1 //! Composable external iteration.
3 //! If you've found yourself with a collection of some kind, and needed to
4 //! perform an operation on the elements of said collection, you'll quickly run
5 //! into 'iterators'. Iterators are heavily used in idiomatic Rust code, so
6 //! it's worth becoming familiar with them.
8 //! Before explaining more, let's talk about how this module is structured:
12 //! This module is largely organized by type:
14 //! * [Traits] are the core portion: these traits define what kind of iterators
15 //! exist and what you can do with them. The methods of these traits are worth
16 //! putting some extra study time into.
17 //! * [Functions] provide some helpful ways to create some basic iterators.
18 //! * [Structs] are often the return types of the various methods on this
19 //! module's traits. You'll usually want to look at the method that creates
20 //! the `struct`, rather than the `struct` itself. For more detail about why,
21 //! see '[Implementing Iterator](#implementing-iterator)'.
24 //! [Functions]: #functions
25 //! [Structs]: #structs
27 //! That's it! Let's dig into iterators.
31 //! The heart and soul of this module is the [`Iterator`] trait. The core of
32 //! [`Iterator`] looks like this:
37 //! fn next(&mut self) -> Option<Self::Item>;
41 //! An iterator has a method, [`next`], which when called, returns an
42 //! [`Option`]`<Item>`. [`next`] will return `Some(Item)` as long as there
43 //! are elements, and once they've all been exhausted, will return `None` to
44 //! indicate that iteration is finished. Individual iterators may choose to
45 //! resume iteration, and so calling [`next`] again may or may not eventually
46 //! start returning `Some(Item)` again at some point (for example, see [`TryIter`]).
48 //! [`Iterator`]'s full definition includes a number of other methods as well,
49 //! but they are default methods, built on top of [`next`], and so you get
52 //! Iterators are also composable, and it's common to chain them together to do
53 //! more complex forms of processing. See the [Adapters](#adapters) section
54 //! below for more details.
56 //! [`Iterator`]: trait.Iterator.html
57 //! [`next`]: trait.Iterator.html#tymethod.next
58 //! [`TryIter`]: ../../std/sync/mpsc/struct.TryIter.html
60 //! # The three forms of iteration
62 //! There are three common methods which can create iterators from a collection:
64 //! * `iter()`, which iterates over `&T`.
65 //! * `iter_mut()`, which iterates over `&mut T`.
66 //! * `into_iter()`, which iterates over `T`.
68 //! Various things in the standard library may implement one or more of the
69 //! three, where appropriate.
71 //! # Implementing Iterator
73 //! Creating an iterator of your own involves two steps: creating a `struct` to
74 //! hold the iterator's state, and then `impl`ementing [`Iterator`] for that
75 //! `struct`. This is why there are so many `struct`s in this module: there is
76 //! one for each iterator and iterator adapter.
78 //! Let's make an iterator named `Counter` which counts from `1` to `5`:
81 //! // First, the struct:
83 //! /// An iterator which counts from one to five
88 //! // we want our count to start at one, so let's add a new() method to help.
89 //! // This isn't strictly necessary, but is convenient. Note that we start
90 //! // `count` at zero, we'll see why in `next()`'s implementation below.
92 //! fn new() -> Counter {
93 //! Counter { count: 0 }
97 //! // Then, we implement `Iterator` for our `Counter`:
99 //! impl Iterator for Counter {
100 //! // we will be counting with usize
101 //! type Item = usize;
103 //! // next() is the only required method
104 //! fn next(&mut self) -> Option<Self::Item> {
105 //! // Increment our count. This is why we started at zero.
108 //! // Check to see if we've finished counting or not.
109 //! if self.count < 6 {
117 //! // And now we can use it!
119 //! let mut counter = Counter::new();
121 //! assert_eq!(counter.next(), Some(1));
122 //! assert_eq!(counter.next(), Some(2));
123 //! assert_eq!(counter.next(), Some(3));
124 //! assert_eq!(counter.next(), Some(4));
125 //! assert_eq!(counter.next(), Some(5));
126 //! assert_eq!(counter.next(), None);
129 //! Calling [`next`] this way gets repetitive. Rust has a construct which can
130 //! call [`next`] on your iterator, until it reaches `None`. Let's go over that
133 //! Also note that `Iterator` provides a default implementation of methods such as `nth` and `fold`
134 //! which call `next` internally. However, it is also possible to write a custom implementation of
135 //! methods like `nth` and `fold` if an iterator can compute them more efficiently without calling
138 //! # for Loops and IntoIterator
140 //! Rust's `for` loop syntax is actually sugar for iterators. Here's a basic
141 //! example of `for`:
144 //! let values = vec![1, 2, 3, 4, 5];
146 //! for x in values {
147 //! println!("{}", x);
151 //! This will print the numbers one through five, each on their own line. But
152 //! you'll notice something here: we never called anything on our vector to
153 //! produce an iterator. What gives?
