diff --git a/src/libcore/iter.rs b/src/libcore/iter.rs
index 5353fcaa3b4..262bfd5de99 100644
--- a/src/libcore/iter.rs
+++ b/src/libcore/iter.rs
@@ -312,46 +312,110 @@ use usize;
fn _assert_is_object_safe(_: &Iterator<Item=()>) {}
-/// An interface for dealing with "external iterators". These types of iterators
-/// can be resumed at any time as all state is stored internally as opposed to
-/// being located on the call stack.
+/// An interface for dealing with iterators.
///
-/// The Iterator protocol states that an iterator yields a (potentially-empty,
-/// potentially-infinite) sequence of values, and returns `None` to signal that
-/// it's finished. The Iterator protocol does not define behavior after `None`
-/// is returned. A concrete Iterator implementation may choose to behave however
-/// it wishes, either by returning `None` infinitely, or by doing something
-/// else.
+/// 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]: index.html
+/// [impl]: index.html#implementing-iterator
#[lang = "iterator"]
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_on_unimplemented = "`{Self}` is not an iterator; maybe try calling \
`.iter()` or a similar method"]
pub trait Iterator {
- /// The type of the elements being iterated
+ /// 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 the
- /// end is reached.
- #[stable(feature = "rust1", since = "1.0.0")]
- fn next(&mut self) -> Option<Self::Item>;
-
- /// Returns a lower and upper bound on the remaining length of the iterator.
+ /// Advances the iterator and returns the next value.
///
- /// An upper bound of `None` means either there is no known upper bound, or
- /// the upper bound does not fit within a `usize`.
+ /// 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.
///
/// # Examples
///
+ /// Basic usage:
+ ///
/// ```
- /// let it = (0..10).filter(|x| x % 2 == 0).chain(15..20);
- /// assert_eq!((5, Some(15)), it.size_hint());
+ /// 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());
+ /// ```
+ #[stable(feature = "rust1", since = "1.0.0")]
+ fn next(&mut self) -> Option<Self::Item>;
+
+ /// 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 `Option<usize>`. A
+ /// `None` here means that either there is no known upper bound, or the
+ /// upper bound is larger than `usize`.
+ ///
+ /// # Examples
+ ///
+ /// Basic usage:
+ ///
+ /// ```
+ /// let a = [1, 2, 3];
+ /// let iter = a.iter();
+ ///
+ /// assert_eq!((3, Some(3)), iter.size_hint());
+ /// ```
+ ///
+ /// A more complex example:
+ ///
+ /// ```
+ /// // The even numbers from zero to ten.
+ /// 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 one 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
+ /// let iter = (0..);
+ ///
+ /// assert_eq!((0, None), iter.size_hint());
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
fn size_hint(&self) -> (usize, Option<usize>) { (0, None) }
- /// Counts the number of elements in this iterator.
+ /// Consumes the iterator, counting the number of iterations and returning it.
+ ///
+ /// This method will evaluate the iterator until its [`next()`] returns
+ /// `None`. Once `None` is encountered, `count()` returns the number of
+ /// times it called [`next()`].
+ ///
+ /// [`next()`]: #method.next
///
/// # Overflow Behavior
///
@@ -362,12 +426,17 @@ pub trait Iterator {
///
/// # Panics
///
- /// This functions might panic if the iterator has more than `usize::MAX`
+ /// 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);
/// ```
@@ -378,11 +447,20 @@ pub trait Iterator {
self.fold(0, |cnt, _| cnt + 1)
}
- /// Loops through the entire iterator, returning the last element.
+ /// 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));
/// ```
@@ -394,15 +472,45 @@ pub trait Iterator {
last
}
- /// Skips the `n` first elements of the iterator and returns the next one.
+ /// Consumes the `n` first elements of the iterator, then returns the
+ /// `next()` one.
+ ///
+ /// This method will evaluate the iterator `n` times, discarding those elements.
+ /// After it does so, it will call [`next()`] and return its value.
+ ///
+ /// [`next()`]: #method.next
+ ///
+ /// Like most indexing operations, the count starts from zero, so `nth(0)`
+ /// returns the first value, `nth(1)` the second, and so on.
+ ///
+ /// `nth()` will return `None` if `n` is larger than the length of the
+ /// iterator.
///
/// # Examples
///
+ /// Basic usage:
+ ///
/// ```
- /// let a = [1, 2, 3, 4, 5];
- /// let mut it = a.iter();
- /// assert_eq!(it.nth(2), Some(&3));
- /// assert_eq!(it.nth(2), None);
+ /// 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` elements:
+ ///
+ /// ```
+ /// let a = [1, 2, 3];
+ /// assert_eq!(a.iter().nth(10), None);
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
@@ -414,19 +522,54 @@ pub trait Iterator {
None
}
- /// Chain this iterator with another, returning a new iterator that will
- /// finish iterating over the current iterator, and then iterate
- /// over the other specified iterator.
