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 //! [`Some(Item)`]: Some
57 //! [`next`]: Iterator::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 implementing [`Iterator`] for that `struct`.
75 //! This is why there are so many `struct`s in this module: there is one for
76 //! 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 //! [`into_iter`]: IntoIterator::into_iter
164 //! let values = vec![1, 2, 3, 4, 5];
166 //! for x in values {
167 //! println!("{}", x);
171 //! Rust de-sugars this into:
174 //! let values = vec![1, 2, 3, 4, 5];
176 //! let result = match IntoIterator::into_iter(values) {
177 //! mut iter => loop {
179 //! match iter.next() {
180 //! Some(val) => next = val,
184 //! let () = { println!("{}", x); };
191 //! First, we call `into_iter()` on the value. Then, we match on the iterator
192 //! that returns, calling [`next`] over and over until we see a `None`. At
193 //! that point, we `break` out of the loop, and we're done iterating.
195 //! There's one more subtle bit here: the standard library contains an
196 //! interesting implementation of [`IntoIterator`]:
198 //! ```ignore (only-for-syntax-highlight)
199 //! impl<I: Iterator> IntoIterator for I
202 //! In other words, all [`Iterator`]s implement [`IntoIterator`], by just
203 //! returning themselves. This means two things:
205 //! 1. If you're writing an [`Iterator`], you can use it with a `for` loop.
206 //! 2. If you're creating a collection, implementing [`IntoIterator`] for it
207 //! will allow your collection to be used with the `for` loop.
209 //! # Iterating by reference
211 //! Since [`into_iter()`] takes `self` by value, using a `for` loop to iterate
212 //! over a collection consumes that collection. Often, you may want to iterate
213 //! over a collection without consuming it. Many collections offer methods that
214 //! provide iterators over references, conventionally called `iter()` and
215 //! `iter_mut()` respectively:
218 //! let mut values = vec![41];
219 //! for x in values.iter_mut() {
222 //! for x in values.iter() {
223 //! assert_eq!(*x, 42);
225 //! assert_eq!(values.len(), 1); // `values` is still owned by this function.
228 //! If a collection type `C` provides `iter()`, it usually also implements
229 //! `IntoIterator` for `&C`, with an implementation that just calls `iter()`.
230 //! Likewise, a collection `C` that provides `iter_mut()` generally implements
231 //! `IntoIterator` for `&mut C` by delegating to `iter_mut()`. This enables a
232 //! convenient shorthand:
235 //! let mut values = vec![41];
236 //! for x in &mut values { // same as `values.iter_mut()`
239 //! for x in &values { // same as `values.iter()`
240 //! assert_eq!(*x, 42);
242 //! assert_eq!(values.len(), 1);
245 //! While many collections offer `iter()`, not all offer `iter_mut()`. For
246 //! example, mutating the keys of a [`HashSet<T>`] or [`HashMap<K, V>`] could
247 //! put the collection into an inconsistent state if the key hashes change, so
248 //! these collections only offer `iter()`.
250 //! [`into_iter()`]: IntoIterator::into_iter
251 //! [`HashSet<T>`]: ../../std/collections/struct.HashSet.html
252 //! [`HashMap<K, V>`]: ../../std/collections/struct.HashMap.html
256 //! Functions which take an [`Iterator`] and return another [`Iterator`] are
257 //! often called 'iterator adapters', as they're a form of the 'adapter
260 //! Common iterator adapters include [`map`], [`take`], and [`filter`].
261 //! For more, see their documentation.
263 //! If an iterator adapter panics, the iterator will be in an unspecified (but
264 //! memory safe) state. This state is also not guaranteed to stay the same
265 //! across versions of Rust, so you should avoid relying on the exact values
266 //! returned by an iterator which panicked.
268 //! [`map`]: Iterator::map
269 //! [`take`]: Iterator::take
270 //! [`filter`]: Iterator::filter
274 //! Iterators (and iterator [adapters](#adapters)) are *lazy*. This means that
275 //! just creating an iterator doesn't _do_ a whole lot. Nothing really happens
276 //! until you call [`next`]. This is sometimes a source of confusion when
277 //! creating an iterator solely for its side effects. For example, the [`map`]
278 //! method calls a closure on each element it iterates over:
281 //! # #![allow(unused_must_use)]
282 //! let v = vec![1, 2, 3, 4, 5];
283 //! v.iter().map(|x| println!("{}", x));
286 //! This will not print any values, as we only created an iterator, rather than
287 //! using it. The compiler will warn us about this kind of behavior:
290 //! warning: unused result that must be used: iterators are lazy and
291 //! do nothing unless consumed
294 //! The idiomatic way to write a [`map`] for its side effects is to use a
295 //! `for` loop or call the [`for_each`] method:
298 //! let v = vec![1, 2, 3, 4, 5];
300 //! v.iter().for_each(|x| println!("{}", x));
303 //! println!("{}", x);
307 //! [`map`]: Iterator::map
308 //! [`for_each`]: Iterator::for_each
310 //! Another common way to evaluate an iterator is to use the [`collect`]
311 //! method to produce a new collection.
