How can I get the last item in a BTreeMap?

Question

If you have a sorted map of key/value pairs (or just keys), one of the obvious operations is to get the first or last pair (or key).

C++'s std::vector has front() and back() for this purpose. std::map doesn't, but *map.begin() and *map.rbegin() (reverse iterator) work for this (assuming one knows the map is not empty).

In Rust, getting the first element of a map seems to require map.iter().next().unwrap() — ugly, but perhaps justified considering some error checking is needed.

How can we get the last element? By stepping over all elements: map.iter().last().unwrap()?

I see that there is Iterator::rev(), so is map.iter().rev().next().unwrap() a reasonable alternative?

Why the unwrap? Can you structure the code so that you use the presence (or non-) of the back value in a conditional? — bluss, Nov 14 '15 at 14:40
Good point; I ended up using `.unwrap_or(0)` for this use-case. For another I needed to know first that there was exactly one element, hence `map.iter().next().unwrap()` — a bit unwieldy way of getting the only element! — dhardy, Nov 14 '15 at 22:13
The `first()` and `last()` methods are coming soon: https://github.com/rust-lang/rust/issues/62924 — Code4R7, Oct 10 '22 at 10:11
https://doc.rust-lang.org/alloc/collections/btree_map/struct.BTreeMap.html#method.last_entry — cloudsurfin, Feb 27 '23 at 17:38

score 22 · Answer 1 · edited Jun 05 '19 at 14:34

22

btree_map::Iter, which is returned by BTreeMap::iter(), implements DoubleEndedIterator, so indeed, either the approach with rev() would work or you can use the next_back() method directly:

let (key, value) = map.iter().next_back().unwrap();

edited Jun 05 '19 at 14:34

Shepmaster

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answered Nov 13 '15 at 18:18

Vladimir Matveev

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rubyu2 · Answer 2 · 2023-04-04T00:53:35.630

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https://github.com/rust-lang/rust/issues/31690#issuecomment-184445033

A dedicated method would improve discoverability, but you can do:

let map: BTreeMap<K, V> = ...;
let min = map.iter().next();
let max = map.iter().next_back();

and the same for BTreeSet.

edited Apr 04 '23 at 00:53

answered Jul 19 '19 at 02:23

rubyu2

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score 1 · Answer 3 · edited Jun 05 '19 at 14:34

1

The Iterator::rev method requires that Self implements DoubleEndedIterator, so it should always be an optimized and correct choice for your use case.

fn rev(self) -> Rev<Self>
where
    Self: DoubleEndedIterator,

edited Jun 05 '19 at 14:34

Shepmaster

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answered Jun 05 '19 at 08:17

Stargateur

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How can I get the last item in a BTreeMap?

3 Answers3