Are there queue and stack collections in Rust?
Both VecDeque
and LinkedList
have push
/pop
_front
/back
.
Matthieu M. has it just about perfect. Vec
is your stack (LIFO) and VecDeque
is a double ended queue that supports all 4 variants (FIFO, FILO, LIFO, and LILO) using:
.push_front(x) | .front() | .pop_front()
.push_back(x) | .back() | .pop_back()
If you're looking to maximize your efficiency, I recommend checking out "Unterstanding Rust’s Vec and its capacity for fast and efficient programs". It goes into a lot more detail about how allocation and reallocation occurs in Vec
and VecDeque
, but the biggest take away is that if you can predict the maximum number of elements you're going to need in the queue you can use VecDeque::with_capacity(x)
if you know when you initialize it, or .reserve_exact(x)
if at some point you know exactly how many more slots you're going to need
I strongly recommend checking out the Rust docs on std::collections
, it has an excellent list of the most common collections used in Rust, along with suggestions on when to pick each
One last thing, VecDeque
isn't part of the default prelude in Rust so if you want to use it you need to include this:
use std::collections::VecDeque;
First of all, Rust does not offer (in the Standard library) any collection with guaranteed latency for adding elements: Rust collections may generally allocate memory when adding new elements, and allocating memory may take an unbounded amount of time in the worst case.
That being said, there are two contenders for each case:
- a stack may be implemented either on top of
Vec
orLinkedList
(both featurepop_back
andpush_back
) - a queue may be implemented either on top of
VecDeque
orLinkedList
(both featurepop_front
andpush_back
)
The difference between Vec*
and LinkedList
is that the latter is simplistic: for each call to push_back
a memory allocation is made. On the one hand, this is great because it means that the cost of push_back
is independent of the number of elements already in the collection, on the other hand... well, a memory allocation may take a really long time.
The former is a bit more complicated:
- it has better throughput, thanks to being more cache-friendly
- it has additional capacity, guaranteeing non-allocating
push_back
as long as there is excess capacity - it still maintains amortized O(1)
push_back
even when not reserving excess capacity ahead of time
In general, I would advise to use Vec
for a stack and VecDeque
for a queue.