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60 lines
2.0 KiB
Markdown
60 lines
2.0 KiB
Markdown
# Iterating over an `Option`
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## Description
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`Option` can be viewed as a container that contains either zero or one
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element. In particular, it implements the `IntoIterator` trait, and as such
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can be used with generic code that needs such a type.
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## Examples
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Since `Option` implements `IntoIterator`, it can be used as an argument to
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[`.extend()`](https://doc.rust-lang.org/std/iter/trait.Extend.html#tymethod.extend):
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```rust
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let turing = Some("Turing");
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let mut logicians = vec!["Curry", "Kleene", "Markov"];
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logicians.extend(turing);
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// equivalent to
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if let Some(turing_inner) = turing {
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logicians.push(turing_inner);
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}
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```
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If you need to tack an `Option` to the end of an existing iterator, you can
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pass it to [`.chain()`](https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.chain):
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```rust
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let turing = Some("Turing");
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let logicians = vec!["Curry", "Kleene", "Markov"];
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for logician in logicians.iter().chain(turing.iter()) {
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println!("{} is a logician", logician);
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}
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```
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Note that if the `Option` is always `Some`, then it is more idiomatic to use
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[`std::iter::once`](https://doc.rust-lang.org/std/iter/fn.once.html) on the
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element instead.
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Also, since `Option` implements `IntoIterator`, it's possible to iterate over
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it using a `for` loop. This is equivalent to matching it with `if let Some(..)`,
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and in most cases you should prefer the latter.
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## See also
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* [`std::iter::once`](https://doc.rust-lang.org/std/iter/fn.once.html) is an
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iterator which yields exactly one element. It's a more readable alternative to
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`Some(foo).into_iter()`.
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* [`Iterator::filter_map`](https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.filter_map)
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is a version of [`Iterator::map`](https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.map),
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specialized to mapping functions which return `Option`.
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* The [`ref_slice`](https://crates.io/crates/ref_slice) crate provides functions
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for converting an `Option` to a zero- or one-element slice.
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* [Documentation for `Option<T>`](https://doc.rust-lang.org/std/option/enum.Option.html)
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