Google Git
Sign in
nest-open-source / manifest_repos / toolchain / 6143c57c644e0da7c4a10d1b4af322f92e74e6f7 / . / x86_64 / usr / lib / rustlib / src / rust / library / core / src / iter / traits / iterator.rs
blob: 83c7e8977e9f3e22e842bef6a0581afe7be174a9 [file] [log] [blame]
use crate::array;
use crate::cmp::{self, Ordering};
use crate::ops::{ChangeOutputType, ControlFlow, FromResidual, Residual, Try};
use super::super::try_process;
use super::super::ByRefSized;
use super::super::TrustedRandomAccessNoCoerce;
use super::super::{ArrayChunks, Chain, Cloned, Copied, Cycle, Enumerate, Filter, FilterMap, Fuse};
use super::super::{FlatMap, Flatten};
use super::super::{FromIterator, Intersperse, IntersperseWith, Product, Sum, Zip};
use super::super::{
Inspect, Map, MapWhile, Peekable, Rev, Scan, Skip, SkipWhile, StepBy, Take, TakeWhile,
};
fn _assert_is_object_safe(_: &dyn Iterator<Item = ()>) {}
/// A trait for dealing with iterators.
///
/// This is the main iterator trait. For more about the concept of iterators
/// generally, please see the [module-level documentation]. In particular, you
/// may want to know how to [implement `Iterator`][impl].
///
/// [module-level documentation]: crate::iter
/// [impl]: crate::iter#implementing-iterator
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_on_unimplemented(
on(
_Self = "std::ops::RangeTo<Idx>",
label = "if you meant to iterate until a value, add a starting value",
note = "`..end` is a `RangeTo`, which cannot be iterated on; you might have meant to have a \
bounded `Range`: `0..end`"
),
on(
_Self = "std::ops::RangeToInclusive<Idx>",
label = "if you meant to iterate until a value (including it), add a starting value",
note = "`..=end` is a `RangeToInclusive`, which cannot be iterated on; you might have meant \
to have a bounded `RangeInclusive`: `0..=end`"
),
on(
_Self = "[]",
label = "`{Self}` is not an iterator; try calling `.into_iter()` or `.iter()`"
),
on(_Self = "&[]", label = "`{Self}` is not an iterator; try calling `.iter()`"),
on(
_Self = "std::vec::Vec<T, A>",
label = "`{Self}` is not an iterator; try calling `.into_iter()` or `.iter()`"
),
on(
_Self = "&str",
label = "`{Self}` is not an iterator; try calling `.chars()` or `.bytes()`"
),
on(
_Self = "std::string::String",
label = "`{Self}` is not an iterator; try calling `.chars()` or `.bytes()`"
),
on(
_Self = "{integral}",
note = "if you want to iterate between `start` until a value `end`, use the exclusive range \
syntax `start..end` or the inclusive range syntax `start..=end`"
),
label = "`{Self}` is not an iterator",
message = "`{Self}` is not an iterator"
)]
#[doc(notable_trait)]
#[rustc_diagnostic_item = "Iterator"]
#[must_use = "iterators are lazy and do nothing unless consumed"]
pub trait Iterator {
/// The type of the elements being iterated over.
#[stable(feature = "rust1", since = "1.0.0")]
type Item;
/// Advances the iterator and returns the next value.
///
/// Returns [`None`] when iteration is finished. Individual iterator
/// implementations may choose to resume iteration, and so calling `next()`
/// again may or may not eventually start returning [`Some(Item)`] again at some
/// point.
///
/// [`Some(Item)`]: Some
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let a = [1, 2, 3];
///
/// let mut iter = a.iter();
///
/// // A call to next() returns the next value...
/// assert_eq!(Some(&1), iter.next());
/// assert_eq!(Some(&2), iter.next());
/// assert_eq!(Some(&3), iter.next());
///
/// // ... and then None once it's over.
/// assert_eq!(None, iter.next());
///
/// // More calls may or may not return `None`. Here, they always will.
/// assert_eq!(None, iter.next());
/// assert_eq!(None, iter.next());
/// ```
#[lang = "next"]
#[stable(feature = "rust1", since = "1.0.0")]
fn next(&mut self) -> Option<Self::Item>;
/// Advances the iterator and returns an array containing the next `N` values.
///
/// If there are not enough elements to fill the array then `Err` is returned
/// containing an iterator over the remaining elements.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// #![feature(iter_next_chunk)]
///
/// let mut iter = "lorem".chars();
///
/// assert_eq!(iter.next_chunk().unwrap(), ['l', 'o']); // N is inferred as 2
/// assert_eq!(iter.next_chunk().unwrap(), ['r', 'e', 'm']); // N is inferred as 3
/// assert_eq!(iter.next_chunk::<4>().unwrap_err().as_slice(), &[]); // N is explicitly 4
/// ```
///
/// Split a string and get the first three items.
///
/// ```
/// #![feature(iter_next_chunk)]
///
/// let quote = "not all those who wander are lost";
/// let [first, second, third] = quote.split_whitespace().next_chunk().unwrap();
/// assert_eq!(first, "not");
/// assert_eq!(second, "all");
/// assert_eq!(third, "those");
/// ```
#[inline]
#[unstable(feature = "iter_next_chunk", reason = "recently added", issue = "98326")]
fn next_chunk<const N: usize>(
&mut self,
) -> Result<[Self::Item; N], array::IntoIter<Self::Item, N>>
where
Self: Sized,
{
array::iter_next_chunk(self)
}
/// Returns the bounds on the remaining length of the iterator.
///
/// Specifically, `size_hint()` returns a tuple where the first element
/// is the lower bound, and the second element is the upper bound.
///
/// The second half of the tuple that is returned is an <code>[Option]<[usize]></code>.
/// A [`None`] here means that either there is no known upper bound, or the
/// upper bound is larger than [`usize`].
///
/// # Implementation notes
///
/// It is not enforced that an iterator implementation yields the declared
/// number of elements. A buggy iterator may yield less than the lower bound
/// or more than the upper bound of elements.
///
/// `size_hint()` is primarily intended to be used for optimizations such as
/// reserving space for the elements of the iterator, but must not be
/// trusted to e.g., omit bounds checks in unsafe code. An incorrect
/// implementation of `size_hint()` should not lead to memory safety
/// violations.
///
/// That said, the implementation should provide a correct estimation,
/// because otherwise it would be a violation of the trait's protocol.
///
/// The default implementation returns <code>(0, [None])</code> which is correct for any
/// iterator.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let a = [1, 2, 3];
/// let mut iter = a.iter();
///
/// assert_eq!((3, Some(3)), iter.size_hint());
/// let _ = iter.next();
/// assert_eq!((2, Some(2)), iter.size_hint());
/// ```
///
/// A more complex example:
///
/// ```
/// // The even numbers in the range of zero to nine.
/// let iter = (0..10).filter(|x| x % 2 == 0);
///
/// // We might iterate from zero to ten times. Knowing that it's five
/// // exactly wouldn't be possible without executing filter().
/// assert_eq!((0, Some(10)), iter.size_hint());
///
/// // Let's add five more numbers with chain()
/// let iter = (0..10).filter(|x| x % 2 == 0).chain(15..20);
///
/// // now both bounds are increased by five
/// assert_eq!((5, Some(15)), iter.size_hint());
/// ```
///
/// Returning `None` for an upper bound:
///
/// ```
/// // an infinite iterator has no upper bound
/// // and the maximum possible lower bound
/// let iter = 0..;
///
/// assert_eq!((usize::MAX, None), iter.size_hint());
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
fn size_hint(&self) -> (usize, Option<usize>) {
(0, None)
}
/// Consumes the iterator, counting the number of iterations and returning it.
///
/// This method will call [`next`] repeatedly until [`None`] is encountered,
/// returning the number of times it saw [`Some`]. Note that [`next`] has to be
/// called at least once even if the iterator does not have any elements.
///
/// [`next`]: Iterator::next
///
/// # Overflow Behavior
///
/// The method does no guarding against overflows, so counting elements of
/// an iterator with more than [`usize::MAX`] elements either produces the
/// wrong result or panics. If debug assertions are enabled, a panic is
/// guaranteed.
///
/// # Panics
///
/// This function might panic if the iterator has more than [`usize::MAX`]
/// elements.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let a = [1, 2, 3];
/// assert_eq!(a.iter().count(), 3);
///
/// let a = [1, 2, 3, 4, 5];
/// assert_eq!(a.iter().count(), 5);
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
fn count(self) -> usize
where
Self: Sized,
{
self.fold(
0,
#[rustc_inherit_overflow_checks]
|count, _| count + 1,
)
}
/// Consumes the iterator, returning the last element.
///
/// This method will evaluate the iterator until it returns [`None`]. While
/// doing so, it keeps track of the current element. After [`None`] is
/// returned, `last()` will then return the last element it saw.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let a = [1, 2, 3];
/// assert_eq!(a.iter().last(), Some(&3));
///
/// let a = [1, 2, 3, 4, 5];
/// assert_eq!(a.iter().last(), Some(&5));
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
fn last(self) -> Option<Self::Item>
where
Self: Sized,
{
#[inline]
fn some<T>(_: Option<T>, x: T) -> Option<T> {
Some(x)
}
self.fold(None, some)
}
/// Advances the iterator by `n` elements.
///
/// This method will eagerly skip `n` elements by calling [`next`] up to `n`
/// times until [`None`] is encountered.
///
/// `advance_by(n)` will return [`Ok(())`][Ok] if the iterator successfully advances by
/// `n` elements, or [`Err(k)`][Err] if [`None`] is encountered, where `k` is the number
/// of elements the iterator is advanced by before running out of elements (i.e. the
/// length of the iterator). Note that `k` is always less than `n`.
///
/// Calling `advance_by(0)` can do meaningful work, for example [`Flatten`]
/// can advance its outer iterator until it finds an inner iterator that is not empty, which
/// then often allows it to return a more accurate `size_hint()` than in its initial state.
///
/// [`Flatten`]: crate::iter::Flatten
/// [`next`]: Iterator::next
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// #![feature(iter_advance_by)]
///
/// let a = [1, 2, 3, 4];
/// let mut iter = a.iter();
///
/// assert_eq!(iter.advance_by(2), Ok(()));
/// assert_eq!(iter.next(), Some(&3));
/// assert_eq!(iter.advance_by(0), Ok(()));
/// assert_eq!(iter.advance_by(100), Err(1)); // only `&4` was skipped
/// ```
#[inline]
#[unstable(feature = "iter_advance_by", reason = "recently added", issue = "77404")]
fn advance_by(&mut self, n: usize) -> Result<(), usize> {
for i in 0..n {
self.next().ok_or(i)?;
}
Ok(())
}
/// Returns the `n`th element of the iterator.
///
/// Like most indexing operations, the count starts from zero, so `nth(0)`
/// returns the first value, `nth(1)` the second, and so on.
///
/// Note that all preceding elements, as well as the returned element, will be
/// consumed from the iterator. That means that the preceding elements will be
/// discarded, and also that calling `nth(0)` multiple times on the same iterator
/// will return different elements.
///
/// `nth()` will return [`None`] if `n` is greater than or equal to the length of the
/// iterator.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let a = [1, 2, 3];
/// assert_eq!(a.iter().nth(1), Some(&2));
/// ```
///
/// Calling `nth()` multiple times doesn't rewind the iterator:
///
/// ```
/// let a = [1, 2, 3];
///
/// let mut iter = a.iter();
///
/// assert_eq!(iter.nth(1), Some(&2));
/// assert_eq!(iter.nth(1), None);
/// ```
///
/// Returning `None` if there are less than `n + 1` elements:
///
/// ```
/// let a = [1, 2, 3];
/// assert_eq!(a.iter().nth(10), None);
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
fn nth(&mut self, n: usize) -> Option<Self::Item> {
self.advance_by(n).ok()?;
self.next()
}
/// Creates an iterator starting at the same point, but stepping by
/// the given amount at each iteration.
///
/// Note 1: The first element of the iterator will always be returned,
/// regardless of the step given.
///
/// Note 2: The time at which ignored elements are pulled is not fixed.
/// `StepBy` behaves like the sequence `self.next()`, `self.nth(step-1)`,
/// `self.nth(step-1)`, …, but is also free to behave like the sequence
/// `advance_n_and_return_first(&mut self, step)`,
/// `advance_n_and_return_first(&mut self, step)`, …
/// Which way is used may change for some iterators for performance reasons.
/// The second way will advance the iterator earlier and may consume more items.
///
/// `advance_n_and_return_first` is the equivalent of:
/// ```
/// fn advance_n_and_return_first<I>(iter: &mut I, n: usize) -> Option<I::Item>
/// where
/// I: Iterator,
/// {
/// let next = iter.next();
/// if n > 1 {
/// iter.nth(n - 2);
/// }
/// next
/// }
/// ```
///
/// # Panics
///
/// The method will panic if the given step is `0`.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let a = [0, 1, 2, 3, 4, 5];
/// let mut iter = a.iter().step_by(2);
///
/// assert_eq!(iter.next(), Some(&0));
/// assert_eq!(iter.next(), Some(&2));
/// assert_eq!(iter.next(), Some(&4));
/// assert_eq!(iter.next(), None);
/// ```
#[inline]
#[stable(feature = "iterator_step_by", since = "1.28.0")]
fn step_by(self, step: usize) -> StepBy<Self>
where
Self: Sized,
{
StepBy::new(self, step)
}
/// Takes two iterators and creates a new iterator over both in sequence.
///
/// `chain()` will return a new iterator which will first iterate over
/// values from the first iterator and then over values from the second
/// iterator.