+ /// 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. 🔗
///
/// # Examples
///
+ /// Basic usage:
+ ///
/// ```
- /// let a = [0];
- /// let b = [1];
- /// let mut it = a.iter().chain(&b);
- /// assert_eq!(it.next(), Some(&0));
- /// assert_eq!(it.next(), Some(&1));
- /// assert!(it.next().is_none());
+ /// 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:
+ ///
+ /// [`IntoIterator`]: trait.IntoIterator.html
+ /// [`Iterator`]: trait.Iterator.html
+ ///
+ /// ```
+ /// 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);
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
@@ -436,34 +579,73 @@ pub trait Iterator {
Chain{a: self, b: other.into_iter(), state: ChainState::Both}
}
- /// Creates an iterator that iterates over both this and the specified
- /// iterators simultaneously, yielding the two elements as pairs. When
- /// either iterator returns `None`, all further invocations of `next()`
+ /// '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. 🤐
+ ///
+ /// When either iterator returns `None`, all further calls to `next()`
/// will return `None`.
///
/// # Examples
///
- /// ```
- /// let a = [0];
- /// let b = [1];
- /// let mut it = a.iter().zip(&b);
- /// assert_eq!(it.next(), Some((&0, &1)));
- /// assert!(it.next().is_none());
- /// ```
- ///
- /// `zip` can provide similar functionality to `enumerate`:
+ /// Basic usage:
///
/// ```
- /// for pair in "foo".chars().enumerate() {
- /// println!("{:?}", pair);
- /// }
+ /// let a1 = [1, 2, 3];
+ /// let a2 = [4, 5, 6];
///
- /// for pair in (0..).zip("foo".chars()) {
- /// println!("{:?}", pair);
- /// }
+ /// 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);
/// ```
///
- /// both produce the same output.
+ /// 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:
+ ///
+ /// [`IntoIterator`]: trait.IntoIterator.html
+ /// [`Iterator`]: trait.Iterator.html
+ ///
+ /// ```
+ /// 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]);
+ /// ```
+ ///
+ /// [`enumerate()`]: trait.Iterator.html#method.enumerate
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
fn zip<U>(self, other: U) -> Zip<Self, U::IntoIter> where
@@ -472,17 +654,52 @@ pub trait Iterator {
Zip{a: self, b: other.into_iter()}
}
- /// Creates a new iterator that will apply the specified function to each
- /// element returned by the first, yielding the mapped element instead.
+ /// 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/loops.html#for
///
/// # Examples
///
+ /// Basic usage:
+ ///
/// ```
- /// let a = [1, 2];
- /// let mut it = a.iter().map(|&x| 2 * x);
- /// assert_eq!(it.next(), Some(2));
- /// assert_eq!(it.next(), Some(4));
- /// assert!(it.next().is_none());
+ /// let a = [1, 2, 3];
+ ///
+ /// let mut iter = a.into_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()`:
+ ///
+ /// ```
+ /// // 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")]
@@ -492,18 +709,66 @@ pub trait Iterator {
Map{iter: self, f: f}
}
- /// Creates an iterator that applies the predicate to each element returned
- /// by this iterator. The only elements that will be yielded are those that
- /// make the predicate evaluate to `true`.
+ /// Creates an iterator which uses a closure to determine if an element
+ /// should be yielded.
+ ///
+ /// The closure must return `true` or `false`. `filter()` creates an
+ /// iterator which calls this closure on each element. If the closure
+ /// returns `true`, then the element is returned. If the closure returns
+ /// `false`, it will try again, and call the closure on the next element,
+ /// seeing if it passes the test.
///
/// # Examples
///
+ /// Basic usage:
+ ///
/// ```
- /// let a = [1, 2];
- /// let mut it = a.iter().filter(|&x| *x > 1);
- /// assert_eq!(it.next(), Some(&2));
- /// assert!(it.next().is_none());
+ /// let a = [0i32, 1, 2];
+ ///
+ /// let mut iter = a.into_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.into_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.into_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.into_iter().filter(|&&x| x > 1); // two &s
+ ///
+ /// assert_eq!(iter.next(), Some(&2));
+ /// assert_eq!(iter.next(), None);
+ /// ```
+ ///
+ /// of these layers.
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
fn filter<P>(self, predicate: P) -> Filter<Self, P> where
@@ -512,18 +777,57 @@ pub trait Iterator {
Filter{iter: self, predicate: predicate}
}
- /// Creates an iterator that both filters and maps elements.
- /// If the specified function returns `None`, the element is skipped.
- /// Otherwise the option is unwrapped and the new value is yielded.
+ /// Creates an iterator that both filters and maps.
