How can I iterate over a sequence multiple times within a function? - rust

Question

I have a function that I would like to take an argument that can be looped over. However I would like to loop over it twice. I tried using the Iterator trait however I can only iterate over it once because it consumes the struct when iterating.
How should I make it so my function can loop twice? I know I could use values: Vec<usize> however I would like to make it generic over any object that is iterable.
Here's an example of what I would like to do: (Please ignore what the loops are actually doing. In my real code I can't condense the two loops into one.)
fn perform<'a, I>(values: I) -> usize
where
I: Iterator<Item = &'a usize>,
{
// Loop one: This works.
let sum = values.sum::<usize>();
// Loop two: This doesn't work due to `error[E0382]: use of moved value:
// `values``.
let max = values.max().unwrap();
sum * max
}
fn main() {
let v: Vec<usize> = vec![1, 2, 3, 4];
let result = perform(v.iter());
print!("Result: {}", result);
}

Answer 1

You can't iterate over the same iterator twice, because iterators are not guaranteed to be randomly accessible. For example, std::iter::from_fn produces an iterator that is most definitely not randomly accessible.
As #mousetail already mentioned, one way to get around this problem is to expect a Cloneable iterator:
fn perform<'a, I>(values: I) -> usize
where
I: Iterator<Item = &'a usize> + Clone,
{
// Loop one: This works.
let sum = values.clone().sum::<usize>();
// Loop two: This doesn't work due to `error[E0382]: use of moved value:
// `values``.
let max = values.max().unwrap();
sum * max
}
fn main() {
let v: Vec<usize> = vec![1, 2, 3, 4];
let result = perform(v.iter());
println!("Result: {}", result);
}
Result: 40
Although in your specific example, I'd compute both sum and max in the same iteration:
fn perform<'a, I>(values: I) -> usize
where
I: Iterator<Item = &'a usize>,
{
let (sum, max) = values.fold((0, usize::MIN), |(sum, max), &el| {
(sum + el, usize::max(max, el))
});
sum * max
}
fn main() {
let v: Vec<usize> = vec![1, 2, 3, 4];
let result = perform(v.iter());
println!("Result: {}", result);
}
Result: 40