// C++ program to demonstrate 
// functionality of unordered_map
#include <iostream>
#include <unordered_map>
using namespace std;
  
// Driver code
int main()
{
  // Declaring umap to be of 
  // <string, int> type key 
  // will be of STRING type 
  // and mapped VALUE will
  // be of int type
  unordered_map<string, int> umap;
  
  // inserting values by using [] operator
  umap["GeeksforGeeks"] = 10;
  umap["Practice"] = 20;
  umap["Contribute"] = 30;
  
  // Traversing an unordered map
  for (auto x : umap)
    cout << x.first << " " << 
            x.second << endl;
}

Contribute 30
Practice 20
GeeksforGeeks 10

//C++程序演示
//初始化、索引、迭代
#include <iostream>
#include <unordered_map>
using namespace std;

//主函数
int main()
{
    //声明umap为
    //<string，double>类型key
    //将是字符串类型的键部分，
    //映射值将是double类型
    unordered_map<string, double> umap = { //直接在映射中插入元素
        {"One", 1},
        {"Two", 2},
        {"Three", 3}
    };

    //通过[]运算符插入值
    umap["PI"] = 3.14;
    umap["root2"] = 1.414;
    umap["root3"] = 1.732;
    umap["log10"] = 2.302;
    umap["loge"] = 1.0;

    //通过insert函数插入值
    umap.insert(make_pair("e", 2.718));

    string key = "PI";

    //如果在map中未找到键，则迭代器将返回“end”
    if (umap.find(key) == umap.end())
        cout << key << " not found\n\n";

    //如果找到了键，则返回到该键的迭代器
    else
        cout << "Found " << key << "\n\n";  

    key = "lambda";
    if (umap.find(key) == umap.end())
        cout << key << " not found\n";
    else
        cout << "Found " << key << endl;

    //遍历umap中的所有值
    unordered_map<string, double> :: iterator itr;
    cout << "\nAll Elements : \n";
    for (itr = umap.begin();
         itr != umap.end(); itr++) 
    {
        //itr作为pair<string,double>类型的指针工作，指向键 和 值
        //itr->first 存储键和 itr->second 存储值
        cout << itr->first << "  " << 
                itr->second << endl; 
    }
}

Found PI

lambda not found

All Elements : 
e  2.718
loge  1
log10  2.302
Two  2
One  1
Three  3
PI  3.14
root2  1.414
root3  1.732

输入: str = “geeks for geeks geeks quiz practice qa for”;  
输出: 各单词的频率为  
(practice，1)   
(for，2)  
(qa，1)  
(quiz，1)  
(geeks，3) 

// C++程序使用unordered_map查找每个单词的出现频率
#include <bits/stdc++.h>
using namespace std;

//以str为参数打印单个单词的频率
void printFrequencies(const string &str)
{
    //声明map为类型，
    //每个单词都映射到其频率
    unordered_map<string, int> wordFreq;

    //使用stringstream将输入拆分成单个单词
    stringstream ss(str);

    //用于拆分单词
    string word; 
    while (ss >&gtword;)
        wordFreq[word]++;

    //现在迭代word, freq键值对并以<,>格式打印它们
    unordered_map<string, int> :: iterator p;
    for (p = wordFreq.begin(); 
         p != wordFreq.end(); p++)
        cout << "(" << p->first << ", " <<
                   p->second << ")\n";
}

//主函数
int main()
{
    string str = "geeks for geeks geeks quiz "
                 "practice qa for";
    printFrequencies(str);
    return 0;
}

(practice, 1)
(for, 2)
(qa, 1)
(quiz, 1)
(geeks, 3)