57 lines
1.9 KiB
Markdown
57 lines
1.9 KiB
Markdown
# `peeking_take_while`
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[](https://travis-ci.org/fitzgen/peeking_take_while)
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Provides the `peeking_take_while` iterator adaptor method.
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The `peeking_take_while` method is very similar to `take_while`, but behaves
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differently when used with a borrowed iterator (perhaps returned by
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`Iterator::by_ref`).
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`peeking_take_while` peeks at the next item in the iterator and runs the
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predicate on that peeked item. This avoids consuming the first item yielded by
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the underlying iterator for which the predicate returns `false`. On the other
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hand, `take_while` will consume that first item for which the predicate returns
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`false`, and it will be lost.
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```rust
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// Bring the `peeking_take_while` method for peekable iterators into
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// scope.
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use peeking_take_while::PeekableExt;
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// Let's say we have two collections we want to iterate through: `xs` and
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// `ys`. We want to perform one operation on all the leading contiguous
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// elements that match some predicate, and a different thing with the rest of
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// the elements. With the `xs`, we will use the normal `take_while`. With the
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// `ys`, we will use `peeking_take_while`.
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let xs: Vec<u8> = (0..100).collect();
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let ys = xs.clone();
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let mut iter_xs = xs.into_iter();
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let mut iter_ys = ys.into_iter().peekable();
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{
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// Let's do one thing with all the items that are less than 10.
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let xs_less_than_ten = iter_xs.by_ref().take_while(|x| *x < 10);
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for x in xs_less_than_ten {
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do_things_with(x);
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}
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let ys_less_than_ten = iter_ys.by_ref().peeking_take_while(|y| *y < 10);
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for y in ys_less_than_ten {
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do_things_with(y);
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}
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}
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// And now we will do some other thing with the items that are greater than
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// or equal to 10.
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// ...except, when using plain old `take_while` we lost 10!
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assert_eq!(iter_xs.next(), Some(11));
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// However, when using `peeking_take_while` we did not! Great!
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assert_eq!(iter_ys.next(), Some(10));
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```
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