///
/// In other words, it links two iterators together, in a chain. 🔗
///
/// [`once`] is commonly used to adapt a single value into a chain of
/// other kinds of iteration.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let a1 = [1, 2, 3];
/// let a2 = [4, 5, 6];
///
/// let mut iter = a1.iter().chain(a2.iter());
///
/// assert_eq!(iter.next(), Some(&1));
/// assert_eq!(iter.next(), Some(&2));
/// assert_eq!(iter.next(), Some(&3));
/// assert_eq!(iter.next(), Some(&4));
/// assert_eq!(iter.next(), Some(&5));
/// assert_eq!(iter.next(), Some(&6));
/// assert_eq!(iter.next(), None);
/// ```
///
/// Since the argument to `chain()` uses [`IntoIterator`], we can pass
/// anything that can be converted into an [`Iterator`], not just an
/// [`Iterator`] itself. For example, slices (`&[T]`) implement
/// [`IntoIterator`], and so can be passed to `chain()` directly:
///
/// ```
/// let s1 = &[1, 2, 3];
/// let s2 = &[4, 5, 6];
///
/// let mut iter = s1.iter().chain(s2);
///
/// assert_eq!(iter.next(), Some(&1));
/// assert_eq!(iter.next(), Some(&2));
/// assert_eq!(iter.next(), Some(&3));
/// assert_eq!(iter.next(), Some(&4));
/// assert_eq!(iter.next(), Some(&5));
/// assert_eq!(iter.next(), Some(&6));
/// assert_eq!(iter.next(), None);
/// ```
///
/// If you work with Windows API, you may wish to convert [`OsStr`] to `Vec<u16>`:
///
/// ```
/// #[cfg(windows)]
/// fn os_str_to_utf16(s: &std::ffi::OsStr) -> Vec<u16> {
/// use std::os::windows::ffi::OsStrExt;
/// s.encode_wide().chain(std::iter::once(0)).collect()
/// }
/// ```
///
/// [`once`]: crate::iter::once
/// [`OsStr`]: ../../std/ffi/struct.OsStr.html
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
fn chain<U>(self, other: U) -> Chain<Self, U::IntoIter>
where
Self: Sized,
U: IntoIterator<Item = Self::Item>,
{
Chain::new(self, other.into_iter())
}
/// 'Zips up' two iterators into a single iterator of pairs.
///
/// `zip()` returns a new iterator that will iterate over two other
/// iterators, returning a tuple where the first element comes from the
/// first iterator, and the second element comes from the second iterator.
///
/// In other words, it zips two iterators together, into a single one.
///
/// If either iterator returns [`None`], [`next`] from the zipped iterator
/// will return [`None`].
/// If the zipped iterator has no more elements to return then each further attempt to advance
/// it will first try to advance the first iterator at most one time and if it still yielded an item
/// try to advance the second iterator at most one time.
///
/// To 'undo' the result of zipping up two iterators, see [`unzip`].
///
/// [`unzip`]: Iterator::unzip
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let a1 = [1, 2, 3];
/// let a2 = [4, 5, 6];
///
/// let mut iter = a1.iter().zip(a2.iter());
///
/// assert_eq!(iter.next(), Some((&1, &4)));
/// assert_eq!(iter.next(), Some((&2, &5)));
/// assert_eq!(iter.next(), Some((&3, &6)));
/// assert_eq!(iter.next(), None);
/// ```
///
/// Since the argument to `zip()` uses [`IntoIterator`], we can pass
/// anything that can be converted into an [`Iterator`], not just an
/// [`Iterator`] itself. For example, slices (`&[T]`) implement
/// [`IntoIterator`], and so can be passed to `zip()` directly:
///
/// ```
/// let s1 = &[1, 2, 3];
/// let s2 = &[4, 5, 6];
///
/// let mut iter = s1.iter().zip(s2);
///
/// assert_eq!(iter.next(), Some((&1, &4)));
/// assert_eq!(iter.next(), Some((&2, &5)));
/// assert_eq!(iter.next(), Some((&3, &6)));
/// assert_eq!(iter.next(), None);
/// ```
///
/// `zip()` is often used to zip an infinite iterator to a finite one.
/// This works because the finite iterator will eventually return [`None`],
/// ending the zipper. Zipping with `(0..)` can look a lot like [`enumerate`]:
///
/// ```
/// let enumerate: Vec<_> = "foo".chars().enumerate().collect();
///
/// let zipper: Vec<_> = (0..).zip("foo".chars()).collect();
///
/// assert_eq!((0, 'f'), enumerate[0]);
/// assert_eq!((0, 'f'), zipper[0]);
///
/// assert_eq!((1, 'o'), enumerate[1]);
/// assert_eq!((1, 'o'), zipper[1]);
///
/// assert_eq!((2, 'o'), enumerate[2]);
/// assert_eq!((2, 'o'), zipper[2]);
/// ```
///
/// If both iterators have roughly equivalent syntax, it may be more readable to use [`zip`]:
///
/// ```
/// use std::iter::zip;
///
/// let a = [1, 2, 3];
/// let b = [2, 3, 4];
///
/// let mut zipped = zip(
/// a.into_iter().map(|x| x * 2).skip(1),
/// b.into_iter().map(|x| x * 2).skip(1),
/// );
///
/// assert_eq!(zipped.next(), Some((4, 6)));
/// assert_eq!(zipped.next(), Some((6, 8)));
/// assert_eq!(zipped.next(), None);
/// ```
///
/// compared to:
///
/// ```
/// # let a = [1, 2, 3];
/// # let b = [2, 3, 4];
/// #
/// let mut zipped = a
/// .into_iter()
/// .map(|x| x * 2)
/// .skip(1)
/// .zip(b.into_iter().map(|x| x * 2).skip(1));
/// #
/// # assert_eq!(zipped.next(), Some((4, 6)));
/// # assert_eq!(zipped.next(), Some((6, 8)));
/// # assert_eq!(zipped.next(), None);
/// ```
///
/// [`enumerate`]: Iterator::enumerate
/// [`next`]: Iterator::next
/// [`zip`]: crate::iter::zip
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
fn zip<U>(self, other: U) -> Zip<Self, U::IntoIter>
where
Self: Sized,
U: IntoIterator,
{
Zip::new(self, other.into_iter())
}
/// Creates a new iterator which places a copy of `separator` between adjacent
/// items of the original iterator.
///
/// In case `separator` does not implement [`Clone`] or needs to be
/// computed every time, use [`intersperse_with`].
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// #![feature(iter_intersperse)]
///
/// let mut a = [0, 1, 2].iter().intersperse(&100);
/// assert_eq!(a.next(), Some(&0)); // The first element from `a`.
/// assert_eq!(a.next(), Some(&100)); // The separator.
/// assert_eq!(a.next(), Some(&1)); // The next element from `a`.
/// assert_eq!(a.next(), Some(&100)); // The separator.
/// assert_eq!(a.next(), Some(&2)); // The last element from `a`.
/// assert_eq!(a.next(), None); // The iterator is finished.
/// ```
///
/// `intersperse` can be very useful to join an iterator's items using a common element:
/// ```
/// #![feature(iter_intersperse)]
///
/// let hello = ["Hello", "World", "!"].iter().copied().intersperse(" ").collect::<String>();
/// assert_eq!(hello, "Hello World !");
/// ```
///
/// [`Clone`]: crate::clone::Clone
/// [`intersperse_with`]: Iterator::intersperse_with
#[inline]
#[unstable(feature = "iter_intersperse", reason = "recently added", issue = "79524")]
fn intersperse(self, separator: Self::Item) -> Intersperse<Self>
where
Self: Sized,
Self::Item: Clone,
{
Intersperse::new(self, separator)
}
/// Creates a new iterator which places an item generated by `separator`
/// between adjacent items of the original iterator.
///
/// The closure will be called exactly once each time an item is placed
/// between two adjacent items from the underlying iterator; specifically,
/// the closure is not called if the underlying iterator yields less than
/// two items and after the last item is yielded.
///
/// If the iterator's item implements [`Clone`], it may be easier to use
/// [`intersperse`].
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// #![feature(iter_intersperse)]
///
/// #[derive(PartialEq, Debug)]
/// struct NotClone(usize);
///
/// let v = [NotClone(0), NotClone(1), NotClone(2)];
/// let mut it = v.into_iter().intersperse_with(|| NotClone(99));
///
/// assert_eq!(it.next(), Some(NotClone(0))); // The first element from `v`.
/// assert_eq!(it.next(), Some(NotClone(99))); // The separator.
/// assert_eq!(it.next(), Some(NotClone(1))); // The next element from `v`.
/// assert_eq!(it.next(), Some(NotClone(99))); // The separator.
/// assert_eq!(it.next(), Some(NotClone(2))); // The last element from `v`.
/// assert_eq!(it.next(), None); // The iterator is finished.
/// ```
///
/// `intersperse_with` can be used in situations where the separator needs
/// to be computed:
/// ```
/// #![feature(iter_intersperse)]
///
/// let src = ["Hello", "to", "all", "people", "!!"].iter().copied();
///
/// // The closure mutably borrows its context to generate an item.
/// let mut happy_emojis = [" ❤️ ", " 😀 "].iter().copied();
/// let separator = || happy_emojis.next().unwrap_or(" 🦀 ");
///
/// let result = src.intersperse_with(separator).collect::<String>();
/// assert_eq!(result, "Hello ❤️ to 😀 all 🦀 people 🦀 !!");
/// ```
/// [`Clone`]: crate::clone::Clone
/// [`intersperse`]: Iterator::intersperse
#[inline]
#[unstable(feature = "iter_intersperse", reason = "recently added", issue = "79524")]
fn intersperse_with<G>(self, separator: G) -> IntersperseWith<Self, G>
where
Self: Sized,
G: FnMut() -> Self::Item,
{
IntersperseWith::new(self, separator)
}
/// Takes a closure and creates an iterator which calls that closure on each
/// element.
///
/// `map()` transforms one iterator into another, by means of its argument:
/// something that implements [`FnMut`]. It produces a new iterator which
/// calls this closure on each element of the original iterator.
///
/// If you are good at thinking in types, you can think of `map()` like this:
/// If you have an iterator that gives you elements of some type `A`, and
/// you want an iterator of some other type `B`, you can use `map()`,
/// passing a closure that takes an `A` and returns a `B`.
///
/// `map()` is conceptually similar to a [`for`] loop. However, as `map()` is
/// lazy, it is best used when you're already working with other iterators.
/// If you're doing some sort of looping for a side effect, it's considered
/// more idiomatic to use [`for`] than `map()`.
///
/// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
/// [`FnMut`]: crate::ops::FnMut
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let a = [1, 2, 3];
///
/// let mut iter = a.iter().map(|x| 2 * x);
///
/// assert_eq!(iter.next(), Some(2));
/// assert_eq!(iter.next(), Some(4));
/// assert_eq!(iter.next(), Some(6));
/// assert_eq!(iter.next(), None);
/// ```
///
/// If you're doing some sort of side effect, prefer [`for`] to `map()`:
///
/// ```
/// # #![allow(unused_must_use)]
/// // don't do this:
/// (0..5).map(|x| println!("{x}"));
///
/// // it won't even execute, as it is lazy. Rust will warn you about this.
///
/// // Instead, use for:
/// for x in 0..5 {
/// println!("{x}");
/// }
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
fn map<B, F>(self, f: F) -> Map<Self, F>
where
Self: Sized,
F: FnMut(Self::Item) -> B,
{
Map::new(self, f)
}
/// Calls a closure on each element of an iterator.
///
/// This is equivalent to using a [`for`] loop on the iterator, although
/// `break` and `continue` are not possible from a closure. It's generally
/// more idiomatic to use a `for` loop, but `for_each` may be more legible
/// when processing items at the end of longer iterator chains. In some
/// cases `for_each` may also be faster than a loop, because it will use
/// internal iteration on adapters like `Chain`.
///
/// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// use std::sync::mpsc::channel;
///
/// let (tx, rx) = channel();
/// (0..5).map(|x| x * 2 + 1)
/// .for_each(move |x| tx.send(x).unwrap());
///
/// let v: Vec<_> = rx.iter().collect();
/// assert_eq!(v, vec![1, 3, 5, 7, 9]);
/// ```
///
/// For such a small example, a `for` loop may be cleaner, but `for_each`
/// might be preferable to keep a functional style with longer iterators:
///
/// ```
/// (0..5).flat_map(|x| x * 100 .. x * 110)
/// .enumerate()
/// .filter(|&(i, x)| (i + x) % 3 == 0)
/// .for_each(|(i, x)| println!("{i}:{x}"));
/// ```
#[inline]
#[stable(feature = "iterator_for_each", since = "1.21.0")]
fn for_each<F>(self, f: F)
where
Self: Sized,
F: FnMut(Self::Item),
{
#[inline]
fn call<T>(mut f: impl FnMut(T)) -> impl FnMut((), T) {
move |(), item| f(item)
}
self.fold((), call(f));
}
/// Creates an iterator which uses a closure to determine if an element
/// should be yielded.
///
/// Given an element the closure must return `true` or `false`. The returned
/// iterator will yield only the elements for which the closure returns
/// true.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let a = [0i32, 1, 2];
///
/// let mut iter = a.iter().filter(|x| x.is_positive());
///
/// assert_eq!(iter.next(), Some(&1));
/// assert_eq!(iter.next(), Some(&2));
/// assert_eq!(iter.next(), None);
/// ```
///
/// Because the closure passed to `filter()` takes a reference, and many
/// iterators iterate over references, this leads to a possibly confusing
/// situation, where the type of the closure is a double reference:
///
/// ```
/// let a = [0, 1, 2];
///
/// let mut iter = a.iter().filter(|x| **x > 1); // need two *s!
///
/// assert_eq!(iter.next(), Some(&2));
/// assert_eq!(iter.next(), None);
/// ```
///
/// It's common to instead use destructuring on the argument to strip away
/// one:
///
/// ```
/// let a = [0, 1, 2];
///
/// let mut iter = a.iter().filter(|&x| *x > 1); // both & and *
///
/// assert_eq!(iter.next(), Some(&2));
/// assert_eq!(iter.next(), None);
/// ```
///
/// or both:
///
/// ```
/// let a = [0, 1, 2];
///
/// let mut iter = a.iter().filter(|&&x| x > 1); // two &s
///
/// assert_eq!(iter.next(), Some(&2));
/// assert_eq!(iter.next(), None);
/// ```
///
/// of these layers.
///
/// Note that `iter.filter(f).next()` is equivalent to `iter.find(f)`.
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
fn filter<P>(self, predicate: P) -> Filter<Self, P>
where
Self: Sized,
P: FnMut(&Self::Item) -> bool,
{
Filter::new(self, predicate)
}
/// Creates an iterator that both filters and maps.
///
/// The returned iterator yields only the `value`s for which the supplied
/// closure returns `Some(value)`.
///
/// `filter_map` can be used to make chains of [`filter`] and [`map`] more
/// concise. The example below shows how a `map().filter().map()` can be
/// shortened to a single call to `filter_map`.
///
/// [`filter`]: Iterator::filter
/// [`map`]: Iterator::map
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let a = ["1", "two", "NaN", "four", "5"];
///
/// let mut iter = a.iter().filter_map(|s| s.parse().ok());
///
/// assert_eq!(iter.next(), Some(1));
/// assert_eq!(iter.next(), Some(5));
/// assert_eq!(iter.next(), None);
/// ```
///
/// Here's the same example, but with [`filter`] and [`map`]:
///
/// ```
/// let a = ["1", "two", "NaN", "four", "5"];
/// let mut iter = a.iter().map(|s| s.parse()).filter(|s| s.is_ok()).map(|s| s.unwrap());
/// assert_eq!(iter.next(), Some(1));
/// assert_eq!(iter.next(), Some(5));
/// assert_eq!(iter.next(), None);
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
fn filter_map<B, F>(self, f: F) -> FilterMap<Self, F>
where
Self: Sized,
F: FnMut(Self::Item) -> Option<B>,
{
FilterMap::new(self, f)
}
/// Creates an iterator which gives the current iteration count as well as
/// the next value.
