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

Question

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

Answer 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);

Answer 2

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

Answer 3

Since the map is just an AVL tree then, it's sorted -in an ascending order-. So, the element with largest key is the last element and you can obtain it using one of the following two methods:

1.

    largestElement = (myMap.rbegin())-> first; // rbegin(): returns an iterator pointing to the last element

2.  largestElement = (--myMap.end())->first; // end(): returns an iterator pointing to the theortical element following the last element 
   

Answer 4

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