How to get the key of a map using the value in C++ STL

Question

Get key by inputting the value of that key in C++ STL

 map<int,int> m;
   m[0]=8;
   m[8]=7;
   m[1562]=4;
   m[100]=1;
 auto i=m.find(1562);
    cout<<endl<<i->first;

Answer 1

You cant. The map works by hashing the key and then using that to find the value stored in memory. It does not allow you to use the value to index the key.

What you can do is, iterate through the map to find the value and get the key from it.

int key = 0;
int value = 4;
for(auto entry : m)
{
    if(entry.second == value)
    {
        key = entry.first;
        break;    // Exit from the loop.
    }
}

Reference: cppreference

Answer 2

std::map is an ordered by key container. So to find a key by value you need to use the sequential search. For example you can use the standard algorithm std::find_if .

Here is a demonstrative program.

#include <iostream>
#include <map>
#include <iterator>
#include <algorithm>

int main() 
{
    std::map<int,int> m =
    {
        { 0, 8 }, { 8, 7 }, { 1562, 4 }, { 100, 1 }
    };
   
    int value = 4;
    
    auto it = std::find_if( std::begin( m ), std::end( m ),
              [&value]( const auto &p )
              {
                return p.second == value; 
              } );
              
    if ( it != std::end( m ) ) 
    {
        std::cout << it->first << ' ' << it->second << '\n';
    }
    
    return 0;
}

The program output is

1562 4

Or you should use a non-standard container that allows a quick access to elements of the container by key and by value.

Answer 3

You may do it like this as well:

#include<iostream>
#include <map>

int findByValue(std::map<int, int> mapOfElemen, int value)
{

    std::map<int, int>::iterator it = mapOfElemen.begin();
    // Iterate through the map
    while(it != mapOfElemen.end())
    {
        // Check if value of this entry matches with given value
        if(it->second == value)
        {
            return it->first;
        }
        // Go to next entry in map
        it++;
    }
   //key for the value is not found hou probably need to change this depending on your key space
    return -9999;
}

int main()
{
std::map<int,int> m;
   m[0]=8;
   m[8]=7;
   m[1562]=4;
   m[100]=1;

   int value = 1562;
   int result = findByValue( m, value);
   std::cout << "key for "<< value << " is " << result << std::endl;

   value = 8;
   result = findByValue( m, value);
   std::cout << "key for "<< value << " is " << result << std::endl;

   value = 7;
   result = findByValue( m, value);
   std::cout << "key for "<< value << " is " << result << std::endl;
}

The result would give this:

key for 1562 is -9999 
key for 8 is 0
key for 7 is 8
key for 4 is 1562

Answer 4

You can use an unordered map:

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

int main()
{
    unordered_map<int,int> m = {{0,8},{8,7},{1562,4},{100,1}};
    cout<<m[1562];

    return 0;
}

This will print the output as 4.