///
/// The iterator returned yields pairs `(i, val)`, where `i` is the
/// current index of iteration and `val` is the value returned by the
/// iterator.
///
/// `enumerate()` keeps its count as a [`usize`]. If you want to count by a
/// different sized integer, the [`zip`] function provides similar
/// functionality.
///
/// # Overflow Behavior
///
/// The method does no guarding against overflows, so enumerating more than
/// [`usize::MAX`] elements either produces the wrong result or panics. If
/// debug assertions are enabled, a panic is guaranteed.
///
/// # Panics
///
/// The returned iterator might panic if the to-be-returned index would
/// overflow a [`usize`].
///
/// [`zip`]: Iterator::zip
///
/// # Examples
///
/// ```
/// let a = ['a', 'b', 'c'];
///
/// let mut iter = a.iter().enumerate();
///
/// assert_eq!(iter.next(), Some((0, &'a')));
/// assert_eq!(iter.next(), Some((1, &'b')));
/// assert_eq!(iter.next(), Some((2, &'c')));
/// assert_eq!(iter.next(), None);
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
fn enumerate(self) -> Enumerate<Self>
where
Self: Sized,
{
Enumerate::new(self)
}
/// Creates an iterator which can use the [`peek`] and [`peek_mut`] methods
/// to look at the next element of the iterator without consuming it. See
/// their documentation for more information.
///
/// Note that the underlying iterator is still advanced when [`peek`] or
/// [`peek_mut`] are called for the first time: In order to retrieve the
/// next element, [`next`] is called on the underlying iterator, hence any
/// side effects (i.e. anything other than fetching the next value) of
/// the [`next`] method will occur.
///
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let xs = [1, 2, 3];
///
/// let mut iter = xs.iter().peekable();
///
/// // peek() lets us see into the future
/// assert_eq!(iter.peek(), Some(&&1));
/// assert_eq!(iter.next(), Some(&1));
///
/// assert_eq!(iter.next(), Some(&2));
///
/// // we can peek() multiple times, the iterator won't advance
/// assert_eq!(iter.peek(), Some(&&3));
/// assert_eq!(iter.peek(), Some(&&3));
///
/// assert_eq!(iter.next(), Some(&3));
///
/// // after the iterator is finished, so is peek()
/// assert_eq!(iter.peek(), None);
/// assert_eq!(iter.next(), None);
/// ```
///
/// Using [`peek_mut`] to mutate the next item without advancing the
/// iterator:
///
/// ```
/// let xs = [1, 2, 3];
///
/// let mut iter = xs.iter().peekable();
///
/// // `peek_mut()` lets us see into the future
/// assert_eq!(iter.peek_mut(), Some(&mut &1));
/// assert_eq!(iter.peek_mut(), Some(&mut &1));
/// assert_eq!(iter.next(), Some(&1));
///
/// if let Some(mut p) = iter.peek_mut() {
/// assert_eq!(*p, &2);
/// // put a value into the iterator
/// *p = &1000;
/// }
///
/// // The value reappears as the iterator continues
/// assert_eq!(iter.collect::<Vec<_>>(), vec![&1000, &3]);
/// ```
/// [`peek`]: Peekable::peek
/// [`peek_mut`]: Peekable::peek_mut
/// [`next`]: Iterator::next
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
fn peekable(self) -> Peekable<Self>
where
Self: Sized,
{
Peekable::new(self)
}
/// Creates an iterator that [`skip`]s elements based on a predicate.
///
/// [`skip`]: Iterator::skip
///
/// `skip_while()` takes a closure as an argument. It will call this
/// closure on each element of the iterator, and ignore elements
/// until it returns `false`.
///
/// After `false` is returned, `skip_while()`'s job is over, and the
/// rest of the elements are yielded.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let a = [-1i32, 0, 1];
///
/// let mut iter = a.iter().skip_while(|x| x.is_negative());
///
/// assert_eq!(iter.next(), Some(&0));
/// assert_eq!(iter.next(), Some(&1));
/// assert_eq!(iter.next(), None);
/// ```
///
/// Because the closure passed to `skip_while()` takes a reference, and many
/// iterators iterate over references, this leads to a possibly confusing
/// situation, where the type of the closure argument is a double reference:
///
/// ```
/// let a = [-1, 0, 1];
///
/// let mut iter = a.iter().skip_while(|x| **x < 0); // need two *s!
///
/// assert_eq!(iter.next(), Some(&0));
/// assert_eq!(iter.next(), Some(&1));
/// assert_eq!(iter.next(), None);
/// ```
///
/// Stopping after an initial `false`:
///
/// ```
/// let a = [-1, 0, 1, -2];
///
/// let mut iter = a.iter().skip_while(|x| **x < 0);
///
/// assert_eq!(iter.next(), Some(&0));
/// assert_eq!(iter.next(), Some(&1));
///
/// // while this would have been false, since we already got a false,
/// // skip_while() isn't used any more
/// assert_eq!(iter.next(), Some(&-2));
///
/// assert_eq!(iter.next(), None);
/// ```
#[inline]
#[doc(alias = "drop_while")]
#[stable(feature = "rust1", since = "1.0.0")]
fn skip_while<P>(self, predicate: P) -> SkipWhile<Self, P>
where
Self: Sized,
P: FnMut(&Self::Item) -> bool,
{
SkipWhile::new(self, predicate)
}
/// Creates an iterator that yields elements based on a predicate.
///
/// `take_while()` takes a closure as an argument. It will call this
/// closure on each element of the iterator, and yield elements
/// while it returns `true`.
///
/// After `false` is returned, `take_while()`'s job is over, and the
/// rest of the elements are ignored.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let a = [-1i32, 0, 1];
///
/// let mut iter = a.iter().take_while(|x| x.is_negative());
///
/// assert_eq!(iter.next(), Some(&-1));
/// assert_eq!(iter.next(), None);
/// ```
///
/// Because the closure passed to `take_while()` takes a reference, and many
/// iterators iterate over references, this leads to a possibly confusing
/// situation, where the type of the closure is a double reference:
///
/// ```
/// let a = [-1, 0, 1];
///
/// let mut iter = a.iter().take_while(|x| **x < 0); // need two *s!
///
/// assert_eq!(iter.next(), Some(&-1));
/// assert_eq!(iter.next(), None);
/// ```
///
/// Stopping after an initial `false`:
///
/// ```
/// let a = [-1, 0, 1, -2];
///
/// let mut iter = a.iter().take_while(|x| **x < 0);
///
/// assert_eq!(iter.next(), Some(&-1));
///
/// // We have more elements that are less than zero, but since we already
/// // got a false, take_while() isn't used any more
/// assert_eq!(iter.next(), None);
/// ```
///
/// Because `take_while()` needs to look at the value in order to see if it
/// should be included or not, consuming iterators will see that it is
/// removed:
///
/// ```
/// let a = [1, 2, 3, 4];
/// let mut iter = a.iter();
///
/// let result: Vec<i32> = iter.by_ref()
/// .take_while(|n| **n != 3)
/// .cloned()
/// .collect();
///
/// assert_eq!(result, &[1, 2]);
///
/// let result: Vec<i32> = iter.cloned().collect();
///
/// assert_eq!(result, &[4]);
/// ```
///
/// The `3` is no longer there, because it was consumed in order to see if
/// the iteration should stop, but wasn't placed back into the iterator.
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
fn take_while<P>(self, predicate: P) -> TakeWhile<Self, P>
where
Self: Sized,
P: FnMut(&Self::Item) -> bool,
{
TakeWhile::new(self, predicate)
}
/// Creates an iterator that both yields elements based on a predicate and maps.
///
/// `map_while()` takes a closure as an argument. It will call this
/// closure on each element of the iterator, and yield elements
/// while it returns [`Some(_)`][`Some`].
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let a = [-1i32, 4, 0, 1];
///
/// let mut iter = a.iter().map_while(|x| 16i32.checked_div(*x));
///
/// assert_eq!(iter.next(), Some(-16));
/// assert_eq!(iter.next(), Some(4));
/// assert_eq!(iter.next(), None);
/// ```
///
/// Here's the same example, but with [`take_while`] and [`map`]:
///
/// [`take_while`]: Iterator::take_while
/// [`map`]: Iterator::map
///
/// ```
/// let a = [-1i32, 4, 0, 1];
///
/// let mut iter = a.iter()
/// .map(|x| 16i32.checked_div(*x))
/// .take_while(|x| x.is_some())
/// .map(|x| x.unwrap());
///
/// assert_eq!(iter.next(), Some(-16));
/// assert_eq!(iter.next(), Some(4));
/// assert_eq!(iter.next(), None);
/// ```
///
/// Stopping after an initial [`None`]:
///
/// ```
/// let a = [0, 1, 2, -3, 4, 5, -6];
///
/// let iter = a.iter().map_while(|x| u32::try_from(*x).ok());
/// let vec = iter.collect::<Vec<_>>();
///
/// // We have more elements which could fit in u32 (4, 5), but `map_while` returned `None` for `-3`
/// // (as the `predicate` returned `None`) and `collect` stops at the first `None` encountered.
/// assert_eq!(vec, vec![0, 1, 2]);
/// ```
///
/// Because `map_while()` needs to look at the value in order to see if it
/// should be included or not, consuming iterators will see that it is
/// removed:
///
/// ```
/// let a = [1, 2, -3, 4];
/// let mut iter = a.iter();
///
/// let result: Vec<u32> = iter.by_ref()
/// .map_while(|n| u32::try_from(*n).ok())
/// .collect();
///
/// assert_eq!(result, &[1, 2]);
///
/// let result: Vec<i32> = iter.cloned().collect();
///
/// assert_eq!(result, &[4]);
/// ```
///
/// The `-3` is no longer there, because it was consumed in order to see if
/// the iteration should stop, but wasn't placed back into the iterator.
///
/// Note that unlike [`take_while`] this iterator is **not** fused.
/// It is also not specified what this iterator returns after the first [`None`] is returned.
/// If you need fused iterator, use [`fuse`].
///
/// [`fuse`]: Iterator::fuse
#[inline]
#[stable(feature = "iter_map_while", since = "1.57.0")]
fn map_while<B, P>(self, predicate: P) -> MapWhile<Self, P>
where
Self: Sized,
P: FnMut(Self::Item) -> Option<B>,
{
MapWhile::new(self, predicate)
}
/// Creates an iterator that skips the first `n` elements.
///
/// `skip(n)` skips elements until `n` elements are skipped or the end of the
/// iterator is reached (whichever happens first). After that, all the remaining
/// elements are yielded. In particular, if the original iterator is too short,
/// then the returned iterator is empty.
///
/// Rather than overriding this method directly, instead override the `nth` method.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let a = [1, 2, 3];
///
/// let mut iter = a.iter().skip(2);
///
/// assert_eq!(iter.next(), Some(&3));
/// assert_eq!(iter.next(), None);
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
fn skip(self, n: usize) -> Skip<Self>
where
Self: Sized,
{
Skip::new(self, n)
}
/// Creates an iterator that yields the first `n` elements, or fewer
/// if the underlying iterator ends sooner.
///
/// `take(n)` yields elements until `n` elements are yielded or the end of
/// the iterator is reached (whichever happens first).
/// The returned iterator is a prefix of length `n` if the original iterator
/// contains at least `n` elements, otherwise it contains all of the
/// (fewer than `n`) elements of the original iterator.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let a = [1, 2, 3];
///
/// let mut iter = a.iter().take(2);
///
/// assert_eq!(iter.next(), Some(&1));
/// assert_eq!(iter.next(), Some(&2));
/// assert_eq!(iter.next(), None);
/// ```
///
/// `take()` is often used with an infinite iterator, to make it finite:
///
/// ```
/// let mut iter = (0..).take(3);
///
/// assert_eq!(iter.next(), Some(0));
/// assert_eq!(iter.next(), Some(1));
/// assert_eq!(iter.next(), Some(2));
/// assert_eq!(iter.next(), None);
/// ```
///
/// If less than `n` elements are available,
/// `take` will limit itself to the size of the underlying iterator:
///
/// ```
/// let v = [1, 2];
/// let mut iter = v.into_iter().take(5);
/// assert_eq!(iter.next(), Some(1));
/// assert_eq!(iter.next(), Some(2));
/// assert_eq!(iter.next(), None);
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
fn take(self, n: usize) -> Take<Self>
where
Self: Sized,
{
Take::new(self, n)
}
/// An iterator adapter similar to [`fold`] that holds internal state and
/// produces a new iterator.
///
/// [`fold`]: Iterator::fold
///
/// `scan()` takes two arguments: an initial value which seeds the internal
/// state, and a closure with two arguments, the first being a mutable
/// reference to the internal state and the second an iterator element.
/// The closure can assign to the internal state to share state between
/// iterations.
///
/// On iteration, the closure will be applied to each element of the
/// iterator and the return value from the closure, an [`Option`], is
/// yielded by the iterator.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let a = [1, 2, 3];
///
/// let mut iter = a.iter().scan(1, |state, &x| {
/// // each iteration, we'll multiply the state by the element
/// *state = *state * x;
///
/// // then, we'll yield the negation of the state
/// Some(-*state)
/// });
///
/// assert_eq!(iter.next(), Some(-1));
/// assert_eq!(iter.next(), Some(-2));
/// assert_eq!(iter.next(), Some(-6));
/// assert_eq!(iter.next(), None);
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
fn scan<St, B, F>(self, initial_state: St, f: F) -> Scan<Self, St, F>
where
Self: Sized,
F: FnMut(&mut St, Self::Item) -> Option<B>,
{
Scan::new(self, initial_state, f)
}
/// Creates an iterator that works like map, but flattens nested structure.
///
/// The [`map`] adapter is very useful, but only when the closure
/// argument produces values. If it produces an iterator instead, there's
/// an extra layer of indirection. `flat_map()` will remove this extra layer
/// on its own.
///
/// You can think of `flat_map(f)` as the semantic equivalent
/// of [`map`]ping, and then [`flatten`]ing as in `map(f).flatten()`.
///
/// Another way of thinking about `flat_map()`: [`map`]'s closure returns
/// one item for each element, and `flat_map()`'s closure returns an
/// iterator for each element.
