How to find an element in a specified range in std::map?

Question

Is there an equivalent version of std::find(first, last) but for a std::map ? Ie, is there a version of std::map 's find method that searches for an element in a map , but restricting the search only to a specified [first, last) range? Ideally, the solution should be logarithmic in the size of [first, last) .

From what I've seen , std::map::find itself doesn't support this functionality (it always searches the whole map).

Answer 1

You can use std::lower_bound , std::upper_bound or std::equal_range for that as std::map iterators and data in the map satisfy the requirement for those functions, though you should be aware that it will be less efficient than std::map::find() due to linear iterator increments.

From std::lower_bound documentation

The number of comparisons performed is logarithmic in the distance between first and last (At most log 2(last - first) + O(1) comparisons). However, for non-LegacyRandomAccessIterators, the number of iterator increments is linear .

emphasis is mine.

Answer 2

If I understood the question correctly, std::map::lower_bound is exactly what you are looking for - it gives you the element which is not less than the key. And there is upper_bound as well for the other end.