155 //! There's a trait in the standard library for converting something into an
156 //! iterator: [`IntoIterator`]. This trait has one method, [`into_iter`],
157 //! which converts the thing implementing [`IntoIterator`] into an iterator.
158 //! Let's take a look at that `for` loop again, and what the compiler converts
161 //! [`IntoIterator`]: trait.IntoIterator.html
162 //! [`into_iter`]: trait.IntoIterator.html#tymethod.into_iter
165 //! let values = vec![1, 2, 3, 4, 5];
167 //! for x in values {
168 //! println!("{}", x);
172 //! Rust de-sugars this into:
175 //! let values = vec![1, 2, 3, 4, 5];
177 //! let result = match IntoIterator::into_iter(values) {
178 //! mut iter => loop {
180 //! match iter.next() {
181 //! Some(val) => next = val,
185 //! let () = { println!("{}", x); };
192 //! First, we call `into_iter()` on the value. Then, we match on the iterator
193 //! that returns, calling [`next`] over and over until we see a `None`. At
194 //! that point, we `break` out of the loop, and we're done iterating.
196 //! There's one more subtle bit here: the standard library contains an
197 //! interesting implementation of [`IntoIterator`]:
199 //! ```ignore (only-for-syntax-highlight)
200 //! impl<I: Iterator> IntoIterator for I
203 //! In other words, all [`Iterator`]s implement [`IntoIterator`], by just
204 //! returning themselves. This means two things:
206 //! 1. If you're writing an [`Iterator`], you can use it with a `for` loop.
207 //! 2. If you're creating a collection, implementing [`IntoIterator`] for it
208 //! will allow your collection to be used with the `for` loop.
212 //! Functions which take an [`Iterator`] and return another [`Iterator`] are
213 //! often called 'iterator adapters', as they're a form of the 'adapter
216 //! Common iterator adapters include [`map`], [`take`], and [`filter`].
217 //! For more, see their documentation.
219 //! If an iterator adapter panics, the iterator will be in an unspecified (but
220 //! memory safe) state. This state is also not guaranteed to stay the same
221 //! across versions of Rust, so you should avoid relying on the exact values
222 //! returned by an iterator which panicked.
224 //! [`map`]: trait.Iterator.html#method.map
225 //! [`take`]: trait.Iterator.html#method.take
226 //! [`filter`]: trait.Iterator.html#method.filter
230 //! Iterators (and iterator [adapters](#adapters)) are *lazy*. This means that
231 //! just creating an iterator doesn't _do_ a whole lot. Nothing really happens
232 //! until you call [`next`]. This is sometimes a source of confusion when
233 //! creating an iterator solely for its side effects. For example, the [`map`]
234 //! method calls a closure on each element it iterates over:
237 //! # #![allow(unused_must_use)]
238 //! let v = vec![1, 2, 3, 4, 5];
239 //! v.iter().map(|x| println!("{}", x));
242 //! This will not print any values, as we only created an iterator, rather than
243 //! using it. The compiler will warn us about this kind of behavior:
246 //! warning: unused result that must be used: iterators are lazy and
247 //! do nothing unless consumed
250 //! The idiomatic way to write a [`map`] for its side effects is to use a
251 //! `for` loop or call the [`for_each`] method:
254 //! let v = vec![1, 2, 3, 4, 5];
256 //! v.iter().for_each(|x| println!("{}", x));
259 //! println!("{}", x);
263 //! [`map`]: trait.Iterator.html#method.map
264 //! [`for_each`]: trait.Iterator.html#method.for_each
266 //! Another common way to evaluate an iterator is to use the [`collect`]
267 //! method to produce a new collection.
269 //! [`collect`]: trait.Iterator.html#method.collect
273 //! Iterators do not have to be finite. As an example, an open-ended range is
274 //! an infinite iterator:
277 //! let numbers = 0..;
280 //! It is common to use the [`take`] iterator adapter to turn an infinite
281 //! iterator into a finite one:
284 //! let numbers = 0..;
285 //! let five_numbers = numbers.take(5);
287 //! for number in five_numbers {
288 //! println!("{}", number);
292 //! This will print the numbers `0` through `4`, each on their own line.