///
/// [`map`]: Iterator::map
/// [`flatten`]: Iterator::flatten
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let words = ["alpha", "beta", "gamma"];
///
/// // chars() returns an iterator
/// let merged: String = words.iter()
/// .flat_map(|s| s.chars())
/// .collect();
/// assert_eq!(merged, "alphabetagamma");
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
fn flat_map<U, F>(self, f: F) -> FlatMap<Self, U, F>
where
Self: Sized,
U: IntoIterator,
F: FnMut(Self::Item) -> U,
{
FlatMap::new(self, f)
}
/// Creates an iterator that flattens nested structure.
///
/// This is useful when you have an iterator of iterators or an iterator of
/// things that can be turned into iterators and you want to remove one
/// level of indirection.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let data = vec![vec![1, 2, 3, 4], vec![5, 6]];
/// let flattened = data.into_iter().flatten().collect::<Vec<u8>>();
/// assert_eq!(flattened, &[1, 2, 3, 4, 5, 6]);
/// ```
///
/// Mapping and then flattening:
///
/// ```
/// let words = ["alpha", "beta", "gamma"];
///
/// // chars() returns an iterator
/// let merged: String = words.iter()
/// .map(|s| s.chars())
/// .flatten()
/// .collect();
/// assert_eq!(merged, "alphabetagamma");
/// ```
///
/// You can also rewrite this in terms of [`flat_map()`], which is preferable
/// in this case since it conveys intent more clearly:
///
/// ```
/// let words = ["alpha", "beta", "gamma"];
///
/// // chars() returns an iterator
/// let merged: String = words.iter()
/// .flat_map(|s| s.chars())
/// .collect();
/// assert_eq!(merged, "alphabetagamma");
/// ```
///
/// Flattening only removes one level of nesting at a time:
///
/// ```
/// let d3 = [[[1, 2], [3, 4]], [[5, 6], [7, 8]]];
///
/// let d2 = d3.iter().flatten().collect::<Vec<_>>();
/// assert_eq!(d2, [&[1, 2], &[3, 4], &[5, 6], &[7, 8]]);
///
/// let d1 = d3.iter().flatten().flatten().collect::<Vec<_>>();
/// assert_eq!(d1, [&1, &2, &3, &4, &5, &6, &7, &8]);
/// ```
///
/// Here we see that `flatten()` does not perform a "deep" flatten.
/// Instead, only one level of nesting is removed. That is, if you
/// `flatten()` a three-dimensional array, the result will be
/// two-dimensional and not one-dimensional. To get a one-dimensional
/// structure, you have to `flatten()` again.
///
/// [`flat_map()`]: Iterator::flat_map
#[inline]
#[stable(feature = "iterator_flatten", since = "1.29.0")]
fn flatten(self) -> Flatten<Self>
where
Self: Sized,
Self::Item: IntoIterator,
{
Flatten::new(self)
}
/// Creates an iterator which ends after the first [`None`].
///
/// After an iterator returns [`None`], future calls may or may not yield
/// [`Some(T)`] again. `fuse()` adapts an iterator, ensuring that after a
/// [`None`] is given, it will always return [`None`] forever.
///
/// Note that the [`Fuse`] wrapper is a no-op on iterators that implement
/// the [`FusedIterator`] trait. `fuse()` may therefore behave incorrectly
/// if the [`FusedIterator`] trait is improperly implemented.
///
/// [`Some(T)`]: Some
/// [`FusedIterator`]: crate::iter::FusedIterator
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// // an iterator which alternates between Some and None
/// struct Alternate {
/// state: i32,
/// }
///
/// impl Iterator for Alternate {
/// type Item = i32;
///
/// fn next(&mut self) -> Option<i32> {
/// let val = self.state;
/// self.state = self.state + 1;
///
/// // if it's even, Some(i32), else None
/// if val % 2 == 0 {
/// Some(val)
/// } else {
/// None
/// }
/// }
/// }
///
/// let mut iter = Alternate { state: 0 };
///
/// // we can see our iterator going back and forth
/// assert_eq!(iter.next(), Some(0));
/// assert_eq!(iter.next(), None);
/// assert_eq!(iter.next(), Some(2));
/// assert_eq!(iter.next(), None);
///
/// // however, once we fuse it...
/// let mut iter = iter.fuse();
///
/// assert_eq!(iter.next(), Some(4));
/// assert_eq!(iter.next(), None);
///
/// // it will always return `None` after the first time.
/// assert_eq!(iter.next(), None);
/// assert_eq!(iter.next(), None);
/// assert_eq!(iter.next(), None);
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
fn fuse(self) -> Fuse<Self>
where
Self: Sized,
{
Fuse::new(self)
}
/// Does something with each element of an iterator, passing the value on.
///
/// When using iterators, you'll often chain several of them together.
/// While working on such code, you might want to check out what's
/// happening at various parts in the pipeline. To do that, insert
/// a call to `inspect()`.
///
/// It's more common for `inspect()` to be used as a debugging tool than to
/// exist in your final code, but applications may find it useful in certain
/// situations when errors need to be logged before being discarded.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let a = [1, 4, 2, 3];
///
/// // this iterator sequence is complex.
/// let sum = a.iter()
/// .cloned()
/// .filter(|x| x % 2 == 0)
/// .fold(0, |sum, i| sum + i);
///
/// println!("{sum}");
///
/// // let's add some inspect() calls to investigate what's happening
/// let sum = a.iter()
/// .cloned()
/// .inspect(|x| println!("about to filter: {x}"))
/// .filter(|x| x % 2 == 0)
/// .inspect(|x| println!("made it through filter: {x}"))
/// .fold(0, |sum, i| sum + i);
///
/// println!("{sum}");
/// ```
///
/// This will print:
///
/// ```text
/// 6
/// about to filter: 1
/// about to filter: 4
/// made it through filter: 4
/// about to filter: 2
/// made it through filter: 2
/// about to filter: 3
/// 6
/// ```
///
/// Logging errors before discarding them:
///
/// ```
/// let lines = ["1", "2", "a"];
///
/// let sum: i32 = lines
/// .iter()
/// .map(|line| line.parse::<i32>())
/// .inspect(|num| {
/// if let Err(ref e) = *num {
/// println!("Parsing error: {e}");
/// }
/// })
/// .filter_map(Result::ok)
/// .sum();
///
/// println!("Sum: {sum}");
/// ```
///
/// This will print:
///
/// ```text
/// Parsing error: invalid digit found in string
/// Sum: 3
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
fn inspect<F>(self, f: F) -> Inspect<Self, F>
where
Self: Sized,
F: FnMut(&Self::Item),
{
Inspect::new(self, f)
}
/// Borrows an iterator, rather than consuming it.
///
/// This is useful to allow applying iterator adapters while still
/// retaining ownership of the original iterator.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let mut words = ["hello", "world", "of", "Rust"].into_iter();
///
/// // Take the first two words.
/// let hello_world: Vec<_> = words.by_ref().take(2).collect();
/// assert_eq!(hello_world, vec!["hello", "world"]);
///
/// // Collect the rest of the words.
/// // We can only do this because we used `by_ref` earlier.
/// let of_rust: Vec<_> = words.collect();
/// assert_eq!(of_rust, vec!["of", "Rust"]);
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
fn by_ref(&mut self) -> &mut Self
where
Self: Sized,
{
self
}
/// Transforms an iterator into a collection.
///
/// `collect()` can take anything iterable, and turn it into a relevant
/// collection. This is one of the more powerful methods in the standard
/// library, used in a variety of contexts.
///
/// The most basic pattern in which `collect()` is used is to turn one
/// collection into another. You take a collection, call [`iter`] on it,
/// do a bunch of transformations, and then `collect()` at the end.
///
/// `collect()` can also create instances of types that are not typical
/// collections. For example, a [`String`] can be built from [`char`]s,
/// and an iterator of [`Result<T, E>`][`Result`] items can be collected
/// into `Result<Collection<T>, E>`. See the examples below for more.
///
/// Because `collect()` is so general, it can cause problems with type
/// inference. As such, `collect()` is one of the few times you'll see
/// the syntax affectionately known as the 'turbofish': `::<>`. This
/// helps the inference algorithm understand specifically which collection
/// you're trying to collect into.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let a = [1, 2, 3];
///
/// let doubled: Vec<i32> = a.iter()
/// .map(|&x| x * 2)
/// .collect();
///
/// assert_eq!(vec![2, 4, 6], doubled);
/// ```
///
/// Note that we needed the `: Vec<i32>` on the left-hand side. This is because
/// we could collect into, for example, a [`VecDeque<T>`] instead:
///
/// [`VecDeque<T>`]: ../../std/collections/struct.VecDeque.html
///
/// ```
/// use std::collections::VecDeque;
///
/// let a = [1, 2, 3];
///
/// let doubled: VecDeque<i32> = a.iter().map(|&x| x * 2).collect();
///
/// assert_eq!(2, doubled[0]);
/// assert_eq!(4, doubled[1]);
/// assert_eq!(6, doubled[2]);
/// ```
///
/// Using the 'turbofish' instead of annotating `doubled`:
///
/// ```
/// let a = [1, 2, 3];
///
/// let doubled = a.iter().map(|x| x * 2).collect::<Vec<i32>>();
///
/// assert_eq!(vec![2, 4, 6], doubled);
/// ```
///
/// Because `collect()` only cares about what you're collecting into, you can
/// still use a partial type hint, `_`, with the turbofish:
///
/// ```
/// let a = [1, 2, 3];
///
/// let doubled = a.iter().map(|x| x * 2).collect::<Vec<_>>();
///
/// assert_eq!(vec![2, 4, 6], doubled);
/// ```
///
/// Using `collect()` to make a [`String`]:
///
/// ```
/// let chars = ['g', 'd', 'k', 'k', 'n'];
///
/// let hello: String = chars.iter()
/// .map(|&x| x as u8)
/// .map(|x| (x + 1) as char)
/// .collect();
///
/// assert_eq!("hello", hello);
/// ```
///
/// If you have a list of [`Result<T, E>`][`Result`]s, you can use `collect()` to
/// see if any of them failed:
///
/// ```
/// let results = [Ok(1), Err("nope"), Ok(3), Err("bad")];
///
/// let result: Result<Vec<_>, &str> = results.iter().cloned().collect();
///
/// // gives us the first error
/// assert_eq!(Err("nope"), result);
///
/// let results = [Ok(1), Ok(3)];
///
/// let result: Result<Vec<_>, &str> = results.iter().cloned().collect();
///
/// // gives us the list of answers
/// assert_eq!(Ok(vec![1, 3]), result);
/// ```
///
/// [`iter`]: Iterator::next
/// [`String`]: ../../std/string/struct.String.html
/// [`char`]: type@char
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
#[must_use = "if you really need to exhaust the iterator, consider `.for_each(drop)` instead"]
fn collect<B: FromIterator<Self::Item>>(self) -> B
where
Self: Sized,
{
FromIterator::from_iter(self)
}
/// Fallibly transforms an iterator into a collection, short circuiting if
/// a failure is encountered.
///
/// `try_collect()` is a variation of [`collect()`][`collect`] that allows fallible
/// conversions during collection. Its main use case is simplifying conversions from
/// iterators yielding [`Option<T>`][`Option`] into `Option<Collection<T>>`, or similarly for other [`Try`]
/// types (e.g. [`Result`]).
///
/// Importantly, `try_collect()` doesn't require that the outer [`Try`] type also implements [`FromIterator`];
/// only the inner type produced on `Try::Output` must implement it. Concretely,
/// this means that collecting into `ControlFlow<_, Vec<i32>>` is valid because `Vec<i32>` implements
/// [`FromIterator`], even though [`ControlFlow`] doesn't.
///
/// Also, if a failure is encountered during `try_collect()`, the iterator is still valid and
/// may continue to be used, in which case it will continue iterating starting after the element that
/// triggered the failure. See the last example below for an example of how this works.
///
/// # Examples
/// Successfully collecting an iterator of `Option<i32>` into `Option<Vec<i32>>`:
/// ```
/// #![feature(iterator_try_collect)]
///
/// let u = vec![Some(1), Some(2), Some(3)];
/// let v = u.into_iter().try_collect::<Vec<i32>>();
/// assert_eq!(v, Some(vec![1, 2, 3]));
/// ```
///
/// Failing to collect in the same way:
/// ```
/// #![feature(iterator_try_collect)]
///
/// let u = vec![Some(1), Some(2), None, Some(3)];
/// let v = u.into_iter().try_collect::<Vec<i32>>();
/// assert_eq!(v, None);
/// ```
///
/// A similar example, but with `Result`:
/// ```
/// #![feature(iterator_try_collect)]
///
/// let u: Vec<Result<i32, ()>> = vec![Ok(1), Ok(2), Ok(3)];
/// let v = u.into_iter().try_collect::<Vec<i32>>();
/// assert_eq!(v, Ok(vec![1, 2, 3]));
///
/// let u = vec![Ok(1), Ok(2), Err(()), Ok(3)];
/// let v = u.into_iter().try_collect::<Vec<i32>>();
/// assert_eq!(v, Err(()));
/// ```
///
/// Finally, even [`ControlFlow`] works, despite the fact that it
/// doesn't implement [`FromIterator`]. Note also that the iterator can
/// continue to be used, even if a failure is encountered:
///
/// ```
/// #![feature(iterator_try_collect)]
///
/// use core::ops::ControlFlow::{Break, Continue};
///
/// let u = [Continue(1), Continue(2), Break(3), Continue(4), Continue(5)];
/// let mut it = u.into_iter();
///
/// let v = it.try_collect::<Vec<_>>();
/// assert_eq!(v, Break(3));
///
/// let v = it.try_collect::<Vec<_>>();
/// assert_eq!(v, Continue(vec![4, 5]));
/// ```
///
/// [`collect`]: Iterator::collect
#[inline]
#[unstable(feature = "iterator_try_collect", issue = "94047")]
fn try_collect<B>(&mut self) -> ChangeOutputType<Self::Item, B>
where
Self: Sized,
<Self as Iterator>::Item: Try,
<<Self as Iterator>::Item as Try>::Residual: Residual<B>,
B: FromIterator<<Self::Item as Try>::Output>,
{
try_process(ByRefSized(self), |i| i.collect())
}
/// Collects all the items from an iterator into a collection.
///
/// This method consumes the iterator and adds all its items to the
/// passed collection. The collection is then returned, so the call chain
/// can be continued.
///
/// This is useful when you already have a collection and wants to add
/// the iterator items to it.
