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<int, int> m; m[1] = 10; m[2] = 5; m[4] = 6; m[6] = 1; 

I'd like to sort that map by m's value. So, if I print the map, I'd like to get the result as follows:

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

How can I sort the map in this way? Is there any way that I can deal with the key and value with sorted values?

Answer 1

Dump out all the key-value pairs into a set<pair<K, V> > first, where the set is constructed with a less-than functor that compares the pair's second value only. That way, your code still works even if your values aren't all distinct.

Or dump the key-value pairs into a vector<pair<K, V> >, then sort that vector with the same less-than functor afterwards.

Answer 2

You can build a second map, with the first map's values as keys and the first map's keys as values.

This works only if all values are distinct. If you cannot assume this, then you need to build a multimap instead of a map.