+ ///
+ /// The closure must return an [`Option<T>`]. `filter_map()` creates an
+ /// iterator which calls this closure on each element. If the closure
+ /// returns `Some(element)`, then that element is returned. If the
+ /// closure returns `None`, it will try again, and call the closure on the
+ /// next element, seeing if it will return `Some`.
+ ///
+ /// [`Option<T>`]: ../option/enum.Option.html
+ ///
+ /// Why `filter_map()` and not just [`filter()`].[`map()`]? The key is in this
+ /// part:
+ ///
+ /// [`filter()`]: #method.filter
+ /// [`map()`]: #method.map
+ ///
+ /// > If the closure returns `Some(element)`, then that element is returned.
+ ///
+ /// In other words, it removes the [`Option<T>`] layer automatically. If your
+ /// mapping is already returning an [`Option<T>`] and you want to skip over
+ /// `None`s, then `filter_map()` is much, much nicer to use.
///
/// # Examples
///
+ /// Basic usage:
+ ///
/// ```
- /// let a = [1, 2];
- /// let mut it = a.iter().filter_map(|&x| if x > 1 {Some(2 * x)} else {None});
- /// assert_eq!(it.next(), Some(4));
- /// assert!(it.next().is_none());
+ /// let a = ["1", "2", "lol"];
+ ///
+ /// let mut iter = a.iter().filter_map(|s| s.parse().ok());
+ ///
+ /// assert_eq!(iter.next(), Some(1));
+ /// assert_eq!(iter.next(), Some(2));
+ /// assert_eq!(iter.next(), None);
/// ```
+ ///
+ /// Here's the same example, but with [`filter()`] and [`map()`]:
+ ///
+ /// ```
+ /// let a = ["1", "2", "lol"];
+ ///
+ /// let mut iter = a.iter()
+ /// .map(|s| s.parse().ok())
+ /// .filter(|s| s.is_some());
+ ///
+ /// assert_eq!(iter.next(), Some(Some(1)));
+ /// assert_eq!(iter.next(), Some(Some(2)));
+ /// assert_eq!(iter.next(), None);
+ /// ```
+ ///
+ /// There's an extra layer of `Some` in there.
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
fn filter_map<B, F>(self, f: F) -> FilterMap<Self, F> where
@@ -532,20 +836,28 @@ pub trait Iterator {
FilterMap { iter: self, f: f }
}
- /// Creates an iterator that yields pairs `(i, val)` where `i` is the
+ /// 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
+ /// `enumerate()` keeps its count as a [`usize`]. If you want to count by a
+ /// different sized integer, the [`zip()`] function provides similar
/// functionality.
///
+ /// [`usize`]: ../primitive.usize.html
+ /// [`zip()`]: #method.zip
+ ///
/// # Overflow Behavior
///
/// The method does no guarding against overflows, so enumerating more than
- /// `usize::MAX` elements either produces the wrong result or panics. If
+ /// [`usize::MAX`] elements either produces the wrong result or panics. If
/// debug assertions are enabled, a panic is guaranteed.
///
+ /// [`usize::MAX`]: ../usize/constant.MAX.html
+ ///
/// # Panics
///
/// The returned iterator might panic if the to-be-returned index would
@@ -554,11 +866,14 @@ pub trait Iterator {
/// # Examples
///
/// ```
- /// let a = [100, 200];
- /// let mut it = a.iter().enumerate();
- /// assert_eq!(it.next(), Some((0, &100)));
- /// assert_eq!(it.next(), Some((1, &200)));
- /// assert!(it.next().is_none());
+ /// let a = [1, 2, 3];
+ ///
+ /// let mut iter = a.iter().enumerate();
+ ///
+ /// assert_eq!(iter.next(), Some((0, &1)));
+ /// assert_eq!(iter.next(), Some((1, &2)));
+ /// assert_eq!(iter.next(), Some((2, &3)));
+ /// assert_eq!(iter.next(), None);
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
@@ -566,22 +881,38 @@ pub trait Iterator {
Enumerate { iter: self, count: 0 }
}
- /// Creates an iterator that has a `.peek()` method
- /// that returns an optional reference to the next element.
+ /// Creates an iterator which can look at the `next()` element without
+ /// consuming it.
+ ///
+ /// Adds a [`peek()`] method to an iterator. See its documentation for
+ /// more information.