///
/// This method is a convenience method to call [Extend::extend](trait.Extend.html),
/// but instead of being called on a collection, it's called on an iterator.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// #![feature(iter_collect_into)]
///
/// let a = [1, 2, 3];
/// let mut vec: Vec::<i32> = vec![0, 1];
///
/// a.iter().map(|&x| x * 2).collect_into(&mut vec);
/// a.iter().map(|&x| x * 10).collect_into(&mut vec);
///
/// assert_eq!(vec![0, 1, 2, 4, 6, 10, 20, 30], vec);
/// ```
///
/// `Vec` can have a manual set capacity to avoid reallocating it:
///
/// ```
/// #![feature(iter_collect_into)]
///
/// let a = [1, 2, 3];
/// let mut vec: Vec::<i32> = Vec::with_capacity(6);
///
/// a.iter().map(|&x| x * 2).collect_into(&mut vec);
/// a.iter().map(|&x| x * 10).collect_into(&mut vec);
///
/// assert_eq!(6, vec.capacity());
/// println!("{:?}", vec);
/// ```
///
/// The returned mutable reference can be used to continue the call chain:
///
/// ```
/// #![feature(iter_collect_into)]
///
/// let a = [1, 2, 3];
/// let mut vec: Vec::<i32> = Vec::with_capacity(6);
///
/// let count = a.iter().collect_into(&mut vec).iter().count();
///
/// assert_eq!(count, vec.len());
/// println!("Vec len is {}", count);
///
/// let count = a.iter().collect_into(&mut vec).iter().count();
///
/// assert_eq!(count, vec.len());
/// println!("Vec len now is {}", count);
/// ```
#[inline]
#[unstable(feature = "iter_collect_into", reason = "new API", issue = "94780")]
fn collect_into<E: Extend<Self::Item>>(self, collection: &mut E) -> &mut E
where
Self: Sized,
{
collection.extend(self);
collection
}
/// Consumes an iterator, creating two collections from it.
///
/// The predicate passed to `partition()` can return `true`, or `false`.
/// `partition()` returns a pair, all of the elements for which it returned
/// `true`, and all of the elements for which it returned `false`.
///
/// See also [`is_partitioned()`] and [`partition_in_place()`].
///
/// [`is_partitioned()`]: Iterator::is_partitioned
/// [`partition_in_place()`]: Iterator::partition_in_place
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let a = [1, 2, 3];
///
/// let (even, odd): (Vec<_>, Vec<_>) = a
/// .into_iter()
/// .partition(|n| n % 2 == 0);
///
/// assert_eq!(even, vec![2]);
/// assert_eq!(odd, vec![1, 3]);
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
fn partition<B, F>(self, f: F) -> (B, B)
where
Self: Sized,
B: Default + Extend<Self::Item>,
F: FnMut(&Self::Item) -> bool,
{
#[inline]
fn extend<'a, T, B: Extend<T>>(
mut f: impl FnMut(&T) -> bool + 'a,
left: &'a mut B,
right: &'a mut B,
) -> impl FnMut((), T) + 'a {
move |(), x| {
if f(&x) {
left.extend_one(x);
} else {
right.extend_one(x);
}
}
}
let mut left: B = Default::default();
let mut right: B = Default::default();
self.fold((), extend(f, &mut left, &mut right));
(left, right)
}
/// Reorders the elements of this iterator *in-place* according to the given predicate,
/// such that all those that return `true` precede all those that return `false`.
/// Returns the number of `true` elements found.
///
/// The relative order of partitioned items is not maintained.
///
/// # Current implementation
///
/// Current algorithms tries finding the first element for which the predicate evaluates
/// to false, and the last element for which it evaluates to true and repeatedly swaps them.
///
/// Time complexity: *O*(*n*)
///
/// See also [`is_partitioned()`] and [`partition()`].
///
/// [`is_partitioned()`]: Iterator::is_partitioned
/// [`partition()`]: Iterator::partition
///
/// # Examples
///
/// ```
/// #![feature(iter_partition_in_place)]
///
/// let mut a = [1, 2, 3, 4, 5, 6, 7];
///
/// // Partition in-place between evens and odds
/// let i = a.iter_mut().partition_in_place(|&n| n % 2 == 0);
///
/// assert_eq!(i, 3);
/// assert!(a[..i].iter().all(|&n| n % 2 == 0)); // evens
/// assert!(a[i..].iter().all(|&n| n % 2 == 1)); // odds
/// ```
#[unstable(feature = "iter_partition_in_place", reason = "new API", issue = "62543")]
fn partition_in_place<'a, T: 'a, P>(mut self, ref mut predicate: P) -> usize
where
Self: Sized + DoubleEndedIterator<Item = &'a mut T>,
P: FnMut(&T) -> bool,
{
// FIXME: should we worry about the count overflowing? The only way to have more than
// `usize::MAX` mutable references is with ZSTs, which aren't useful to partition...
// These closure "factory" functions exist to avoid genericity in `Self`.
#[inline]
fn is_false<'a, T>(
predicate: &'a mut impl FnMut(&T) -> bool,
true_count: &'a mut usize,
) -> impl FnMut(&&mut T) -> bool + 'a {
move |x| {
let p = predicate(&**x);
*true_count += p as usize;
!p
}
}
#[inline]
fn is_true<T>(predicate: &mut impl FnMut(&T) -> bool) -> impl FnMut(&&mut T) -> bool + '_ {
move |x| predicate(&**x)
}
// Repeatedly find the first `false` and swap it with the last `true`.
let mut true_count = 0;
while let Some(head) = self.find(is_false(predicate, &mut true_count)) {
if let Some(tail) = self.rfind(is_true(predicate)) {
crate::mem::swap(head, tail);
true_count += 1;
} else {
break;
}
}
true_count
}
/// Checks if the elements of this iterator are partitioned according to the given predicate,
/// such that all those that return `true` precede all those that return `false`.
///
/// See also [`partition()`] and [`partition_in_place()`].
///
/// [`partition()`]: Iterator::partition
/// [`partition_in_place()`]: Iterator::partition_in_place
///
/// # Examples
///
/// ```
/// #![feature(iter_is_partitioned)]
///
/// assert!("Iterator".chars().is_partitioned(char::is_uppercase));
/// assert!(!"IntoIterator".chars().is_partitioned(char::is_uppercase));
/// ```
#[unstable(feature = "iter_is_partitioned", reason = "new API", issue = "62544")]
fn is_partitioned<P>(mut self, mut predicate: P) -> bool
where
Self: Sized,
P: FnMut(Self::Item) -> bool,
{
// Either all items test `true`, or the first clause stops at `false`
// and we check that there are no more `true` items after that.
self.all(&mut predicate) || !self.any(predicate)
}
/// An iterator method that applies a function as long as it returns
/// successfully, producing a single, final value.
///
/// `try_fold()` takes two arguments: an initial value, and a closure with
/// two arguments: an 'accumulator', and an element. The closure either
/// returns successfully, with the value that the accumulator should have
/// for the next iteration, or it returns failure, with an error value that
/// is propagated back to the caller immediately (short-circuiting).
///
/// The initial value is the value the accumulator will have on the first
/// call. If applying the closure succeeded against every element of the
/// iterator, `try_fold()` returns the final accumulator as success.
///
/// Folding is useful whenever you have a collection of something, and want
/// to produce a single value from it.
///
/// # Note to Implementors
///
/// Several of the other (forward) methods have default implementations in
/// terms of this one, so try to implement this explicitly if it can
/// do something better than the default `for` loop implementation.
///
/// In particular, try to have this call `try_fold()` on the internal parts
/// from which this iterator is composed. If multiple calls are needed,
/// the `?` operator may be convenient for chaining the accumulator value
/// along, but beware any invariants that need to be upheld before those
/// early returns. This is a `&mut self` method, so iteration needs to be
/// resumable after hitting an error here.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let a = [1, 2, 3];
///
/// // the checked sum of all of the elements of the array
/// let sum = a.iter().try_fold(0i8, |acc, &x| acc.checked_add(x));
///
/// assert_eq!(sum, Some(6));
/// ```
///
/// Short-circuiting:
///
/// ```
/// let a = [10, 20, 30, 100, 40, 50];
/// let mut it = a.iter();
///
/// // This sum overflows when adding the 100 element
/// let sum = it.try_fold(0i8, |acc, &x| acc.checked_add(x));
/// assert_eq!(sum, None);
///
/// // Because it short-circuited, the remaining elements are still
/// // available through the iterator.
/// assert_eq!(it.len(), 2);
/// assert_eq!(it.next(), Some(&40));
/// ```
///
/// While you cannot `break` from a closure, the [`ControlFlow`] type allows
/// a similar idea:
///
/// ```
/// use std::ops::ControlFlow;
///
/// let triangular = (1..30).try_fold(0_i8, |prev, x| {
/// if let Some(next) = prev.checked_add(x) {
/// ControlFlow::Continue(next)
/// } else {
/// ControlFlow::Break(prev)
/// }
/// });
/// assert_eq!(triangular, ControlFlow::Break(120));
///
/// let triangular = (1..30).try_fold(0_u64, |prev, x| {
/// if let Some(next) = prev.checked_add(x) {
/// ControlFlow::Continue(next)
/// } else {
/// ControlFlow::Break(prev)
/// }
/// });
/// assert_eq!(triangular, ControlFlow::Continue(435));
/// ```
#[inline]
#[stable(feature = "iterator_try_fold", since = "1.27.0")]
fn try_fold<B, F, R>(&mut self, init: B, mut f: F) -> R
where
Self: Sized,
F: FnMut(B, Self::Item) -> R,
R: Try<Output = B>,
{
let mut accum = init;
while let Some(x) = self.next() {
accum = f(accum, x)?;
}
try { accum }
}
/// An iterator method that applies a fallible function to each item in the
/// iterator, stopping at the first error and returning that error.
///
/// This can also be thought of as the fallible form of [`for_each()`]
/// or as the stateless version of [`try_fold()`].
///
/// [`for_each()`]: Iterator::for_each
/// [`try_fold()`]: Iterator::try_fold
///
/// # Examples
///
/// ```
/// use std::fs::rename;
/// use std::io::{stdout, Write};
/// use std::path::Path;
///
/// let data = ["no_tea.txt", "stale_bread.json", "torrential_rain.png"];
///
/// let res = data.iter().try_for_each(|x| writeln!(stdout(), "{x}"));
/// assert!(res.is_ok());
///
/// let mut it = data.iter().cloned();
/// let res = it.try_for_each(|x| rename(x, Path::new(x).with_extension("old")));
/// assert!(res.is_err());
/// // It short-circuited, so the remaining items are still in the iterator:
/// assert_eq!(it.next(), Some("stale_bread.json"));
/// ```
///
/// The [`ControlFlow`] type can be used with this method for the situations
/// in which you'd use `break` and `continue` in a normal loop:
///
/// ```
/// use std::ops::ControlFlow;
///
/// let r = (2..100).try_for_each(|x| {
/// if 323 % x == 0 {
/// return ControlFlow::Break(x)
/// }
///
/// ControlFlow::Continue(())
/// });
/// assert_eq!(r, ControlFlow::Break(17));
/// ```
#[inline]
#[stable(feature = "iterator_try_fold", since = "1.27.0")]
fn try_for_each<F, R>(&mut self, f: F) -> R
where
Self: Sized,
F: FnMut(Self::Item) -> R,
R: Try<Output = ()>,
{
#[inline]
fn call<T, R>(mut f: impl FnMut(T) -> R) -> impl FnMut((), T) -> R {
move |(), x| f(x)
}
self.try_fold((), call(f))
}
/// Folds every element into an accumulator by applying an operation,
/// returning the final result.
///
/// `fold()` takes two arguments: an initial value, and a closure with two
/// arguments: an 'accumulator', and an element. The closure returns the value that
/// the accumulator should have for the next iteration.
///
/// The initial value is the value the accumulator will have on the first
/// call.
///
/// After applying this closure to every element of the iterator, `fold()`
/// returns the accumulator.
///
/// This operation is sometimes called 'reduce' or 'inject'.
///
/// Folding is useful whenever you have a collection of something, and want
/// to produce a single value from it.
///
/// Note: `fold()`, and similar methods that traverse the entire iterator,
/// might not terminate for infinite iterators, even on traits for which a
/// result is determinable in finite time.
///
/// Note: [`reduce()`] can be used to use the first element as the initial
/// value, if the accumulator type and item type is the same.
///
/// Note: `fold()` combines elements in a *left-associative* fashion. For associative
/// operators like `+`, the order the elements are combined in is not important, but for non-associative
/// operators like `-` the order will affect the final result.
/// For a *right-associative* version of `fold()`, see [`DoubleEndedIterator::rfold()`].
///
/// # Note to Implementors
///
/// Several of the other (forward) methods have default implementations in
/// terms of this one, so try to implement this explicitly if it can
/// do something better than the default `for` loop implementation.
///
/// In particular, try to have this call `fold()` on the internal parts
/// from which this iterator is composed.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let a = [1, 2, 3];
///
/// // the sum of all of the elements of the array
/// let sum = a.iter().fold(0, |acc, x| acc + x);
///
/// assert_eq!(sum, 6);
/// ```
///
/// Let's walk through each step of the iteration here:
///
/// | element | acc | x | result |
/// |---------|-----|---|--------|
/// | | 0 | | |
/// | 1 | 0 | 1 | 1 |
/// | 2 | 1 | 2 | 3 |
/// | 3 | 3 | 3 | 6 |
///
/// And so, our final result, `6`.
///
/// This example demonstrates the left-associative nature of `fold()`:
/// it builds a string, starting with an initial value
/// and continuing with each element from the front until the back:
///
/// ```
/// let numbers = [1, 2, 3, 4, 5];
///
/// let zero = "0".to_string();
///
/// let result = numbers.iter().fold(zero, |acc, &x| {
/// format!("({acc} + {x})")
/// });
///
/// assert_eq!(result, "(((((0 + 1) + 2) + 3) + 4) + 5)");
/// ```
/// It's common for people who haven't used iterators a lot to
/// use a `for` loop with a list of things to build up a result. Those
/// can be turned into `fold()`s:
///
/// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
///
/// ```
/// let numbers = [1, 2, 3, 4, 5];
///
/// let mut result = 0;
///
/// // for loop:
/// for i in &numbers {
/// result = result + i;
/// }
///
/// // fold:
/// let result2 = numbers.iter().fold(0, |acc, &x| acc + x);
///
/// // they're the same
/// assert_eq!(result, result2);
/// ```
///
/// [`reduce()`]: Iterator::reduce
#[doc(alias = "inject", alias = "foldl")]
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
fn fold<B, F>(mut self, init: B, mut f: F) -> B
where
Self: Sized,
F: FnMut(B, Self::Item) -> B,
{
let mut accum = init;
while let Some(x) = self.next() {
accum = f(accum, x);
}
accum
}
/// Reduces the elements to a single one, by repeatedly applying a reducing
/// operation.
///
/// If the iterator is empty, returns [`None`]; otherwise, returns the
/// result of the reduction.
