How can I find the minimum value in a map?

Question

I have a map and I want to find the minimum value (right-hand side) in the map. Here is how I did it:

bool compare(std::pair

        

Answer 1

The problem is that this:

bool MyClass::compare

Requires an instance of the class to be called on. That is, you can't just call MyClass::compare, but you need someInstance.compare. However, min_element needs the former.

The easy solution is to make it static:

static bool MyClass::compare

// ...

min_element(mymap.begin(), mymap.end(), &MyClass::compare);

This no longer requires an instance to be called on, and your code will be fine. You can make it more general with a functor, though:

struct compare2nd
{
    template 
    bool operator()(const T& pLhs, const T& pRhs)
    {
        return pLhs.second < pRhs.second;
    }
};

min_element(mymap.begin(), mymap.end(), compare2nd());

All this does is grab the second from each pair and grab them, works with any pair. It could be made for general, but that's a bit too much.

If you need to look up by value enough, I recommend you use Boost's Bimap. It's a bi-directional map, so both the key and value can be used to lookup. You would simply get the front of the value-key map.

Lastly, you can always just keep track of the minimum element going into your map. Every time you insert a new value, check if it's lower than your current value (and that should be probably be a pointer to a map pair, start it as null), and if it's lower, point to the new lowest. Requesting the lowest becomes as simple as dereferencing a pointer.