C++ map按照key或者value排序小结

曾经终败给现在 2022-03-06 07:54 403阅读 0赞

在STL中，map 默认是按照 key 的 less 方法进行排序的。

如果希望改变map的KEY排序规则，可以用到以下两种方法：

#include <iostream>
    #include <string>
    #include <map>
    using namespace std;
    
    
    typedef pair<string, double> PAIR;
    
    ostream& operator<<(ostream& out, const PAIR& p) {
    	return out << p.first << "\t: " << p.second;
    }
    
    struct CmpByKeyLength {
    	bool operator()(const string& left, const string& right) {
    		return left.substr(0,1) < right.substr(0,1);
    	}
    };
    
    
    int _tmain(int argc, _TCHAR* argv[])
    {
    	//默认按照KEY的less方法进行排序
    	cout << "less 排序后：" << endl;
    	map<string, double> fruit_price_map;
    	fruit_price_map["Apple"] = 10.5;
    	fruit_price_map["Grape"] = 15;
    	fruit_price_map["Banana"] = 8;
    	fruit_price_map.insert(make_pair("Mango", 9.5));
    	fruit_price_map.insert(make_pair("Orange", 4.5));
    
    	map<string, double>::iterator iter = fruit_price_map.begin();
    	for (iter; iter != fruit_price_map.end(); ++iter) 
    	{
    		cout << *iter << endl;
    	}
    
    	
    	//
    	cout << "\r\ngreater 排序后：" << endl;
    	map<string, double, greater<string>> fruitAndPrice;
    	fruitAndPrice["Apple"] = 10.5;
    	fruitAndPrice["Grape"] = 15;
    	fruitAndPrice["Banana"] = 8;
    	fruitAndPrice.insert(make_pair("Mango", 9.5));
    	fruitAndPrice.insert(make_pair("Orange", 4.5));
    
    	map<string, double, greater<string>>::iterator itr = fruitAndPrice.begin();
    	for (itr; itr != fruitAndPrice.end(); ++itr)
    	{
    		cout << itr->first << " : " << itr->second << endl;
    	}
    
    	
    	//
    	cout << "\r\n自定义排序：" << endl;
    	map<string, double, CmpByKeyLength> fruitPriceMap;
    	fruitPriceMap["Apple"] = 10.5;
    	fruitPriceMap["Grape"] = 15;
    	fruitPriceMap["Banana"] = 8;
    	fruitPriceMap.insert(make_pair("Mango", 9.5));
    	fruitPriceMap.insert(make_pair("Orange", 4.5));
    
    	map<string, double, CmpByKeyLength>::iterator it = fruitPriceMap.begin();
    	for (it; it != fruitPriceMap.end(); ++it)
    	{
    		cout << it->first << " : " << it->second << endl;
    	}
    
    	return 0;
    }

如果我们想对 map 按照 value 进行排序，该如何实现？

我们知道，在map里面，是成对的pair，也可以理解为 pair 就是 map 的一个子集，那么我们只要把 pair 取出来，然后放入vector，再使用 sort 就可以实现自定义排序了！

#include "stdafx.h"
    #include <string>
    #include <map>
    #include <vector>
    #include <algorithm>
    #include <iostream>
    
    namespace
    {
    	typedef std::pair<std::string, double> PAIR;
    	bool fun_cmp_by_value(const PAIR& left, const PAIR& right) {
    		return left.second > right.second;
    	}
    
    	struct funCmpByValue {
    		bool operator()(const PAIR& left, const PAIR& right) {
    			return left.second > right.second;
    		}
    	};
    }
    
    int _tmain(int argc, _TCHAR* argv[])
    {
    	std::map<std::string, double> nameScoreMap;
    	nameScoreMap["B"] = 90.5;
    	nameScoreMap["A"] = 83.5;
    	nameScoreMap["D"] = 95;
    	nameScoreMap.insert(std::make_pair("E",92.5));
    	nameScoreMap.insert(std::make_pair("C",88));
    
    	//输出未按照VALUE排序之前的MAP
    	std::cout << "sort by key:" << std::endl;
    	std::map<std::string, double>::iterator itr = nameScoreMap.begin();
    	for (itr; itr!=nameScoreMap.end(); itr++)
    	{
    		std::cout << itr->first << " : " << itr->second << std::endl;//默认按照KEY进行排序
    	}
    
    	//输出按照VALUE排序后的MAP,
    	//直接用sort对MAP排序是做不到的.可以把map装进pair里,然后将pair放入vector,自定义sort实现排序
    	std::cout << "\r\nsort by value:" << std::endl;
    	std::vector<PAIR> nameScoreArray(nameScoreMap.begin(), nameScoreMap.end());
    	sort(nameScoreArray.begin(), nameScoreArray.end(), funCmpByValue());
    	//sort(nameScoreArray.begin(), nameScoreArray.end(), fun_cmp_by_value);//
    	for (size_t i=0; i<nameScoreArray.size(); i++)
    	{
    		PAIR _pair = nameScoreArray[i];
    		std::cout << _pair.first << " : " << _pair.second << std::endl;
    	}
    
    	return 0;
    }

输出结果：

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2hlbGxva2FuZHk_size_16_color_FFFFFF_t_70][]

[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2hlbGxva2FuZHk_size_16_color_FFFFFF_t_70]: /images/20220306/87711d911287438fbf61d3c7f3fb1b9f.png