294 //! Bear in mind that methods on infinite iterators, even those for which a
295 //! result can be determined mathematically in finite time, may not terminate.
296 //! Specifically, methods such as [`min`], which in the general case require
297 //! traversing every element in the iterator, are likely not to return
298 //! successfully for any infinite iterators.
301 //! let ones = std::iter::repeat(1);
302 //! let least = ones.min().unwrap(); // Oh no! An infinite loop!
303 //! // `ones.min()` causes an infinite loop, so we won't reach this point!
304 //! println!("The smallest number one is {}.", least);
307 //! [`take`]: trait.Iterator.html#method.take
308 //! [`min`]: trait.Iterator.html#method.min
310 #![stable(feature = "rust1", since = "1.0.0")]
314 #[stable(feature = "rust1", since = "1.0.0")]
315 pub use self::traits::Iterator;
318 feature = "step_trait",
319 reason = "likely to be replaced by finer-grained traits",
322 pub use self::range::Step;
324 #[stable(feature = "iter_empty", since = "1.2.0")]
325 pub use self::sources::{empty, Empty};
326 #[stable(feature = "iter_from_fn", since = "1.34.0")]
327 pub use self::sources::{from_fn, FromFn};
328 #[stable(feature = "iter_once", since = "1.2.0")]
329 pub use self::sources::{once, Once};
330 #[stable(feature = "iter_once_with", since = "1.43.0")]
331 pub use self::sources::{once_with, OnceWith};
332 #[stable(feature = "rust1", since = "1.0.0")]
333 pub use self::sources::{repeat, Repeat};
334 #[stable(feature = "iterator_repeat_with", since = "1.28.0")]
335 pub use self::sources::{repeat_with, RepeatWith};
336 #[stable(feature = "iter_successors", since = "1.34.0")]
337 pub use self::sources::{successors, Successors};
339 #[stable(feature = "fused", since = "1.26.0")]
340 pub use self::traits::FusedIterator;
341 #[unstable(feature = "trusted_len", issue = "37572")]
342 pub use self::traits::TrustedLen;
343 #[stable(feature = "rust1", since = "1.0.0")]
344 pub use self::traits::{DoubleEndedIterator, Extend, FromIterator, IntoIterator};
345 #[stable(feature = "rust1", since = "1.0.0")]
346 pub use self::traits::{ExactSizeIterator, Product, Sum};
348 #[stable(feature = "iter_cloned", since = "1.1.0")]
349 pub use self::adapters::Cloned;
350 #[stable(feature = "iter_copied", since = "1.36.0")]
351 pub use self::adapters::Copied;
352 #[stable(feature = "iterator_flatten", since = "1.29.0")]
353 pub use self::adapters::Flatten;
354 #[unstable(feature = "iter_map_while", reason = "recently added", issue = "68537")]
355 pub use self::adapters::MapWhile;
356 #[stable(feature = "iterator_step_by", since = "1.28.0")]
357 pub use self::adapters::StepBy;
358 #[stable(feature = "rust1", since = "1.0.0")]
359 pub use self::adapters::{Chain, Cycle, Enumerate, Filter, FilterMap, Map, Rev, Zip};
360 #[stable(feature = "rust1", since = "1.0.0")]
361 pub use self::adapters::{FlatMap, Peekable, Scan, Skip, SkipWhile, Take, TakeWhile};
362 #[stable(feature = "rust1", since = "1.0.0")]
363 pub use self::adapters::{Fuse, Inspect};
365 pub(crate) use self::adapters::{process_results, TrustedRandomAccess};
372 /// Used to make try_fold closures more like normal loops
374 enum LoopState<C, B> {
379 impl<C, B> Try for LoopState<C, B> {
383 fn into_result(self) -> Result<Self::Ok, Self::Error> {
385 LoopState::Continue(y) => Ok(y),
386 LoopState::Break(x) => Err(x),
390 fn from_error(v: Self::Error) -> Self {
394 fn from_ok(v: Self::Ok) -> Self {
395 LoopState::Continue(v)
399 impl<C, B> LoopState<C, B> {
401 fn break_value(self) -> Option<B> {
403 LoopState::Continue(..) => None,
404 LoopState::Break(x) => Some(x),
409 impl<R: Try> LoopState<R::Ok, R> {
411 fn from_try(r: R) -> Self {
412 match Try::into_result(r) {
413 Ok(v) => LoopState::Continue(v),
414 Err(v) => LoopState::Break(Try::from_error(v)),
418 fn into_try(self) -> R {
420 LoopState::Continue(v) => Try::from_ok(v),
421 LoopState::Break(v) => v,