C++ How to find the biggest key in a std::map?

问题:

At the moment my solution is to iterate through the map to solve this.

I see there is a upper_bound method which can make this loop faster, but is there a quicker or more succinct way?

回答1:

The end:

m.rbegin();

Maps (and sets) are sorted, so the first element is the smallest, and the last element is the largest. By default maps use std::less, but you can switch the comparer and this would of course change the position of the largest element. (For example, using std::greater would place it at begin().

Keep in mind rbegin returns an iterator. To get the actual key, use m.rbegin()->first. You might wrap it up into a function for clarity, though I"m not sure if it's worth it:

template <typename T>
inline const typename T::key_type& last_key(const T& pMap)
{
    return pMap.rbegin()->first;
}

typedef std::map</* types */> map_type;

map_type myMap;
// populate

map_type::key_type k = last_key(myMap);

回答2:

The entries in a std::map are sorted, so for a std::map m (assuming m.empty() is false), you can get the biggest key easily: (--m.end())->first

回答3:

As std::map is assosiative array one can easily find biggest or smallest key very easily. By defualt compare function is less(<) operator so biggest key will be last element in map. Similarly if someone has different requirement anyone can modify compare function while declaring map.

std::map< key, Value, compare< key,Value > >

By default compare=std::less