///
/// The reducing function is a closure with two arguments: an 'accumulator', and an element.
/// For iterators with at least one element, this is the same as [`fold()`]
/// with the first element of the iterator as the initial accumulator value, folding
/// every subsequent element into it.
///
/// [`fold()`]: Iterator::fold
///
/// # Example
///
/// ```
/// let reduced: i32 = (1..10).reduce(|acc, e| acc + e).unwrap();
/// assert_eq!(reduced, 45);
///
/// // Which is equivalent to doing it with `fold`:
/// let folded: i32 = (1..10).fold(0, |acc, e| acc + e);
/// assert_eq!(reduced, folded);
/// ```
#[inline]
#[stable(feature = "iterator_fold_self", since = "1.51.0")]
fn reduce<F>(mut self, f: F) -> Option<Self::Item>
where
Self: Sized,
F: FnMut(Self::Item, Self::Item) -> Self::Item,
{
let first = self.next()?;
Some(self.fold(first, f))
}
/// Reduces the elements to a single one by repeatedly applying a reducing operation. If the
/// closure returns a failure, the failure is propagated back to the caller immediately.
///
/// The return type of this method depends on the return type of the closure. If the closure
/// returns `Result<Self::Item, E>`, then this function will return `Result<Option<Self::Item>,
/// E>`. If the closure returns `Option<Self::Item>`, then this function will return
/// `Option<Option<Self::Item>>`.
///
/// When called on an empty iterator, this function will return either `Some(None)` or
/// `Ok(None)` depending on the type of the provided closure.
///
/// For iterators with at least one element, this is essentially the same as calling
/// [`try_fold()`] with the first element of the iterator as the initial accumulator value.
///
/// [`try_fold()`]: Iterator::try_fold
///
/// # Examples
///
/// Safely calculate the sum of a series of numbers:
///
/// ```
/// #![feature(iterator_try_reduce)]
///
/// let numbers: Vec<usize> = vec![10, 20, 5, 23, 0];
/// let sum = numbers.into_iter().try_reduce(|x, y| x.checked_add(y));
/// assert_eq!(sum, Some(Some(58)));
/// ```
///
/// Determine when a reduction short circuited:
///
/// ```
/// #![feature(iterator_try_reduce)]
///
/// let numbers = vec![1, 2, 3, usize::MAX, 4, 5];
/// let sum = numbers.into_iter().try_reduce(|x, y| x.checked_add(y));
/// assert_eq!(sum, None);
/// ```
///
/// Determine when a reduction was not performed because there are no elements:
///
/// ```
/// #![feature(iterator_try_reduce)]
///
/// let numbers: Vec<usize> = Vec::new();
/// let sum = numbers.into_iter().try_reduce(|x, y| x.checked_add(y));
/// assert_eq!(sum, Some(None));
/// ```
///
/// Use a [`Result`] instead of an [`Option`]:
///
/// ```
/// #![feature(iterator_try_reduce)]
///
/// let numbers = vec!["1", "2", "3", "4", "5"];
/// let max: Result<Option<_>, <usize as std::str::FromStr>::Err> =
/// numbers.into_iter().try_reduce(|x, y| {
/// if x.parse::<usize>()? > y.parse::<usize>()? { Ok(x) } else { Ok(y) }
/// });
/// assert_eq!(max, Ok(Some("5")));
/// ```
#[inline]
#[unstable(feature = "iterator_try_reduce", reason = "new API", issue = "87053")]
fn try_reduce<F, R>(&mut self, f: F) -> ChangeOutputType<R, Option<R::Output>>
where
Self: Sized,
F: FnMut(Self::Item, Self::Item) -> R,
R: Try<Output = Self::Item>,
R::Residual: Residual<Option<Self::Item>>,
{
let first = match self.next() {
Some(i) => i,
None => return Try::from_output(None),
};
match self.try_fold(first, f).branch() {
ControlFlow::Break(r) => FromResidual::from_residual(r),
ControlFlow::Continue(i) => Try::from_output(Some(i)),
}
}
/// Tests if every element of the iterator matches a predicate.
///
/// `all()` takes a closure that returns `true` or `false`. It applies
/// this closure to each element of the iterator, and if they all return
/// `true`, then so does `all()`. If any of them return `false`, it
/// returns `false`.
///
/// `all()` is short-circuiting; in other words, it will stop processing
/// as soon as it finds a `false`, given that no matter what else happens,
/// the result will also be `false`.
///
/// An empty iterator returns `true`.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let a = [1, 2, 3];
///
/// assert!(a.iter().all(|&x| x > 0));
///
/// assert!(!a.iter().all(|&x| x > 2));
/// ```
///
/// Stopping at the first `false`:
///
/// ```
/// let a = [1, 2, 3];
///
/// let mut iter = a.iter();
///
/// assert!(!iter.all(|&x| x != 2));
///
/// // we can still use `iter`, as there are more elements.
/// assert_eq!(iter.next(), Some(&3));
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
fn all<F>(&mut self, f: F) -> bool
where
Self: Sized,
F: FnMut(Self::Item) -> bool,
{
#[inline]
fn check<T>(mut f: impl FnMut(T) -> bool) -> impl FnMut((), T) -> ControlFlow<()> {
move |(), x| {
if f(x) { ControlFlow::CONTINUE } else { ControlFlow::BREAK }
}
}
self.try_fold((), check(f)) == ControlFlow::CONTINUE
}
/// Tests if any element of the iterator matches a predicate.
///
/// `any()` takes a closure that returns `true` or `false`. It applies
/// this closure to each element of the iterator, and if any of them return
/// `true`, then so does `any()`. If they all return `false`, it
/// returns `false`.
///
/// `any()` is short-circuiting; in other words, it will stop processing
/// as soon as it finds a `true`, given that no matter what else happens,
/// the result will also be `true`.
///
/// An empty iterator returns `false`.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let a = [1, 2, 3];
///
/// assert!(a.iter().any(|&x| x > 0));
///
/// assert!(!a.iter().any(|&x| x > 5));
/// ```
///
/// Stopping at the first `true`:
///
/// ```
/// let a = [1, 2, 3];
///
/// let mut iter = a.iter();
///
/// assert!(iter.any(|&x| x != 2));
///
/// // we can still use `iter`, as there are more elements.
/// assert_eq!(iter.next(), Some(&2));
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
fn any<F>(&mut self, f: F) -> bool
where
Self: Sized,
F: FnMut(Self::Item) -> bool,
{
#[inline]
fn check<T>(mut f: impl FnMut(T) -> bool) -> impl FnMut((), T) -> ControlFlow<()> {
move |(), x| {
if f(x) { ControlFlow::BREAK } else { ControlFlow::CONTINUE }
}
}
self.try_fold((), check(f)) == ControlFlow::BREAK
}
/// Searches for an element of an iterator that satisfies a predicate.
///
/// `find()` takes a closure that returns `true` or `false`. It applies
/// this closure to each element of the iterator, and if any of them return
/// `true`, then `find()` returns [`Some(element)`]. If they all return
/// `false`, it returns [`None`].
///
/// `find()` is short-circuiting; in other words, it will stop processing
/// as soon as the closure returns `true`.
///
/// Because `find()` takes a reference, and many iterators iterate over
/// references, this leads to a possibly confusing situation where the
/// argument is a double reference. You can see this effect in the
/// examples below, with `&&x`.
///
/// [`Some(element)`]: Some
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let a = [1, 2, 3];
///
/// assert_eq!(a.iter().find(|&&x| x == 2), Some(&2));
///
/// assert_eq!(a.iter().find(|&&x| x == 5), None);
/// ```
///
/// Stopping at the first `true`:
///
/// ```
/// let a = [1, 2, 3];
///
/// let mut iter = a.iter();
///
/// assert_eq!(iter.find(|&&x| x == 2), Some(&2));
///
/// // we can still use `iter`, as there are more elements.
/// assert_eq!(iter.next(), Some(&3));
/// ```
///
/// Note that `iter.find(f)` is equivalent to `iter.filter(f).next()`.
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
fn find<P>(&mut self, predicate: P) -> Option<Self::Item>
where
Self: Sized,
P: FnMut(&Self::Item) -> bool,
{
#[inline]
fn check<T>(mut predicate: impl FnMut(&T) -> bool) -> impl FnMut((), T) -> ControlFlow<T> {
move |(), x| {
if predicate(&x) { ControlFlow::Break(x) } else { ControlFlow::CONTINUE }
}
}
self.try_fold((), check(predicate)).break_value()
}
/// Applies function to the elements of iterator and returns
/// the first non-none result.
///
/// `iter.find_map(f)` is equivalent to `iter.filter_map(f).next()`.
///
/// # Examples
///
/// ```
/// let a = ["lol", "NaN", "2", "5"];
///
/// let first_number = a.iter().find_map(|s| s.parse().ok());
///
/// assert_eq!(first_number, Some(2));
/// ```
#[inline]
#[stable(feature = "iterator_find_map", since = "1.30.0")]
fn find_map<B, F>(&mut self, f: F) -> Option<B>
where
Self: Sized,
F: FnMut(Self::Item) -> Option<B>,
{
#[inline]
fn check<T, B>(mut f: impl FnMut(T) -> Option<B>) -> impl FnMut((), T) -> ControlFlow<B> {
move |(), x| match f(x) {
Some(x) => ControlFlow::Break(x),
None => ControlFlow::CONTINUE,
}
}
self.try_fold((), check(f)).break_value()
}
/// Applies function to the elements of iterator and returns
/// the first true result or the first error.
///
/// The return type of this method depends on the return type of the closure.
/// If you return `Result<bool, E>` from the closure, you'll get a `Result<Option<Self::Item>; E>`.
/// If you return `Option<bool>` from the closure, you'll get an `Option<Option<Self::Item>>`.
///
/// # Examples
///
/// ```
/// #![feature(try_find)]
///
/// let a = ["1", "2", "lol", "NaN", "5"];
///
/// let is_my_num = |s: &str, search: i32| -> Result<bool, std::num::ParseIntError> {
/// Ok(s.parse::<i32>()? == search)
/// };
///
/// let result = a.iter().try_find(|&&s| is_my_num(s, 2));
/// assert_eq!(result, Ok(Some(&"2")));
///
/// let result = a.iter().try_find(|&&s| is_my_num(s, 5));
/// assert!(result.is_err());
/// ```
///
/// This also supports other types which implement `Try`, not just `Result`.
/// ```
/// #![feature(try_find)]
///
/// use std::num::NonZeroU32;
/// let a = [3, 5, 7, 4, 9, 0, 11];
/// let result = a.iter().try_find(|&&x| NonZeroU32::new(x).map(|y| y.is_power_of_two()));
/// assert_eq!(result, Some(Some(&4)));
/// let result = a.iter().take(3).try_find(|&&x| NonZeroU32::new(x).map(|y| y.is_power_of_two()));
/// assert_eq!(result, Some(None));
/// let result = a.iter().rev().try_find(|&&x| NonZeroU32::new(x).map(|y| y.is_power_of_two()));
/// assert_eq!(result, None);
/// ```
#[inline]
#[unstable(feature = "try_find", reason = "new API", issue = "63178")]
fn try_find<F, R>(&mut self, f: F) -> ChangeOutputType<R, Option<Self::Item>>
where
Self: Sized,
F: FnMut(&Self::Item) -> R,
R: Try<Output = bool>,
R::Residual: Residual<Option<Self::Item>>,
{
#[inline]
fn check<I, V, R>(
mut f: impl FnMut(&I) -> V,
) -> impl FnMut((), I) -> ControlFlow<R::TryType>
where
V: Try<Output = bool, Residual = R>,
R: Residual<Option<I>>,
{
move |(), x| match f(&x).branch() {
ControlFlow::Continue(false) => ControlFlow::CONTINUE,
ControlFlow::Continue(true) => ControlFlow::Break(Try::from_output(Some(x))),
ControlFlow::Break(r) => ControlFlow::Break(FromResidual::from_residual(r)),
}
}
match self.try_fold((), check(f)) {
ControlFlow::Break(x) => x,
ControlFlow::Continue(()) => Try::from_output(None),
}
}
/// Searches for an element in an iterator, returning its index.
///
/// `position()` takes a closure that returns `true` or `false`. It applies
/// this closure to each element of the iterator, and if one of them
/// returns `true`, then `position()` returns [`Some(index)`]. If all of
/// them return `false`, it returns [`None`].
///
/// `position()` is short-circuiting; in other words, it will stop
/// processing as soon as it finds a `true`.
///
/// # Overflow Behavior
///
/// The method does no guarding against overflows, so if there are more
/// than [`usize::MAX`] non-matching elements, it either produces the wrong
/// result or panics. If debug assertions are enabled, a panic is
/// guaranteed.
///
/// # Panics
///
/// This function might panic if the iterator has more than `usize::MAX`
/// non-matching elements.
///
/// [`Some(index)`]: Some
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let a = [1, 2, 3];
///
/// assert_eq!(a.iter().position(|&x| x == 2), Some(1));
///
/// assert_eq!(a.iter().position(|&x| x == 5), None);
/// ```
///
/// Stopping at the first `true`:
///
/// ```
/// let a = [1, 2, 3, 4];
///
/// let mut iter = a.iter();
///
/// assert_eq!(iter.position(|&x| x >= 2), Some(1));
///
/// // we can still use `iter`, as there are more elements.
/// assert_eq!(iter.next(), Some(&3));
///
/// // The returned index depends on iterator state
/// assert_eq!(iter.position(|&x| x == 4), Some(0));
///
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
fn position<P>(&mut self, predicate: P) -> Option<usize>
where
Self: Sized,
P: FnMut(Self::Item) -> bool,
{
#[inline]
fn check<T>(
mut predicate: impl FnMut(T) -> bool,
) -> impl FnMut(usize, T) -> ControlFlow<usize, usize> {
#[rustc_inherit_overflow_checks]
move |i, x| {
if predicate(x) { ControlFlow::Break(i) } else { ControlFlow::Continue(i + 1) }
}
}
self.try_fold(0, check(predicate)).break_value()
}
/// Searches for an element in an iterator from the right, returning its
/// index.
///
/// `rposition()` takes a closure that returns `true` or `false`. It applies
/// this closure to each element of the iterator, starting from the end,
/// and if one of them returns `true`, then `rposition()` returns
/// [`Some(index)`]. If all of them return `false`, it returns [`None`].
///
/// `rposition()` is short-circuiting; in other words, it will stop
/// processing as soon as it finds a `true`.
