如何在哈希图中访问键的值

Question

I am trying to access the value of particular hash key. The example code is below. Test Here

// unordered_map::at
#include <iostream>
#include <string>
#include <unordered_map>

int main ()
{
  std::unordered_map<std::string,int> hashmap = {
                { "Apple", 300},
                { "Banana", 60},
                { "Orange", 70 } };

  std::cout << "Value :" << hashmap[300]<<std::endl;

  return 0;
}

However when I try to access key of a particular value, it works fine, like hashmap["Apple"] , it give 300 i,e the key of Apple . How to make it work otherwise like hashmap[300] to give "Apple" .

Answer 1

How to make it work otherwise like hashmap[300] to give "Apple" .

Hash maps are unidirectional: key --> value . If you need both directions to be fast, you'll need a different data structure (like Boost.Bimap ).

If you just want that lookup to work period and are okay with linear performance, than you can just use std::find_if :

auto it = std::find_if(hashmap.begin(), hashmap.end(),
    [](auto const& pr){ return pr.second == 300; });
if (it != hashmap.end()) {
    std::cout << "Key for 300: " << it->first;
}

There's nothing in std::unordered_map that based on looking up values.

Answer 2

There is no direct access as you expected.

One of the ways is to use std::find_if .

//DATA
std::unordered_map<std::string,int> hashmap = {

            {"Apple", 300},
            {"Banana",60},
            {"Orange",70}
};


//FIND BASED ON GIVEN INPUT
const auto& foundItem = std::find_if(hashmap.begin(),hashmap.end(),
                [](const std::pair<std::string,int>& item)
                {
                    return item.second == 300;
                });

std::cout<<foundItem->first<<std::endl;

And moreover, when you are looking for value in a unordered map, there is a possibility of another key having similar value.

for example another element {"grape",300} . If you want all the keys with value 300....

//DATA
std::unordered_map<std::string,int> hashmap = {

              {"Apple", 300},
              {"Banana",60},
              {"Orange",70},
              {"Grape",300}
};

//RESULT
std::vector<std::string> foundValues;

//FIND BASED ON GIVEN INPUT
std::for_each(hashmap.begin(),hashmap.end(),
                 [&foundValues](const std::pair<std::string,int>& item)
                 {
                       if(item.second == 300)
                           foundValues.push_back(item.first);

                 });

 //JUST TO TEST
 std::cout<<foundValues[0]<<"   "<<foundValues[1]<<std::endl;