+ ///
+ /// [`peek()`]: struct.Peekable.html#method.peek
///
/// # Examples
///
+ /// Basic usage:
+ ///
/// ```
- /// let xs = [100, 200, 300];
- /// let mut it = xs.iter().cloned().peekable();
- /// assert_eq!(*it.peek().unwrap(), 100);
- /// assert_eq!(it.next().unwrap(), 100);
- /// assert_eq!(it.next().unwrap(), 200);
- /// assert_eq!(*it.peek().unwrap(), 300);
- /// assert_eq!(*it.peek().unwrap(), 300);
- /// assert_eq!(it.next().unwrap(), 300);
- /// assert!(it.peek().is_none());
- /// assert!(it.next().is_none());
+ /// 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 itererator won't advance
+ /// assert_eq!(iter.peek(), Some(&&3));
+ /// assert_eq!(iter.peek(), Some(&&3));
+ ///
+ /// assert_eq!(iter.next(), Some(&3));
+ ///
+ /// // after the itererator is finished, so is peek()
+ /// assert_eq!(iter.peek(), None);
+ /// assert_eq!(iter.next(), None);
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
@@ -589,19 +920,60 @@ pub trait Iterator {
Peekable{iter: self, peeked: None}
}
- /// Creates an iterator that invokes the predicate on elements
- /// until it returns false. Once the predicate returns false, that
- /// element and all further elements are yielded.
+ /// Creates an iterator that [`skip()`]s elements based on a predicate.
+ ///
+ /// [`skip()`]: #method.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 = [1, 2, 3, 4, 5];
- /// let mut it = a.iter().skip_while(|&a| *a < 3);
- /// assert_eq!(it.next(), Some(&3));
- /// assert_eq!(it.next(), Some(&4));
- /// assert_eq!(it.next(), Some(&5));
- /// assert!(it.next().is_none());
+ /// let a = [-1i32, 0, 1];
+ ///
+ /// let mut iter = a.into_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 is a double reference:
+ ///
+ /// ```
+ /// let a = [-1, 0, 1];
+ ///
+ /// let mut iter = a.into_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.into_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]
#[stable(feature = "rust1", since = "1.0.0")]
@@ -611,18 +983,53 @@ pub trait Iterator {
SkipWhile{iter: self, flag: false, predicate: predicate}
}
- /// Creates an iterator that yields elements so long as the predicate
- /// returns true. After the predicate returns false for the first time, no
- /// further elements will be yielded.
+ /// 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 = [1, 2, 3, 4, 5];
- /// let mut it = a.iter().take_while(|&a| *a < 3);
- /// assert_eq!(it.next(), Some(&1));
- /// assert_eq!(it.next(), Some(&2));
- /// assert!(it.next().is_none());
+ /// let a = [-1i32, 0, 1];
+ ///
+ /// let mut iter = a.into_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.into_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.into_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);
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
@@ -632,17 +1039,21 @@ pub trait Iterator {
TakeWhile{iter: self, flag: false, predicate: predicate}
}
- /// Creates an iterator that skips the first `n` elements of this iterator,
- /// and then yields all further items.
+ /// Creates an iterator that skips the first `n` elements.
+ ///
+ /// After they have been consumed, the rest of the elements are yielded.
///
/// # Examples
///
+ /// Basic usage:
+ ///
/// ```
- /// let a = [1, 2, 3, 4, 5];
- /// let mut it = a.iter().skip(3);
- /// assert_eq!(it.next(), Some(&4));
- /// assert_eq!(it.next(), Some(&5));
- /// assert!(it.next().is_none());
+ /// 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")]
@@ -650,18 +1061,31 @@ pub trait Iterator {
Skip{iter: self, n: n}
}
- /// Creates an iterator that yields the first `n` elements of this
- /// iterator.
+ /// Creates an iterator that yields its first `n` elements.
///
/// # Examples
///
+ /// Basic usage:
+ ///
/// ```
- /// let a = [1, 2, 3, 4, 5];
- /// let mut it = a.iter().take(3);
- /// assert_eq!(it.next(), Some(&1));
- /// assert_eq!(it.next(), Some(&2));
- /// assert_eq!(it.next(), Some(&3));
- /// assert!(it.next().is_none());
+ /// 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);
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
@@ -669,25 +1093,36 @@ pub trait Iterator {
Take{iter: self, n: n}
}
- /// Creates a new iterator that behaves in a similar fashion to fold.
- /// There is a state which is passed between each iteration and can be
- /// mutated as necessary. The yielded values from the closure are yielded
- /// from the Scan instance when not `None`.
+ /// An iterator similar to `fold()`, with internal state.
+ ///
+ /// `scan()` accumulates a final value, similar to [`fold()`], but instead
+ /// of passing along an accumulator, it maintains the accumulator internally.
+ ///
+ /// [`fold()`]: #method.fold
+ ///
+ /// On each iteraton of `scan()`, you can assign to the internal state, and
+ /// a mutable reference to the state is passed as the first argument to the
+ /// closure, allowing you to modify it on each iteration.
///
/// # Examples
///
+ /// Basic usage:
+ ///
/// ```
- /// let a = [1, 2, 3, 4, 5];
- /// let mut it = a.iter().scan(1, |fac, &x| {
- /// *fac = *fac * x;
- /// Some(*fac)
+ /// 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;
+ ///
+ /// // the value passed on to the next iteration
+ /// Some(*state)
/// });
- /// assert_eq!(it.next(), Some(1));
- /// assert_eq!(it.next(), Some(2));
- /// assert_eq!(it.next(), Some(6));
- /// assert_eq!(it.next(), Some(24));
- /// assert_eq!(it.next(), Some(120));
- /// assert!(it.next().is_none());
+ ///
+ /// 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")]
@@ -697,15 +1132,27 @@ pub trait Iterator {
Scan{iter: self, f: f, state: initial_state}
}
- /// Takes a function that maps each element to a new iterator and yields
- /// all the elements of the produced iterators.