///
/// [`Some(index)`]: Some
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let a = [1, 2, 3];
///
/// assert_eq!(a.iter().rposition(|&x| x == 3), Some(2));
///
/// assert_eq!(a.iter().rposition(|&x| x == 5), None);
/// ```
///
/// Stopping at the first `true`:
///
/// ```
/// let a = [-1, 2, 3, 4];
///
/// let mut iter = a.iter();
///
/// assert_eq!(iter.rposition(|&x| x >= 2), Some(3));
///
/// // we can still use `iter`, as there are more elements.
/// assert_eq!(iter.next(), Some(&-1));
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
fn rposition<P>(&mut self, predicate: P) -> Option<usize>
where
P: FnMut(Self::Item) -> bool,
Self: Sized + ExactSizeIterator + DoubleEndedIterator,
{
// No need for an overflow check here, because `ExactSizeIterator`
// implies that the number of elements fits into a `usize`.
#[inline]
fn check<T>(
mut predicate: impl FnMut(T) -> bool,
) -> impl FnMut(usize, T) -> ControlFlow<usize, usize> {
move |i, x| {
let i = i - 1;
if predicate(x) { ControlFlow::Break(i) } else { ControlFlow::Continue(i) }
}
}
let n = self.len();
self.try_rfold(n, check(predicate)).break_value()
}
/// Returns the maximum element of an iterator.
///
/// If several elements are equally maximum, the last element is
/// returned. If the iterator is empty, [`None`] is returned.
///
/// Note that [`f32`]/[`f64`] doesn't implement [`Ord`] due to NaN being
/// incomparable. You can work around this by using [`Iterator::reduce`]:
/// ```
/// assert_eq!(
/// [2.4, f32::NAN, 1.3]
/// .into_iter()
/// .reduce(f32::max)
/// .unwrap(),
/// 2.4
/// );
/// ```
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let a = [1, 2, 3];
/// let b: Vec<u32> = Vec::new();
///
/// assert_eq!(a.iter().max(), Some(&3));
/// assert_eq!(b.iter().max(), None);
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
fn max(self) -> Option<Self::Item>
where
Self: Sized,
Self::Item: Ord,
{
self.max_by(Ord::cmp)
}
/// Returns the minimum element of an iterator.
///
/// If several elements are equally minimum, the first element is returned.
/// If the iterator is empty, [`None`] is returned.
///
/// Note that [`f32`]/[`f64`] doesn't implement [`Ord`] due to NaN being
/// incomparable. You can work around this by using [`Iterator::reduce`]:
/// ```
/// assert_eq!(
/// [2.4, f32::NAN, 1.3]
/// .into_iter()
/// .reduce(f32::min)
/// .unwrap(),
/// 1.3
/// );
/// ```
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let a = [1, 2, 3];
/// let b: Vec<u32> = Vec::new();
///
/// assert_eq!(a.iter().min(), Some(&1));
/// assert_eq!(b.iter().min(), None);
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
fn min(self) -> Option<Self::Item>
where
Self: Sized,
Self::Item: Ord,
{
self.min_by(Ord::cmp)
}
/// Returns the element that gives the maximum value from the
/// specified function.
///
/// If several elements are equally maximum, the last element is
/// returned. If the iterator is empty, [`None`] is returned.
///
/// # Examples
///
/// ```
/// let a = [-3_i32, 0, 1, 5, -10];
/// assert_eq!(*a.iter().max_by_key(|x| x.abs()).unwrap(), -10);
/// ```
#[inline]
#[stable(feature = "iter_cmp_by_key", since = "1.6.0")]
fn max_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item>
where
Self: Sized,
F: FnMut(&Self::Item) -> B,
{
#[inline]
fn key<T, B>(mut f: impl FnMut(&T) -> B) -> impl FnMut(T) -> (B, T) {
move |x| (f(&x), x)
}
#[inline]
fn compare<T, B: Ord>((x_p, _): &(B, T), (y_p, _): &(B, T)) -> Ordering {
x_p.cmp(y_p)
}
let (_, x) = self.map(key(f)).max_by(compare)?;
Some(x)
}
/// Returns the element that gives the maximum value with respect to the
/// specified comparison function.
///
/// If several elements are equally maximum, the last element is
/// returned. If the iterator is empty, [`None`] is returned.
///
/// # Examples
///
/// ```
/// let a = [-3_i32, 0, 1, 5, -10];
/// assert_eq!(*a.iter().max_by(|x, y| x.cmp(y)).unwrap(), 5);
/// ```
#[inline]
#[stable(feature = "iter_max_by", since = "1.15.0")]
fn max_by<F>(self, compare: F) -> Option<Self::Item>
where
Self: Sized,
F: FnMut(&Self::Item, &Self::Item) -> Ordering,
{
#[inline]
fn fold<T>(mut compare: impl FnMut(&T, &T) -> Ordering) -> impl FnMut(T, T) -> T {
move |x, y| cmp::max_by(x, y, &mut compare)
}
self.reduce(fold(compare))
}
/// Returns the element that gives the minimum value from the
/// specified function.
///
/// If several elements are equally minimum, the first element is
/// returned. If the iterator is empty, [`None`] is returned.
///
/// # Examples
///
/// ```
/// let a = [-3_i32, 0, 1, 5, -10];
/// assert_eq!(*a.iter().min_by_key(|x| x.abs()).unwrap(), 0);
/// ```
#[inline]
#[stable(feature = "iter_cmp_by_key", since = "1.6.0")]
fn min_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item>
where
Self: Sized,
F: FnMut(&Self::Item) -> B,
{
#[inline]
fn key<T, B>(mut f: impl FnMut(&T) -> B) -> impl FnMut(T) -> (B, T) {
move |x| (f(&x), x)
}
#[inline]
fn compare<T, B: Ord>((x_p, _): &(B, T), (y_p, _): &(B, T)) -> Ordering {
x_p.cmp(y_p)
}
let (_, x) = self.map(key(f)).min_by(compare)?;
Some(x)
}
/// Returns the element that gives the minimum value with respect to the
/// specified comparison function.
///
/// If several elements are equally minimum, the first element is
/// returned. If the iterator is empty, [`None`] is returned.
///
/// # Examples
///
/// ```
/// let a = [-3_i32, 0, 1, 5, -10];
/// assert_eq!(*a.iter().min_by(|x, y| x.cmp(y)).unwrap(), -10);
/// ```
#[inline]
#[stable(feature = "iter_min_by", since = "1.15.0")]
fn min_by<F>(self, compare: F) -> Option<Self::Item>
where
Self: Sized,
F: FnMut(&Self::Item, &Self::Item) -> Ordering,
{
#[inline]
fn fold<T>(mut compare: impl FnMut(&T, &T) -> Ordering) -> impl FnMut(T, T) -> T {
move |x, y| cmp::min_by(x, y, &mut compare)
}
self.reduce(fold(compare))
}
/// Reverses an iterator's direction.
///
/// Usually, iterators iterate from left to right. After using `rev()`,
/// an iterator will instead iterate from right to left.
///
/// This is only possible if the iterator has an end, so `rev()` only
/// works on [`DoubleEndedIterator`]s.
///
/// # Examples
///
/// ```
/// let a = [1, 2, 3];
///
/// let mut iter = a.iter().rev();
///
/// assert_eq!(iter.next(), Some(&3));
/// assert_eq!(iter.next(), Some(&2));
/// assert_eq!(iter.next(), Some(&1));
///
/// assert_eq!(iter.next(), None);
/// ```
#[inline]
#[doc(alias = "reverse")]
#[stable(feature = "rust1", since = "1.0.0")]
fn rev(self) -> Rev<Self>
where
Self: Sized + DoubleEndedIterator,
{
Rev::new(self)
}
/// Converts an iterator of pairs into a pair of containers.
///
/// `unzip()` consumes an entire iterator of pairs, producing two
/// collections: one from the left elements of the pairs, and one
/// from the right elements.
///
/// This function is, in some sense, the opposite of [`zip`].
///
/// [`zip`]: Iterator::zip
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let a = [(1, 2), (3, 4), (5, 6)];
///
/// let (left, right): (Vec<_>, Vec<_>) = a.iter().cloned().unzip();
///
/// assert_eq!(left, [1, 3, 5]);
/// assert_eq!(right, [2, 4, 6]);
///
/// // you can also unzip multiple nested tuples at once
/// let a = [(1, (2, 3)), (4, (5, 6))];
///
/// let (x, (y, z)): (Vec<_>, (Vec<_>, Vec<_>)) = a.iter().cloned().unzip();
/// assert_eq!(x, [1, 4]);
/// assert_eq!(y, [2, 5]);
/// assert_eq!(z, [3, 6]);
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
fn unzip<A, B, FromA, FromB>(self) -> (FromA, FromB)
where
FromA: Default + Extend<A>,
FromB: Default + Extend<B>,
Self: Sized + Iterator<Item = (A, B)>,
{
let mut unzipped: (FromA, FromB) = Default::default();
unzipped.extend(self);
unzipped
}
/// Creates an iterator which copies all of its elements.
///
/// This is useful when you have an iterator over `&T`, but you need an
/// iterator over `T`.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let a = [1, 2, 3];
///
/// let v_copied: Vec<_> = a.iter().copied().collect();
///
/// // copied is the same as .map(|&x| x)
/// let v_map: Vec<_> = a.iter().map(|&x| x).collect();
///
/// assert_eq!(v_copied, vec![1, 2, 3]);
/// assert_eq!(v_map, vec![1, 2, 3]);
/// ```
#[stable(feature = "iter_copied", since = "1.36.0")]
fn copied<'a, T: 'a>(self) -> Copied<Self>
where
Self: Sized + Iterator<Item = &'a T>,
T: Copy,
{
Copied::new(self)
}
/// Creates an iterator which [`clone`]s all of its elements.
///
/// This is useful when you have an iterator over `&T`, but you need an
/// iterator over `T`.
///
/// There is no guarantee whatsoever about the `clone` method actually
/// being called *or* optimized away. So code should not depend on
/// either.
///
/// [`clone`]: Clone::clone
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let a = [1, 2, 3];
///
/// let v_cloned: Vec<_> = a.iter().cloned().collect();
///
/// // cloned is the same as .map(|&x| x), for integers
/// let v_map: Vec<_> = a.iter().map(|&x| x).collect();
///
/// assert_eq!(v_cloned, vec![1, 2, 3]);
/// assert_eq!(v_map, vec![1, 2, 3]);
/// ```
///
/// To get the best performance, try to clone late:
///
/// ```
/// let a = [vec![0_u8, 1, 2], vec![3, 4], vec![23]];
/// // don't do this:
/// let slower: Vec<_> = a.iter().cloned().filter(|s| s.len() == 1).collect();
/// assert_eq!(&[vec![23]], &slower[..]);
/// // instead call `cloned` late
/// let faster: Vec<_> = a.iter().filter(|s| s.len() == 1).cloned().collect();
/// assert_eq!(&[vec![23]], &faster[..]);
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
fn cloned<'a, T: 'a>(self) -> Cloned<Self>
where
Self: Sized + Iterator<Item = &'a T>,
T: Clone,
{
Cloned::new(self)
}
/// Repeats an iterator endlessly.
///
/// Instead of stopping at [`None`], the iterator will instead start again,
/// from the beginning. After iterating again, it will start at the
/// beginning again. And again. And again. Forever. Note that in case the
/// original iterator is empty, the resulting iterator will also be empty.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let a = [1, 2, 3];
///
/// let mut it = a.iter().cycle();
///
/// assert_eq!(it.next(), Some(&1));
/// assert_eq!(it.next(), Some(&2));
/// assert_eq!(it.next(), Some(&3));
/// assert_eq!(it.next(), Some(&1));
/// assert_eq!(it.next(), Some(&2));
/// assert_eq!(it.next(), Some(&3));
/// assert_eq!(it.next(), Some(&1));
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
#[inline]
fn cycle(self) -> Cycle<Self>
where
Self: Sized + Clone,
{
Cycle::new(self)
}
/// Returns an iterator over `N` elements of the iterator at a time.
///
/// The chunks do not overlap. If `N` does not divide the length of the
/// iterator, then the last up to `N-1` elements will be omitted and can be
/// retrieved from the [`.into_remainder()`][ArrayChunks::into_remainder]
/// function of the iterator.
///
/// # Panics
///
/// Panics if `N` is 0.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// #![feature(iter_array_chunks)]
///
/// let mut iter = "lorem".chars().array_chunks();
/// assert_eq!(iter.next(), Some(['l', 'o']));
/// assert_eq!(iter.next(), Some(['r', 'e']));
/// assert_eq!(iter.next(), None);
/// assert_eq!(iter.into_remainder().unwrap().as_slice(), &['m']);
/// ```
///
/// ```
/// #![feature(iter_array_chunks)]
///
/// let data = [1, 1, 2, -2, 6, 0, 3, 1];
/// // ^-----^ ^------^
/// for [x, y, z] in data.iter().array_chunks() {
/// assert_eq!(x + y + z, 4);
/// }
/// ```
#[track_caller]
#[unstable(feature = "iter_array_chunks", reason = "recently added", issue = "100450")]
fn array_chunks<const N: usize>(self) -> ArrayChunks<Self, N>
where
Self: Sized,
{
ArrayChunks::new(self)
}
/// Sums the elements of an iterator.
///
/// Takes each element, adds them together, and returns the result.
///
/// An empty iterator returns the zero value of the type.
///
/// # Panics
///
/// When calling `sum()` and a primitive integer type is being returned, this
/// method will panic if the computation overflows and debug assertions are
/// enabled.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let a = [1, 2, 3];
/// let sum: i32 = a.iter().sum();
///
/// assert_eq!(sum, 6);
/// ```
#[stable(feature = "iter_arith", since = "1.11.0")]
fn sum<S>(self) -> S
where
Self: Sized,
S: Sum<Self::Item>,
{
Sum::sum(self)
}
/// Iterates over the entire iterator, multiplying all the elements
///
/// An empty iterator returns the one value of the type.
///
/// # Panics
///
/// When calling `product()` and a primitive integer type is being returned,
/// method will panic if the computation overflows and debug assertions are
/// enabled.
///
/// # Examples
///
/// ```
/// fn factorial(n: u32) -> u32 {
/// (1..=n).product()
/// }
/// assert_eq!(factorial(0), 1);
/// assert_eq!(factorial(1), 1);
/// assert_eq!(factorial(5), 120);
/// ```
#[stable(feature = "iter_arith", since = "1.11.0")]
fn product<P>(self) -> P
where
Self: Sized,
P: Product<Self::Item>,
{
Product::product(self)
}
/// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
/// of another.
