How can I sort an STL map by value?

Question

How can I implement STL map sorting by value?

For example, I have a map m:

map m;
m[1] = 10;
m[2] = 5;
m[4] = 6;
m[6] =          


        

Answer 1

I've just done a similar question in my c++ book. The answer I came up with might not be very efficient though:

int main()
{
    string s;
    map counters;

    while(cin >> s)
        ++counters[s];

    //Get the largest and smallest values from map
    int beginPos = smallest_map_value(counters);
    int endPos = largest_map_value(counters);

    //Increment through smallest value to largest values found
    for(int i = beginPos; i <= endPos; ++i)
    {
        //For each increment, go through the map...
        for(map::const_iterator it = counters.begin(); it != counters.end(); ++it)
        {
            //...and print out any pairs with matching values
            if(it->second == i)
            {
                cout << it->first << "\t" << it->second << endl;
            }
        }
    }
    return 0;
}

//Find the smallest value for a map
int smallest_map_value(const map& m)
{
    map::const_iterator it = m.begin();
    int lowest = it->second;
    for(map::const_iterator it = m.begin(); it != m.end(); ++it)
    {
        if(it->second < lowest)
            lowest = it->second;
    }
    return lowest;
}

//Find the largest value for a map
int largest_map_value(const map& m)
{
    map::const_iterator it = m.begin();
    int highest = it->second;
    for(map::const_iterator it = m.begin(); it != m.end(); ++it)
    {
        if(it->second > highest)
            highest = it->second;
    }
    return highest;
}