+ /// Creates an iterator that works like map, but flattens nested structure.
///
- /// This is useful for unraveling nested structures.
+ /// 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.
+ ///
+ /// [`map()`]: #method.map
+ ///
+ /// 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.
///
/// # Examples
///
+ /// Basic usage:
+ ///
/// ```
/// let words = ["alpha", "beta", "gamma"];
+ ///
+ /// // chars() returns an iterator
/// let merged: String = words.iter()
/// .flat_map(|s| s.chars())
/// .collect();
@@ -719,32 +1166,56 @@ pub trait Iterator {
FlatMap{iter: self, f: f, frontiter: None, backiter: None }
}
- /// Creates an iterator that yields `None` forever after the underlying
- /// iterator yields `None`. Random-access iterator behavior is not
- /// affected, only single and double-ended iterator behavior.
+ /// 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.
///
/// # Examples
///
+ /// Basic usage:
+ ///
/// ```
- /// fn process<U: Iterator<Item=i32>>(it: U) -> i32 {
- /// let mut it = it.fuse();
- /// let mut sum = 0;
- /// for x in it.by_ref() {
- /// if x > 5 {
- /// break;
- /// }
- /// sum += x;
- /// }
- /// // did we exhaust the iterator?
- /// if it.next().is_none() {
- /// sum += 1000;
- /// }
- /// sum
+ /// // an iterator which alternates between Some and None
+ /// struct Alternate {
+ /// state: i32,
/// }
- /// let x = vec![1, 2, 3, 7, 8, 9];
- /// assert_eq!(process(x.into_iter()), 6);
- /// let x = vec![1, 2, 3];
- /// assert_eq!(process(x.into_iter()), 1006);
+ ///
+ /// 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")]
@@ -752,21 +1223,52 @@ pub trait Iterator {
Fuse{iter: self, done: false}
}
- /// Creates an iterator that calls a function with a reference to each
- /// element before yielding it. This is often useful for debugging an
- /// iterator pipeline.
+ /// Do 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 much more common for `inspect()` to be used as a debugging tool
+ /// than to exist in your final code, but never say never.
///
/// # Examples
///
+ /// Basic usage:
+ ///
/// ```
- /// let a = [1, 4, 2, 3, 8, 9, 6];
- /// let sum: i32 = a.iter()
- /// .map(|x| *x)
- /// .inspect(|&x| println!("filtering {}", x))
- /// .filter(|&x| x % 2 == 0)
- /// .inspect(|&x| println!("{} made it through", x))
- /// .fold(0, |sum, i| sum + i);
+ /// 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
+ /// 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
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
@@ -776,32 +1278,162 @@ pub trait Iterator {
Inspect{iter: self, f: f}
}
- /// Creates a wrapper around a mutable reference to the iterator.
+ /// Borrows an iterator, rather than consuming it.
///
/// This is useful to allow applying iterator adaptors while still
- /// retaining ownership of the original iterator value.
+ /// retaining ownership of the original iterator.
///
/// # Examples
///
+ /// Basic usage:
+ ///
/// ```
- /// let mut it = 0..10;
- /// // sum the first five values
- /// let partial_sum = it.by_ref().take(5).fold(0, |a, b| a + b);
- /// assert_eq!(partial_sum, 10);
- /// assert_eq!(it.next(), Some(5));
+ /// let a = [1, 2, 3];
+ ///
+ /// let iter = a.into_iter();
+ ///
+ /// let sum: i32 = iter.take(5)
+ /// .fold(0, |acc, &i| acc + i );
+ ///
+ /// assert_eq!(sum, 6);
+ ///
+ /// // if we try to use iter again, it won't work. The following line
+ /// // gives "error: use of moved value: `iter`
+ /// // assert_eq!(iter.next(), None);
+ ///
+ /// // let's try that again
+ /// let a = [1, 2, 3];
+ ///
+ /// let mut iter = a.into_iter();
+ ///
+ /// // instead, we add in a .by_ref()
+ /// let sum: i32 = iter.by_ref()
+ /// .take(2)
+ /// .fold(0, |acc, &i| acc + i );
+ ///
+ /// assert_eq!(sum, 3);
+ ///
+ /// // now this is just fine:
+ /// assert_eq!(iter.next(), Some(&3));
+ /// assert_eq!(iter.next(), None);
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
fn by_ref(&mut self) -> &mut Self where Self: Sized { self }
- /// Loops through the entire iterator, collecting all of the elements into
- /// a container implementing `FromIterator`.