///
/// # Examples
///
/// ```
/// use std::cmp::Ordering;
///
/// assert_eq!([1].iter().cmp([1].iter()), Ordering::Equal);
/// assert_eq!([1].iter().cmp([1, 2].iter()), Ordering::Less);
/// assert_eq!([1, 2].iter().cmp([1].iter()), Ordering::Greater);
/// ```
#[stable(feature = "iter_order", since = "1.5.0")]
fn cmp<I>(self, other: I) -> Ordering
where
I: IntoIterator<Item = Self::Item>,
Self::Item: Ord,
Self: Sized,
{
self.cmp_by(other, |x, y| x.cmp(&y))
}
/// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
/// of another with respect to the specified comparison function.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// #![feature(iter_order_by)]
///
/// use std::cmp::Ordering;
///
/// let xs = [1, 2, 3, 4];
/// let ys = [1, 4, 9, 16];
///
/// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| x.cmp(&y)), Ordering::Less);
/// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| (x * x).cmp(&y)), Ordering::Equal);
/// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| (2 * x).cmp(&y)), Ordering::Greater);
/// ```
#[unstable(feature = "iter_order_by", issue = "64295")]
fn cmp_by<I, F>(self, other: I, cmp: F) -> Ordering
where
Self: Sized,
I: IntoIterator,
F: FnMut(Self::Item, I::Item) -> Ordering,
{
#[inline]
fn compare<X, Y, F>(mut cmp: F) -> impl FnMut(X, Y) -> ControlFlow<Ordering>
where
F: FnMut(X, Y) -> Ordering,
{
move |x, y| match cmp(x, y) {
Ordering::Equal => ControlFlow::CONTINUE,
non_eq => ControlFlow::Break(non_eq),
}
}
match iter_compare(self, other.into_iter(), compare(cmp)) {
ControlFlow::Continue(ord) => ord,
ControlFlow::Break(ord) => ord,
}
}
/// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
/// of another.
///
/// # Examples
///
/// ```
/// use std::cmp::Ordering;
///
/// assert_eq!([1.].iter().partial_cmp([1.].iter()), Some(Ordering::Equal));
/// assert_eq!([1.].iter().partial_cmp([1., 2.].iter()), Some(Ordering::Less));
/// assert_eq!([1., 2.].iter().partial_cmp([1.].iter()), Some(Ordering::Greater));
///
/// assert_eq!([f64::NAN].iter().partial_cmp([1.].iter()), None);
/// ```
#[stable(feature = "iter_order", since = "1.5.0")]
fn partial_cmp<I>(self, other: I) -> Option<Ordering>
where
I: IntoIterator,
Self::Item: PartialOrd<I::Item>,
Self: Sized,
{
self.partial_cmp_by(other, |x, y| x.partial_cmp(&y))
}
/// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
/// of another with respect to the specified comparison function.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// #![feature(iter_order_by)]
///
/// use std::cmp::Ordering;
///
/// let xs = [1.0, 2.0, 3.0, 4.0];
/// let ys = [1.0, 4.0, 9.0, 16.0];
///
/// assert_eq!(
/// xs.iter().partial_cmp_by(&ys, |&x, &y| x.partial_cmp(&y)),
/// Some(Ordering::Less)
/// );
/// assert_eq!(
/// xs.iter().partial_cmp_by(&ys, |&x, &y| (x * x).partial_cmp(&y)),
/// Some(Ordering::Equal)
/// );
/// assert_eq!(
/// xs.iter().partial_cmp_by(&ys, |&x, &y| (2.0 * x).partial_cmp(&y)),
/// Some(Ordering::Greater)
/// );
/// ```
#[unstable(feature = "iter_order_by", issue = "64295")]
fn partial_cmp_by<I, F>(self, other: I, partial_cmp: F) -> Option<Ordering>
where
Self: Sized,
I: IntoIterator,
F: FnMut(Self::Item, I::Item) -> Option<Ordering>,
{
#[inline]
fn compare<X, Y, F>(mut partial_cmp: F) -> impl FnMut(X, Y) -> ControlFlow<Option<Ordering>>
where
F: FnMut(X, Y) -> Option<Ordering>,
{
move |x, y| match partial_cmp(x, y) {
Some(Ordering::Equal) => ControlFlow::CONTINUE,
non_eq => ControlFlow::Break(non_eq),
}
}
match iter_compare(self, other.into_iter(), compare(partial_cmp)) {
ControlFlow::Continue(ord) => Some(ord),
ControlFlow::Break(ord) => ord,
}
}
/// Determines if the elements of this [`Iterator`] are equal to those of
/// another.
///
/// # Examples
///
/// ```
/// assert_eq!([1].iter().eq([1].iter()), true);
/// assert_eq!([1].iter().eq([1, 2].iter()), false);
/// ```
#[stable(feature = "iter_order", since = "1.5.0")]
fn eq<I>(self, other: I) -> bool
where
I: IntoIterator,
Self::Item: PartialEq<I::Item>,
Self: Sized,
{
self.eq_by(other, |x, y| x == y)
}
/// Determines if the elements of this [`Iterator`] are equal to those of
/// another with respect to the specified equality function.
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// #![feature(iter_order_by)]
///
/// let xs = [1, 2, 3, 4];
/// let ys = [1, 4, 9, 16];
///
/// assert!(xs.iter().eq_by(&ys, |&x, &y| x * x == y));
/// ```
#[unstable(feature = "iter_order_by", issue = "64295")]
fn eq_by<I, F>(self, other: I, eq: F) -> bool
where
Self: Sized,
I: IntoIterator,
F: FnMut(Self::Item, I::Item) -> bool,
{
#[inline]
fn compare<X, Y, F>(mut eq: F) -> impl FnMut(X, Y) -> ControlFlow<()>
where
F: FnMut(X, Y) -> bool,
{
move |x, y| {
if eq(x, y) { ControlFlow::CONTINUE } else { ControlFlow::BREAK }
}
}
match iter_compare(self, other.into_iter(), compare(eq)) {
ControlFlow::Continue(ord) => ord == Ordering::Equal,
ControlFlow::Break(()) => false,
}
}
/// Determines if the elements of this [`Iterator`] are unequal to those of
/// another.
///
/// # Examples
///
/// ```
/// assert_eq!([1].iter().ne([1].iter()), false);
/// assert_eq!([1].iter().ne([1, 2].iter()), true);
/// ```
#[stable(feature = "iter_order", since = "1.5.0")]
fn ne<I>(self, other: I) -> bool
where
I: IntoIterator,
Self::Item: PartialEq<I::Item>,
Self: Sized,
{
!self.eq(other)
}
/// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
/// less than those of another.
///
/// # Examples
///
/// ```
/// assert_eq!([1].iter().lt([1].iter()), false);
/// assert_eq!([1].iter().lt([1, 2].iter()), true);
/// assert_eq!([1, 2].iter().lt([1].iter()), false);
/// assert_eq!([1, 2].iter().lt([1, 2].iter()), false);
/// ```
#[stable(feature = "iter_order", since = "1.5.0")]
fn lt<I>(self, other: I) -> bool
where
I: IntoIterator,
Self::Item: PartialOrd<I::Item>,
Self: Sized,
{
self.partial_cmp(other) == Some(Ordering::Less)
}
/// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
/// less or equal to those of another.
///
/// # Examples
///
/// ```
/// assert_eq!([1].iter().le([1].iter()), true);
/// assert_eq!([1].iter().le([1, 2].iter()), true);
/// assert_eq!([1, 2].iter().le([1].iter()), false);
/// assert_eq!([1, 2].iter().le([1, 2].iter()), true);
/// ```
#[stable(feature = "iter_order", since = "1.5.0")]
fn le<I>(self, other: I) -> bool
where
I: IntoIterator,
Self::Item: PartialOrd<I::Item>,
Self: Sized,
{
matches!(self.partial_cmp(other), Some(Ordering::Less | Ordering::Equal))
}
/// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
/// greater than those of another.
///
/// # Examples
///
/// ```
/// assert_eq!([1].iter().gt([1].iter()), false);
/// assert_eq!([1].iter().gt([1, 2].iter()), false);
/// assert_eq!([1, 2].iter().gt([1].iter()), true);
/// assert_eq!([1, 2].iter().gt([1, 2].iter()), false);
/// ```
#[stable(feature = "iter_order", since = "1.5.0")]
fn gt<I>(self, other: I) -> bool
where
I: IntoIterator,
Self::Item: PartialOrd<I::Item>,
Self: Sized,
{
self.partial_cmp(other) == Some(Ordering::Greater)
}
/// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
/// greater than or equal to those of another.
///
/// # Examples
///
/// ```
/// assert_eq!([1].iter().ge([1].iter()), true);
/// assert_eq!([1].iter().ge([1, 2].iter()), false);
/// assert_eq!([1, 2].iter().ge([1].iter()), true);
/// assert_eq!([1, 2].iter().ge([1, 2].iter()), true);
/// ```
#[stable(feature = "iter_order", since = "1.5.0")]
fn ge<I>(self, other: I) -> bool
where
I: IntoIterator,
Self::Item: PartialOrd<I::Item>,
Self: Sized,
{
matches!(self.partial_cmp(other), Some(Ordering::Greater | Ordering::Equal))
}
/// Checks if the elements of this iterator are sorted.
///
/// That is, for each element `a` and its following element `b`, `a <= b` must hold. If the
/// iterator yields exactly zero or one element, `true` is returned.
///
/// Note that if `Self::Item` is only `PartialOrd`, but not `Ord`, the above definition
/// implies that this function returns `false` if any two consecutive items are not
/// comparable.
///
/// # Examples
///
/// ```
/// #![feature(is_sorted)]
///
/// assert!([1, 2, 2, 9].iter().is_sorted());
/// assert!(![1, 3, 2, 4].iter().is_sorted());
/// assert!([0].iter().is_sorted());
/// assert!(std::iter::empty::<i32>().is_sorted());
/// assert!(![0.0, 1.0, f32::NAN].iter().is_sorted());
/// ```
#[inline]
#[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
fn is_sorted(self) -> bool
where
Self: Sized,
Self::Item: PartialOrd,
{
self.is_sorted_by(PartialOrd::partial_cmp)
}
/// Checks if the elements of this iterator are sorted using the given comparator function.
///
/// Instead of using `PartialOrd::partial_cmp`, this function uses the given `compare`
/// function to determine the ordering of two elements. Apart from that, it's equivalent to
/// [`is_sorted`]; see its documentation for more information.
///
/// # Examples
///
/// ```
/// #![feature(is_sorted)]
///
/// assert!([1, 2, 2, 9].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
/// assert!(![1, 3, 2, 4].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
/// assert!([0].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
/// assert!(std::iter::empty::<i32>().is_sorted_by(|a, b| a.partial_cmp(b)));
/// assert!(![0.0, 1.0, f32::NAN].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
/// ```
///
/// [`is_sorted`]: Iterator::is_sorted
#[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
fn is_sorted_by<F>(mut self, compare: F) -> bool
where
Self: Sized,
F: FnMut(&Self::Item, &Self::Item) -> Option<Ordering>,
{
#[inline]
fn check<'a, T>(
last: &'a mut T,
mut compare: impl FnMut(&T, &T) -> Option<Ordering> + 'a,
) -> impl FnMut(T) -> bool + 'a {
move |curr| {
if let Some(Ordering::Greater) | None = compare(&last, &curr) {
return false;
}
*last = curr;
true
}
}
let mut last = match self.next() {
Some(e) => e,
None => return true,
};
self.all(check(&mut last, compare))
}
/// Checks if the elements of this iterator are sorted using the given key extraction
/// function.
///
/// Instead of comparing the iterator's elements directly, this function compares the keys of
/// the elements, as determined by `f`. Apart from that, it's equivalent to [`is_sorted`]; see
/// its documentation for more information.
///
/// [`is_sorted`]: Iterator::is_sorted
///
/// # Examples
///
/// ```
/// #![feature(is_sorted)]
///
/// assert!(["c", "bb", "aaa"].iter().is_sorted_by_key(|s| s.len()));
/// assert!(![-2i32, -1, 0, 3].iter().is_sorted_by_key(|n| n.abs()));
/// ```
#[inline]
#[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
fn is_sorted_by_key<F, K>(self, f: F) -> bool
where
Self: Sized,
F: FnMut(Self::Item) -> K,
K: PartialOrd,
{
self.map(f).is_sorted()
}
/// See [TrustedRandomAccess][super::super::TrustedRandomAccess]
// The unusual name is to avoid name collisions in method resolution
// see #76479.
#[inline]
#[doc(hidden)]
#[unstable(feature = "trusted_random_access", issue = "none")]
unsafe fn __iterator_get_unchecked(&mut self, _idx: usize) -> Self::Item
where
Self: TrustedRandomAccessNoCoerce,
{
unreachable!("Always specialized");
}
}
/// Compares two iterators element-wise using the given function.
///
/// If `ControlFlow::CONTINUE` is returned from the function, the comparison moves on to the next
/// elements of both iterators. Returning `ControlFlow::Break(x)` short-circuits the iteration and
/// returns `ControlFlow::Break(x)`. If one of the iterators runs out of elements,
/// `ControlFlow::Continue(ord)` is returned where `ord` is the result of comparing the lengths of
/// the iterators.
///
/// Isolates the logic shared by ['cmp_by'](Iterator::cmp_by),
/// ['partial_cmp_by'](Iterator::partial_cmp_by), and ['eq_by'](Iterator::eq_by).
#[inline]
fn iter_compare<A, B, F, T>(mut a: A, mut b: B, f: F) -> ControlFlow<T, Ordering>
where
A: Iterator,
B: Iterator,
F: FnMut(A::Item, B::Item) -> ControlFlow<T>,
{
#[inline]
fn compare<'a, B, X, T>(
b: &'a mut B,
mut f: impl FnMut(X, B::Item) -> ControlFlow<T> + 'a,
) -> impl FnMut(X) -> ControlFlow<ControlFlow<T, Ordering>> + 'a
where
B: Iterator,
{
move |x| match b.next() {
None => ControlFlow::Break(ControlFlow::Continue(Ordering::Greater)),
Some(y) => f(x, y).map_break(ControlFlow::Break),
}
}
match a.try_for_each(compare(&mut b, f)) {
ControlFlow::Continue(()) => ControlFlow::Continue(match b.next() {
None => Ordering::Equal,
Some(_) => Ordering::Less,
}),
ControlFlow::Break(x) => x,
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<I: Iterator + ?Sized> Iterator for &mut I {
type Item = I::Item;
#[inline]
fn next(&mut self) -> Option<I::Item> {
(**self).next()
}
fn size_hint(&self) -> (usize, Option<usize>) {
(**self).size_hint()
}
fn advance_by(&mut self, n: usize) -> Result<(), usize> {
(**self).advance_by(n)
}
fn nth(&mut self, n: usize) -> Option<Self::Item> {
(**self).nth(n)
}
}