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

A variation on @Robᵩ's answer above that uses a lambda:

map<char, int> m = {{'a', 1}, {'b', 2}, {'c', 3}, {'d', 4}, {'e', 5}};

int findVal = 3;
auto it = find_if(m.begin(), m.end(), [findVal](const Pair & p) {
    return p.second == findVal;
});
if (it == m.end()) {
    /*value not found*/
    cout << "*value not found*";
}
else {
    Pair v = *it;
    cout << v.second << "->" << v.first << "\n";
}

(thanks to @Nawaz for his/her contribution here: https://stackoverflow.com/a/19828596/1650814)

Answer 2

I know this is a really old question but this codeproject article (http://www.codeproject.com/Articles/3016/An-STL-like-bidirectional-map) is a pretty good example of a bidirectional map.

This is an example program that shows how easy it is:

 #pragma warning(disable:4503)

    #include "bimap.h"
    #include <iostream>
    #include <string>

    using codeproject::bimap;

    int main(void)
    {
      bimap<int,std::string> bm;

      bm[1]="Monday";
      bm[2]="Tuesday";
      bm[3]="Wednesday";
      bm[4]="Thursday";
      bm[5]="Friday";
      bm[6]="Saturday";
      bm[7]="Sunday";

      std::cout<<"Thursday occupies place #"<<bm["Thursday"]<<
                 " in the week (european style)"<<std::endl;
      return 0;
    }