+ /// 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.
+ ///
+ /// One of the keys to `collect()`'s power is that many things you might
+ /// not think of as 'collections' actually are. For example, a [`String`]
+ /// is a collection of [`char`]s. And a collection of [`Result<T, E>`] can
+ /// be thought of as single [`Result<Collection<T>, E>`]. See the examples
+ /// below for more.
+ ///
+ /// [`String`]: ../string/struct.String.html
+ /// [`Result<T, E>`]: ../result/enum.Result.html
+ /// [`char`]: ../primitive.char.html
+ ///
+ /// 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 expected = [1, 2, 3, 4, 5];
- /// let actual: Vec<_> = expected.iter().cloned().collect();
- /// assert_eq!(actual, expected);
+ /// 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>`]: ../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 annotationg `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()` 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>`]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);
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
@@ -809,16 +1441,24 @@ pub trait Iterator {
FromIterator::from_iter(self)
}
- /// Loops through the entire iterator, collecting all of the elements into
- /// one of two containers, depending on a predicate. The elements of the
- /// first container satisfy the predicate, while the elements of the second
- /// do not.
+ /// 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`.
+ ///
+ /// # Examples
+ ///
+ /// Basic usage:
///
/// ```
- /// let vec = vec![1, 2, 3, 4];
- /// let (even, odd): (Vec<_>, Vec<_>) = vec.into_iter().partition(|&n| n % 2 == 0);
- /// assert_eq!(even, [2, 4]);
- /// assert_eq!(odd, [1, 3]);
+ /// let a = [1, 2, 3];
+ ///
+ /// let (even, odd): (Vec<i32>, Vec<i32>) = 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, mut f: F) -> (B, B) where
@@ -840,16 +1480,67 @@ pub trait Iterator {
(left, right)
}
- /// Performs a fold operation over the entire iterator, returning the
- /// eventual state at the end of the iteration.
+ /// An iterator adaptor that applies a function, producing a single, final value.
+ ///
+ /// `fold()` takes two arguments: an initial value, and a closure with two
+ /// arguments: an 'accumulator', and an element. It 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.
+ ///
/// # Examples
///
+ /// Basic usage:
+ ///
/// ```
- /// let a = [1, 2, 3, 4, 5];
- /// assert_eq!(a.iter().fold(0, |acc, &item| acc + item), 15);
+ /// let a = [1, 2, 3];
+ ///
+ /// // the sum of all of the elements of a
+ /// 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`.
+ ///
+ /// 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:
+ ///
+ /// ```
+ /// 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);
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
@@ -863,16 +1554,40 @@ pub trait Iterator {
accum
}
- /// Tests whether the predicate holds true for all elements in the iterator.
+ /// Tests if every element of the iterator matches a predicate.
///
- /// Does not consume the iterator past the first non-matching element.
+ /// `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-circuting; 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`.
///
/// # Examples
///
+ /// Basic usage:
+ ///
/// ```
- /// let a = [1, 2, 3, 4, 5];
- /// assert!(a.iter().all(|x| *x > 0));
- /// assert!(!a.iter().all(|x| *x > 2));
+ /// 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")]
@@ -887,18 +1602,40 @@ pub trait Iterator {
true
}
- /// Tests whether any element of an iterator satisfies the specified
- /// predicate.
+ /// Tests if any element of the iterator matches a predicate.
///
- /// Does not consume the iterator past the first found element.
+ /// `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-circuting; 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`.
///
/// # Examples
///
+ /// Basic usage:
+ ///
/// ```
- /// let a = [1, 2, 3, 4, 5];
- /// let mut it = a.iter();
- /// assert!(it.any(|x| *x == 3));
- /// assert_eq!(it.collect::<Vec<_>>(), [&4, &5]);
+ /// 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")]
@@ -914,17 +1651,45 @@ pub trait Iterator {
false
}
- /// Returns the first element satisfying the specified predicate.
+ /// Searches for an element of an iterator that satisfies a predicate.
///
- /// Does not consume the iterator past the first found element.
+ /// `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-circuting; 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`.
///
/// # 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));
/// ```
- /// let a = [1, 2, 3, 4, 5];
- /// let mut it = a.iter();
- /// assert_eq!(it.find(|&x| *x == 3), Some(&3));
- /// assert_eq!(it.collect::<Vec<_>>(), [&4, &5]);
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
fn find<P>(&mut self, mut predicate: P) -> Option<Self::Item> where
@@ -937,9 +1702,15 @@ pub trait Iterator {
None
}
- /// Returns the index of the first element satisfying the specified predicate
+ /// Searches for an element in an iterator, returning its index.
///
- /// Does not consume the iterator past the first found element.
