Last key in a std::map

Question

I am looking for the highest key value (a defined by the comparison operator) of a std::map.

Is this guaranteed to be

map.rbegin()->first

?

(I am a bit shaky on reverse iterators, and how much freedom there is in the implementation of std::map)

If not, please advise. I cannot change the data structure.

Steve Jessop · Accepted Answer · 2008-11-14T13:47:27.923

72

Yes. Map is a sorted container, the reverse iterator must return the elements in reverse (i.e. decreasing) order of their keys.

[Edit: as Charles Bailey points out in his answer, your code gives the greatest key if it exists - i.e. if the map is non-empty]

edited Nov 14 '08 at 13:47

answered Nov 14 '08 at 10:45

Steve Jessop

score 38 · Answer 2 · answered Nov 14 '08 at 11:28

38

Yes, but remember to check that map.rbegin() != map.rend().

answered Nov 14 '08 at 11:28

CB Bailey

They are all equivalent checks. – CB Bailey Nov 14 '08 at 13:42
24

... while .empty() is guaranteed to be O(1). (just to clarify what Andreas said) – Thomas Jul 01 '09 at 12:12

birubisht · Answer 3 · 2018-01-03T09:42:11.113

18

You can use following method :-

if(!map.empty())
    (--map.end())->first;

edited Jan 03 '18 at 09:42

answered Nov 22 '16 at 04:56

birubisht

2

This is not guaranteed to work. `std::prev(map.end())->first` is safer. – Kai Petzke Feb 05 '21 at 17:07
@KaiPetzke yes that is why I checked the map first. what else can go wrong here please elaborate. thanks – birubisht Jun 23 '21 at 08:17
1

The problem is, that the C++ standard does not require, that you can apply the `--` operator to the rvalue `map.end()`. For further explanation, see the notes on https://en.cppreference.com/w/cpp/iterator/prev – Kai Petzke Jun 28 '21 at 20:07
@KaiPetzke That's a good point. Then what's the difference between `std::prev(map.end())->first` and `map.rbegin()->first`? They seem pretty much the same. – starriet Sep 23 '22 at 15:21
1

@starriet There is no difference between the two variants you are asking about. Because @birubisht introduced the non-portable `(--map.end())`, I recommended to use the portable `std::prev(map.end())` instead. `map.rbegin()` yields an iterator to exactly the same element. – Kai Petzke Sep 24 '22 at 19:11

score 16 · Answer 4 · answered Nov 15 '20 at 04:06

16

One more way -

std::prev(map.end())->first;

answered Nov 15 '20 at 04:06

Alam

What will this return, if the map is empty? – Melroy van den Berg May 09 '23 at 20:51

score 1 · Answer 5 · answered Nov 12 '21 at 11:29

1

Map store the key value pairs in sorted order so we can access the last element by :-

auto it=m.end();
it--;
int element=it->first;

answered Nov 12 '21 at 11:29

kuldeep singh

5 Answers5