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Add custom nth_back to Skip #60454

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Jun 20, 2019
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Add custom nth_back to Skip #60454

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14 changes: 14 additions & 0 deletions src/libcore/iter/adapters/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1509,6 +1509,20 @@ impl<I> DoubleEndedIterator for Skip<I> where I: DoubleEndedIterator + ExactSize
}
}

#[inline]
fn nth_back(&mut self, n: usize) -> Option<I::Item> {
let len = self.len();
if n < len {
self.iter.nth_back(n)
} else {
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@sinkuu sinkuu May 28, 2019 •
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In case n >= self.len() && self.len() != 0, additionally you need to do self.iter.nth_back(self.len() - 1); (or whatever that takes self.len()-items from the back) since the underlying iterator could have side effects.

let mut v = vec![];
let mut it = [1, 2, 3, 4, 5]
    .iter()
    .cloned()
    .inspect(|x| v.push(*x))
    .skip(1);
assert_eq!(it.len(), 4);
assert_eq!(it.nth_back(4), None);
assert_eq!(v, vec![5, 4, 3, 2]);

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Why let _ = and not just the call to nth_back?

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@sinkuu sinkuu May 28, 2019 •
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It was just for expressing intent to throw away the return value of nth_back, and I don't meant it is actually needed.

if len > 0 {
// consume the original iterator
self.iter.nth_back(len-1);
}
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}
}

fn try_rfold<Acc, Fold, R>(&mut self, init: Acc, mut fold: Fold) -> R where
Self: Sized, Fold: FnMut(Acc, Self::Item) -> R, R: Try<Ok=Acc>
{
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34 changes: 34 additions & 0 deletions src/libcore/tests/iter.rs
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Expand Up @@ -2280,6 +2280,40 @@ fn test_skip_try_folds() {
assert_eq!(iter.next_back(), Some(24));
}

#[test]
fn test_skip_nth_back() {
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I don't think the comment above was addressed; if disagree please add a test that demonstrates that the underlying iterator is still advanced.

(You could do that with inspect, or something like let mut it = ...; it.by_ref().skip(10).nth_back(100); assert_eq!(it.next_back(), ...);)

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Ok, I didn't understand you meant that.

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You're now calling self.len() three times, maybe it's better to store it in a variable first.

let xs = [0, 1, 2, 3, 4, 5];
let mut it = xs.iter().skip(2);
assert_eq!(it.nth_back(0), Some(&5));
assert_eq!(it.nth_back(1), Some(&3));
assert_eq!(it.nth_back(0), Some(&2));
assert_eq!(it.nth_back(0), None);

let ys = [2, 3, 4, 5];
let mut ity = ys.iter();
let mut it = xs.iter().skip(2);
assert_eq!(it.nth_back(1), ity.nth_back(1));
assert_eq!(it.clone().nth(0), ity.clone().nth(0));
assert_eq!(it.nth_back(0), ity.nth_back(0));
assert_eq!(it.clone().nth(0), ity.clone().nth(0));
assert_eq!(it.nth_back(0), ity.nth_back(0));
assert_eq!(it.clone().nth(0), ity.clone().nth(0));
assert_eq!(it.nth_back(0), ity.nth_back(0));
assert_eq!(it.clone().nth(0), ity.clone().nth(0));

let mut it = xs.iter().skip(2);
assert_eq!(it.nth_back(4), None);
assert_eq!(it.nth_back(0), None);

let mut it = xs.iter();
it.by_ref().skip(2).nth_back(3);
assert_eq!(it.next_back(), Some(&1));

let mut it = xs.iter();
it.by_ref().skip(2).nth_back(10);
assert_eq!(it.next_back(), Some(&1));
}

#[test]
fn test_take_try_folds() {
let f = &|acc, x| i32::checked_add(2*acc, x);
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