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47 lines
2.0 KiB
Markdown
47 lines
2.0 KiB
Markdown
8 years ago
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# 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 elements. In particular, it implements the `IntoIterator` trait, and as such 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 [`.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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bar.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 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 [`std::iter::once`](https://doc.rust-lang.org/std/iter/fn.once.html) on the element instead.
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Also, since `Option` implements `IntoIterator`, it's possible to iterate over it using a `for` loop. This is equivalent to matching it with `if let Some(..)`, 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 iterator which yields exactly one element. It's a more readable alternative to `Some(foo).into_iter()`.
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* [`Iterator::filter_map`](https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.filter_map) is a version of [`Iterator::flat_map`](https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.flat_map), specialized to mapping functions which return `Option`.
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* The [`ref_slice`](https://crates.io/crates/ref_slice) crate provides functions 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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