=0;i<v.size();i++){
		cout<<v[i].first<<" "<<v[i].second<<endl;
	}
	return 0;
}