+ /// `position()` takes a closure that returns `true` or `false`. It applies
+ /// this closure to each element of the iterator, and if if one of them
+ /// returns `true`, then `position()` returns `Some(index)`. If all of
+ /// them return `false`, it returns `None`.
+ ///
+ /// `position()` is short-circuting; in other words, it will stop
+ /// processing as soon as it finds a `true`.
///
/// # Overflow Behavior
///
@@ -950,16 +1721,33 @@ pub trait Iterator {
///
/// # Panics
///
- /// This functions might panic if the iterator has more than `usize::MAX`
+ /// This function might panic if the iterator has more than `usize::MAX`
/// non-matching elements.
///
/// # 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];
+ ///
+ /// 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));
/// ```
- /// let a = [1, 2, 3, 4, 5];
- /// let mut it = a.iter();
- /// assert_eq!(it.position(|x| *x == 3), Some(2));
- /// assert_eq!(it.collect::<Vec<_>>(), [&4, &5]);
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
fn position<P>(&mut self, mut predicate: P) -> Option<usize> where
@@ -975,19 +1763,41 @@ pub trait Iterator {
None
}
- /// Returns the index of the last element satisfying the specified predicate
+ /// Searches for an element in an iterator from the right, returning its
+ /// index.
///
- /// If no element matches, `None` is returned.
+ /// `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 if one of them returns `true`, then `rposition()` returns
+ /// `Some(index)`. If all of them return `false`, it returns `None`.
///
- /// Does not consume the iterator *before* the first found element.
+ /// `rposition()` is short-circuting; in other words, it will stop
+ /// processing as soon as it finds a `true`.
///
/// # 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];
+ ///
+ /// let mut iter = a.iter();
+ ///
+ /// assert_eq!(iter.rposition(|&x| x == 2), Some(1));
+ ///
+ /// // we can still use `iter`, as there are more elements.
+ /// assert_eq!(iter.next(), Some(&1));
/// ```
- /// let a = [1, 2, 2, 4, 5];
- /// let mut it = a.iter();
- /// assert_eq!(it.rposition(|x| *x == 2), Some(2));
- /// assert_eq!(it.collect::<Vec<_>>(), [&1, &2]);
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
fn rposition<P>(&mut self, mut predicate: P) -> Option<usize> where
@@ -1007,16 +1817,19 @@ pub trait Iterator {
None
}
- /// Consumes the entire iterator to return the maximum element.
+ /// Returns the maximum element of an iterator.
///
- /// Returns the rightmost element if the comparison determines two elements
- /// to be equally maximum.
+ /// If the two elements are equally maximum, the latest element is
+ /// returned.
///
/// # Examples
///
+ /// Basic usage:
+ ///
/// ```
- /// let a = [1, 2, 3, 4, 5];
- /// assert_eq!(a.iter().max(), Some(&5));
+ /// let a = [1, 2, 3];
+ ///
+ /// assert_eq!(a.iter().max(), Some(&3));
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
@@ -1030,15 +1843,18 @@ pub trait Iterator {
.map(|(_, x)| x)
}
- /// Consumes the entire iterator to return the minimum element.
+ /// Returns the minimum element of an iterator.
///
- /// Returns the leftmost element if the comparison determines two elements
- /// to be equally minimum.
+ /// If the two elements are equally minimum, the first element is
+ /// returned.
///
/// # Examples
///
+ /// Basic usage:
+ ///
/// ```
- /// let a = [1, 2, 3, 4, 5];
+ /// let a = [1, 2, 3];
+ ///
/// assert_eq!(a.iter().min(), Some(&1));
/// ```
#[inline]
@@ -1086,7 +1902,7 @@ pub trait Iterator {
/// Returns the element that gives the minimum value from the
/// specified function.
///
- /// Returns the leftmost element if the comparison determines two elements
+ /// Returns the latest element if the comparison determines two elements
/// to be equally minimum.
///
/// # Examples
@@ -1113,18 +1929,29 @@ pub trait Iterator {
.map(|(_, x)| x)
}
- /// Change the direction of the iterator
+ /// Reverses an iterator's direction.
///
- /// The flipped iterator swaps the ends on an iterator that can already
- /// be iterated from the front and from the back.
+ /// 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.
///
- /// If the iterator also implements RandomAccessIterator, the flipped
- /// iterator is also random access, with the indices starting at the back
- /// of the original iterator.
+ /// [`DoubleEndedIterator`]: trait.DoubleEndedIterator.html
///
- /// Note: Random access with flipped indices still only applies to the first
- /// `std::usize::MAX` elements of the original iterator.
+ /// # 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]
#[stable(feature = "rust1", since = "1.0.0")]
fn rev(self) -> Rev<Self> where Self: Sized + DoubleEndedIterator {
@@ -1133,14 +1960,23 @@ pub trait Iterator {
/// Converts an iterator of pairs into a pair of containers.
///
- /// Loops through the entire iterator, collecting the first component of
- /// each item into one new container, and the second component into another.
