Reverse map lookup

Question

I have a 1 to 1 map. What\'s the best way to find keys from values,

i.e.

For examples if the map is this

KEY VALUE

a    1
b    2
c          


        

Answer 1

Unless the map is huge, or you have some other way of knowing that linear search is too slow, I'd start with linear search:

#include <iostream>
using std::cout;

#include <map>
using std::map;

#include <algorithm>
using std::find_if;

#include <boost/assign/list_of.hpp>
using boost::assign::map_list_of;

typedef map<char, int> Map;
typedef Map::key_type Key;
typedef Map::value_type Pair;
typedef Map::mapped_type Value;


struct finder {
    const Value v;
    finder(const Value& v) : v(v) {}
    bool operator()(const Pair& p) {
        return p.second == v;
    }
};

Map m = map_list_of('a', 1)('b', 2)('c', 3)('d', 4)('e', 5);

int main() {
    Pair v = *find_if(m.begin(), m.end(), finder(3));
    cout << v.second << "->" << v.first << "\n";
}

Answer 2

Given a std::map from keys to values, the following function will return a reverse lookup table, a std::map from values to keys.

    /// Given a map from keys to values, creates a new map from values to keys 
    template<typename K, typename V>
    static map<V, K> reverse_map(const map<K, V>& m) {
        map<V, K> r;
        for (const auto& kv : m)
            r[kv.second] = kv.first;
        return r;
    }

Answer 3

Say you have a map<X,Y>. Build a second structure, perhaps a map<Y*,X*,Deref> that enables the reverse lookup but avoids doubling the storage overhead, because, using pointers, there's no need to store each X and Y twice. The second structure simply has pointers into the first.

Answer 4

Another solution would be to use (the less known?) Boost.Bimap:

Boost.Bimap is a bidirectional maps library for C++. With Boost.Bimap you can create associative containers in which both types can be used as key. A bimap<X,Y> can be thought of as a combination of a std::map<X,Y> and a std::map<Y,X>. The learning curve of bimap is almost flat if you know how to use standard containers. A great deal of effort has been put into mapping the naming scheme of the STL in Boost.Bimap. The library is designed to match the common STL containers.

Answer 5

The most direct way would be to maintain a parallel map where the values and keys are reversed (since the relationship is one to one).

Answer 6

There isn't much you can do about it. Your have options to work with two maps, use multi-key map like one from Boost Multi-Index library, or do linear search.

UPDATE: The most lightweight out of the box solution seems to be Boost.Bimap, which stands for bi-directional map.