How do I process a range in slices in Rust? - vector

Question

I understand that the preferred way to iterate in Rust is through the for var in (range) syntax, but sometimes I'd like to work on more than one of the elements in that range at a time.
From a Ruby perspective, I'm trying to find a way of doing (1..100).each_slice(5) do |this_slice| in Rust.
I'm trying things like
for mut segment_start in (segment_size..max_val).step_by(segment_size) {
let this_segment = segment_start..(segment_start + segment_size).iter().take(segment_size);
}
but I keep getting errors that suggest I'm barking up the wrong type tree. The docs aren't helpful either--they just don't contain this use case.
What's the Rust way to do this?

Answer 1

Use chunks (or chunks_mut if you need mutability):
fn main() {
let things = [5, 4, 3, 2, 1];
for slice in things.chunks(2) {
println!("{:?}", slice);
}
}
Outputs:
[5, 4]
[3, 2]
[1]
The easiest way to combine this with a Range would be to collect the range to a Vec first (which dereferences to a slice):
fn main() {
let things: Vec<_> = (1..100).collect();
for slice in things.chunks(5) {
println!("{:?}", slice);
}
}
Another solution that is pure-iterator would be to use Itertools::chunks_lazy:
extern crate itertools;
use itertools::Itertools;
fn main() {
for chunk in &(1..100).chunks_lazy(5) {
for val in chunk {
print!("{}, ", val);
}
println!("");
}
}
Which suggests a similar solution that only requires the standard library:
fn main() {
let mut range = (1..100).peekable();
while range.peek().is_some() {
for value in range.by_ref().take(5) {
print!("{}, ", value);
}
println!("");
}
}
One trick is that Ruby and Rust have different handling here, mostly centered around efficiency.
In Ruby Enumerable can create new arrays to stuff values in without worrying about ownership and return a new array each time (check with this_slice.object_id).
In Rust, allocating a new vector each time would be pretty unusual. Additionally, you can't easily return a reference to a vector that the iterator holds due to complicated lifetime concerns.
A solution that's very similar to Ruby's is:
fn main() {
let mut range = (1..100).peekable();
while range.peek().is_some() {
let chunk: Vec<_> = range.by_ref().take(5).collect();
println!("{:?}", chunk);
}
}
Which could be wrapped up in a new iterator that hides the details:
use std::iter::Peekable;
struct InefficientChunks<I>
where I: Iterator
{
iter: Peekable<I>,
size: usize,
}
impl<I> Iterator for InefficientChunks<I>
where I: Iterator
{
type Item = Vec<I::Item>;
fn next(&mut self) -> Option<Self::Item> {
if self.iter.peek().is_some() {
Some(self.iter.by_ref().take(self.size).collect())
} else {
None
}
}
}
trait Awesome: Iterator + Sized {
fn inefficient_chunks(self, size: usize) -> InefficientChunks<Self> {
InefficientChunks {
iter: self.peekable(),
size: size,
}
}
}
impl<I> Awesome for I where I: Iterator {}
fn main() {
for chunk in (1..100).inefficient_chunks(5) {
println!("{:?}", chunk);
}
}

Answer 2

Collecting into a vec can easily kill your performance. An approach similar to in the question is perfectly fine.
fn chunk_range(range: Range<usize>, chunk_size: usize) -> impl Iterator<Item=Range<usize>> {
range.clone().step_by(chunk_size).map(move |block_start| {
let block_end = (block_start + chunk_size).min(range.end);
block_start..block_end
})
}