Define a compare function for std::map which uses values instead of keys

Question

I can define a compare class for a map like this:

struct classcomp {
    bool operator() (const string& lhs, const string& rhs) const
    {
        if(lhs < rhs)
            return true;
        else 
            return false;
    }
};

but here lhs and rhs represent keys. What if I want to compare by values instead of key? How will I do that?

[Boost.Bimap](http://www.boost.org/libs/bimap/doc/html/index.html) — Praetorian, Jan 14 '13 at 17:58
That compare class doesn't work. What about when `lhs >= rhs`? — Fred Larson, Jan 14 '13 at 17:58
@crush How do I retrive the values using key inside the compare class? — Jatin, Jan 14 '13 at 18:02
@FredLarson `return false` in that case. Should have written than. — Jatin, Jan 14 '13 at 18:05
It is not clear what you want to do. Can you provide a simple description of the problem you are trying to solve and how 'comparing by values instead of keys' would help? As it stands the question does not make much sense, *key* is what is used to compare, whether it is part of the value or not is another issue. — David Rodríguez - dribeas, Jan 14 '13 at 18:06
@Jatin: Yes. `return lhs < rhs;` would be more concise. But isn't this just the default comparison anyway? — Fred Larson, Jan 14 '13 at 18:07
@FredLarson He doesn't want to compare lhs to rhs. He wants to compare the value in some arbitrary map, corresponding to the key of lhs to the same of rhs. I think his code is just filler for the moment? — crush, Jan 14 '13 at 18:09

score 4 · Accepted Answer · answered Jan 14 '13 at 18:29

4

It's not about what you want; it's about what std::map wants. The ordering is based on the layout of elements in memory (usually in a tree structure) and it is this process that uses the comparator. Wishing this were not the case does not make it so!

It sounds to me like std::map as a container choice does not fit your requirements. Consult a container choice flow chart to decide what to do next.

answered Jan 14 '13 at 18:29

Lightness Races in Orbit

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1

Flowchart link : http://stackoverflow.com/questions/4010097/general-use-cases-for-c-containers – Jatin Jan 14 '13 at 18:49
Yep and http://stackoverflow.com/a/471461/560648 and http://stackoverflow.com/a/6215960/560648 :D – Lightness Races in Orbit Jan 14 '13 at 20:27

score 3 · Answer 2 · answered Jan 14 '13 at 17:59

The std::map type does not support comparison by values. If you want to compare by value, you should consider making a new multimap whose keys represent values in the old map and whose values represent keys in the old map.

That said, it sounds like you're trying to reorder the keys dynamically based on their values. In that case, you might want to look at priority queues supporting decrease-key, since that might be more aligned with what you're trying to do.

Hope this helps!

All wrapped up nicely in [Boost.MultiIndex](http://www.boost.org/libs/multi_index/doc/index.html). — GManNickG, Jan 14 '13 at 18:00

Define a compare function for std::map which uses values instead of keys

2 Answers2