313 //! [`collect`]: Iterator::collect
317 //! Iterators do not have to be finite. As an example, an open-ended range is
318 //! an infinite iterator:
321 //! let numbers = 0..;
324 //! It is common to use the [`take`] iterator adapter to turn an infinite
325 //! iterator into a finite one:
328 //! let numbers = 0..;
329 //! let five_numbers = numbers.take(5);
331 //! for number in five_numbers {
332 //! println!("{}", number);
336 //! This will print the numbers `0` through `4`, each on their own line.
338 //! Bear in mind that methods on infinite iterators, even those for which a
339 //! result can be determined mathematically in finite time, may not terminate.
340 //! Specifically, methods such as [`min`], which in the general case require
341 //! traversing every element in the iterator, are likely not to return
342 //! successfully for any infinite iterators.
345 //! let ones = std::iter::repeat(1);
346 //! let least = ones.min().unwrap(); // Oh no! An infinite loop!
347 //! // `ones.min()` causes an infinite loop, so we won't reach this point!
348 //! println!("The smallest number one is {}.", least);
351 //! [`take`]: Iterator::take
352 //! [`min`]: Iterator::min
354 #![stable(feature = "rust1", since = "1.0.0")]
356 #[stable(feature = "rust1", since = "1.0.0")]
357 pub use self::traits
::Iterator
;
360 feature
= "step_trait",
361 reason
= "likely to be replaced by finer-grained traits",
364 pub use self::range
::Step
;
366 #[stable(feature = "iter_empty", since = "1.2.0")]
367 pub use self::sources
::{empty, Empty}
;
368 #[stable(feature = "iter_from_fn", since = "1.34.0")]
369 pub use self::sources
::{from_fn, FromFn}
;
370 #[stable(feature = "iter_once", since = "1.2.0")]
371 pub use self::sources
::{once, Once}
;
372 #[stable(feature = "iter_once_with", since = "1.43.0")]
373 pub use self::sources
::{once_with, OnceWith}
;
374 #[stable(feature = "rust1", since = "1.0.0")]
375 pub use self::sources
::{repeat, Repeat}
;
376 #[stable(feature = "iterator_repeat_with", since = "1.28.0")]
377 pub use self::sources
::{repeat_with, RepeatWith}
;
378 #[stable(feature = "iter_successors", since = "1.34.0")]
379 pub use self::sources
::{successors, Successors}
;
381 #[stable(feature = "fused", since = "1.26.0")]
382 pub use self::traits
::FusedIterator
;
383 #[unstable(issue = "none", feature = "inplace_iteration")]
384 pub use self::traits
::InPlaceIterable
;
385 #[unstable(feature = "trusted_len", issue = "37572")]
386 pub use self::traits
::TrustedLen
;
387 #[stable(feature = "rust1", since = "1.0.0")]
388 pub use self::traits
::{
389 DoubleEndedIterator
, ExactSizeIterator
, Extend
, FromIterator
, IntoIterator
, Product
, Sum
,
392 #[stable(feature = "iter_cloned", since = "1.1.0")]
393 pub use self::adapters
::Cloned
;
394 #[stable(feature = "iter_copied", since = "1.36.0")]
395 pub use self::adapters
::Copied
;
396 #[stable(feature = "iterator_flatten", since = "1.29.0")]
397 pub use self::adapters
::Flatten
;
398 #[unstable(feature = "iter_map_while", reason = "recently added", issue = "68537")]
399 pub use self::adapters
::MapWhile
;
400 #[unstable(feature = "inplace_iteration", issue = "none")]
401 pub use self::adapters
::SourceIter
;
402 #[stable(feature = "iterator_step_by", since = "1.28.0")]
403 pub use self::adapters
::StepBy
;
404 #[unstable(feature = "trusted_random_access", issue = "none")]
405 pub use self::adapters
::TrustedRandomAccess
;
406 #[stable(feature = "rust1", since = "1.0.0")]
407 pub use self::adapters
::{
408 Chain
, Cycle
, Enumerate
, Filter
, FilterMap
, FlatMap
, Fuse
, Inspect
, Map
, Peekable
, Rev
, Scan
,
409 Skip
, SkipWhile
, Take
, TakeWhile
, Zip
,
411 #[unstable(feature = "iter_intersperse", reason = "recently added", issue = "79524")]
412 pub use self::adapters
::{Intersperse, IntersperseWith}
;
414 pub(crate) use self::adapters
::process_results
;