+ /// `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()`]: #method.zip
///
/// # Examples
///
+ /// Basic usage:
+ ///
/// ```
/// let a = [(1, 2), (3, 4)];
+ ///
/// let (left, right): (Vec<_>, Vec<_>) = a.iter().cloned().unzip();
+ ///
/// assert_eq!(left, [1, 3]);
/// assert_eq!(right, [2, 4]);
/// ```
@@ -1175,18 +2011,25 @@ pub trait Iterator {
(ts, us)
}
- /// Creates an iterator that clones the elements it yields.
+ /// Creates an iterator which clone()s all of its elements.
///
- /// This is useful for converting an `Iterator<&T>` to an`Iterator<T>`,
- /// so it's a more convenient form of `map(|&x| x)`.
+ /// This is useful when you have an iterator over `&T`, but you need an
+ /// iterator over `T`.
///
/// # Examples
///
+ /// Basic usage:
+ ///
/// ```
- /// let a = [0, 1, 2];
+ /// 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, v_map);
+ ///
+ /// assert_eq!(v_cloned, vec![1, 2, 3]);
+ /// assert_eq!(v_map, vec![1, 2, 3]);
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
fn cloned<'a, T: 'a>(self) -> Cloned<Self>
@@ -1195,15 +2038,27 @@ pub trait Iterator {
Cloned { it: self }
}
- /// Repeats an iterator endlessly
+ /// 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.
///
/// # Examples
///
+ /// Basic usage:
+ ///
/// ```
- /// let a = [1, 2];
+ /// 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")]
@@ -1212,16 +2067,21 @@ pub trait Iterator {
Cycle{orig: self.clone(), iter: self}
}
- /// Iterates over the entire iterator, summing up all the elements
+ /// Sums the elements of an iterator.
+ ///
+ /// Takes each element, adds them together, and returns the result.
///
/// # Examples
///
+ /// Basic usage:
+ ///
/// ```
/// #![feature(iter_arith)]
///
- /// let a = [1, 2, 3, 4, 5];
- /// let it = a.iter();
- /// assert_eq!(it.sum::<i32>(), 15);
+ /// let a = [1, 2, 3];
+ /// let sum: i32 = a.iter().sum();
+ ///
+ /// assert_eq!(sum, 6);
/// ```
#[unstable(feature = "iter_arith", reason = "bounds recently changed",
issue = "27739")]
@@ -2520,8 +3380,45 @@ impl<I: ExactSizeIterator> ExactSizeIterator for Peekable<I> {}
#[stable(feature = "rust1", since = "1.0.0")]
impl<I: Iterator> Peekable<I> {
- /// Returns a reference to the next element of the iterator with out
- /// advancing it, or None if the iterator is exhausted.
+ /// Returns a reference to the next() value without advancing the iterator.
+ ///
+ /// The `peek()` method will return the value that a call to [`next()`] would
+ /// return, but does not advance the iterator. Like [`next()`], if there is
+ /// a value, it's wrapped in a `Some(T)`, but if the iterator is over, it
+ /// will return `None`.
+ ///
+ /// [`next()`]: trait.Iterator.html#tymethod.next
+ ///
+ /// Because `peek()` returns reference, and many iterators iterate over
+ /// references, this leads to a possibly confusing situation where the
+ /// return value is a double reference. You can see this effect in the
+ /// examples below, with `&&i32`.
+ ///
+ /// # 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 itererator won't advance
+ /// assert_eq!(iter.peek(), Some(&&3));
+ /// assert_eq!(iter.peek(), Some(&&3));
+ ///
+ /// assert_eq!(iter.next(), Some(&3));
+ ///
+ /// // after the itererator is finished, so is peek()
+ /// assert_eq!(iter.peek(), None);
+ /// assert_eq!(iter.next(), None);
+ /// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn peek(&mut self) -> Option<&I::Item> {
@@ -2534,7 +3431,32 @@ impl<I: Iterator> Peekable<I> {
}
}
- /// Checks whether peekable iterator is empty or not.
+ /// Checks if the iterator has finished iterating.
+ ///
+ /// Returns `true` if there are no more elements in the iterator, and
+ /// `false` if there are.
+ ///
+ /// # Examples
+ ///
+ /// Basic usage:
+ ///
+ /// ```
+ /// #![feature(core)]
+ ///
+ /// let xs = [1, 2, 3];
+ ///
+ /// let mut iter = xs.iter().peekable();
+ ///
+ /// // there are still elements to iterate over
+ /// assert_eq!(iter.is_empty(), false);
+ ///
+ /// // let's consume the iterator
+ /// iter.next();
+ /// iter.next();
+ /// iter.next();
+ ///
+ /// assert_eq!(iter.is_empty(), true);
+ /// ```
#[inline]
pub fn is_empty(&mut self) -> bool {
self.peek().is_none()