Trait itertools::Itertools

pub trait Itertools: Iterator {
    fn interleave<J>(self, other: J) -> Interleave<Self, J::IntoIter>
    where
        J: IntoIterator<Item = Self::Item>,
        Self: Sized,
    { ... }
    fn interleave_shortest<J>(
        self, 
        other: J
    ) -> InterleaveShortest<Self, J::IntoIter>
    where
        J: IntoIterator<Item = Self::Item>,
        Self: Sized,
    { ... }
    fn intersperse(self, element: Self::Item) -> Intersperse<Self>
    where
        Self: Sized,
        Self::Item: Clone,
    { ... }
    fn zip_longest<J>(self, other: J) -> ZipLongest<Self, J::IntoIter>
    where
        J: IntoIterator,
        Self: Sized,
    { ... }
    fn zip_eq<J>(self, other: J) -> ZipEq<Self, J::IntoIter>
    where
        J: IntoIterator,
        Self: Sized,
    { ... }
    fn batching<B, F>(self, f: F) -> Batching<Self, F>
    where
        F: FnMut(&mut Self) -> Option<B>,
        Self: Sized,
    { ... }
    fn group_by<K, F>(self, key: F) -> GroupBy<K, Self, F>
    where
        Self: Sized,
        F: FnMut(&Self::Item) -> K,
        K: PartialEq,
    { ... }
    fn chunks(self, size: usize) -> IntoChunks<Self>
    where
        Self: Sized,
    { ... }
    fn tuple_windows<T>(self) -> TupleWindows<Self, T>
    where
        Self: Sized + Iterator<Item = T::Item>,
        T: TupleCollect,
        T::Item: Clone,
    { ... }
    fn tuples<T>(self) -> Tuples<Self, T>
    where
        Self: Sized + Iterator<Item = T::Item>,
        T: TupleCollect,
    { ... }
    fn tee(self) -> (Tee<Self>, Tee<Self>)
    where
        Self: Sized,
        Self::Item: Clone,
    { ... }
    fn step(self, n: usize) -> Step<Self>
    where
        Self: Sized,
    { ... }
    fn map_into<R>(self) -> MapInto<Self, R>
    where
        Self: Sized,
        Self::Item: Into<R>,
    { ... }
    fn map_results<F, T, U, E>(self, f: F) -> MapResults<Self, F>
    where
        Self: Iterator<Item = Result<T, E>> + Sized,
        F: FnMut(T) -> U,
    { ... }
    fn merge<J>(self, other: J) -> Merge<Self, J::IntoIter>
    where
        Self: Sized,
        Self::Item: PartialOrd,
        J: IntoIterator<Item = Self::Item>,
    { ... }
    fn merge_by<J, F>(
        self, 
        other: J, 
        is_first: F
    ) -> MergeBy<Self, J::IntoIter, F>
    where
        Self: Sized,
        J: IntoIterator<Item = Self::Item>,
        F: FnMut(&Self::Item, &Self::Item) -> bool,
    { ... }
    fn merge_join_by<J, F>(
        self, 
        other: J, 
        cmp_fn: F
    ) -> MergeJoinBy<Self, J::IntoIter, F>
    where
        J: IntoIterator,
        F: FnMut(&Self::Item, &J::Item) -> Ordering,
        Self: Sized,
    { ... }
    fn kmerge(self) -> KMerge<<Self::Item as IntoIterator>::IntoIter>
    where
        Self: Sized,
        Self::Item: IntoIterator,
        <Self::Item as IntoIterator>::Item: PartialOrd,
    { ... }
    fn kmerge_by<F>(
        self, 
        first: F
    ) -> KMergeBy<<Self::Item as IntoIterator>::IntoIter, F>
    where
        Self: Sized,
        Self::Item: IntoIterator,
        F: FnMut(&<Self::Item as IntoIterator>::Item, &<Self::Item as IntoIterator>::Item) -> bool,
    { ... }
    fn cartesian_product<J>(self, other: J) -> Product<Self, J::IntoIter>
    where
        Self: Sized,
        Self::Item: Clone,
        J: IntoIterator,
        J::IntoIter: Clone,
    { ... }
    fn multi_cartesian_product(
        self
    ) -> MultiProduct<<Self::Item as IntoIterator>::IntoIter>
    where
        Self: Iterator + Sized,
        Self::Item: IntoIterator,
        <Self::Item as IntoIterator>::IntoIter: Clone,
        <Self::Item as IntoIterator>::Item: Clone,
    { ... }
    fn coalesce<F>(self, f: F) -> Coalesce<Self, F>
    where
        Self: Sized,
        F: FnMut(Self::Item, Self::Item) -> Result<Self::Item, (Self::Item, Self::Item)>,
    { ... }
    fn dedup(self) -> Dedup<Self>
    where
        Self: Sized,
        Self::Item: PartialEq,
    { ... }
    fn unique(self) -> Unique<Self>
    where
        Self: Sized,
        Self::Item: Clone + Eq + Hash,
    { ... }
    fn unique_by<V, F>(self, f: F) -> UniqueBy<Self, V, F>
    where
        Self: Sized,
        V: Eq + Hash,
        F: FnMut(&Self::Item) -> V,
    { ... }
    fn peeking_take_while<F>(&mut self, accept: F) -> PeekingTakeWhile<Self, F>
    where
        Self: Sized + PeekingNext,
        F: FnMut(&Self::Item) -> bool,
    { ... }
    fn take_while_ref<F>(&mut self, accept: F) -> TakeWhileRef<Self, F>
    where
        Self: Clone,
        F: FnMut(&Self::Item) -> bool,
    { ... }
    fn while_some<A>(self) -> WhileSome<Self>
    where
        Self: Sized + Iterator<Item = Option<A>>,
    { ... }
    fn tuple_combinations<T>(self) -> TupleCombinations<Self, T>
    where
        Self: Sized + Clone,
        Self::Item: Clone,
        T: HasCombination<Self>,
    { ... }
    fn combinations(self, n: usize) -> Combinations<Self>
    where
        Self: Sized,
        Self::Item: Clone,
    { ... }
    fn pad_using<F>(self, min: usize, f: F) -> PadUsing<Self, F>
    where
        Self: Sized,
        F: FnMut(usize) -> Self::Item,
    { ... }
    fn with_position(self) -> WithPosition<Self>
    where
        Self: Sized,
    { ... }
    fn positions<P>(self, predicate: P) -> Positions<Self, P>
    where
        Self: Sized,
        P: FnMut(Self::Item) -> bool,
    { ... }
    fn update<F>(self, updater: F) -> Update<Self, F>
    where
        Self: Sized,
        F: FnMut(&mut Self::Item),
    { ... }
    fn next_tuple<T>(&mut self) -> Option<T>
    where
        Self: Sized + Iterator<Item = T::Item>,
        T: TupleCollect,
    { ... }
    fn collect_tuple<T>(self) -> Option<T>
    where
        Self: Sized + Iterator<Item = T::Item>,
        T: TupleCollect,
    { ... }
    fn find_position<P>(&mut self, pred: P) -> Option<(usize, Self::Item)>
    where
        P: FnMut(&Self::Item) -> bool,
    { ... }
    fn all_equal(&mut self) -> bool
    where
        Self::Item: PartialEq,
    { ... }
    fn dropping(self, n: usize) -> Self
    where
        Self: Sized,
    { ... }
    fn dropping_back(self, n: usize) -> Self
    where
        Self: Sized,
        Self: DoubleEndedIterator,
    { ... }
    fn foreach<F>(self, f: F)
    where
        F: FnMut(Self::Item),
        Self: Sized,
    { ... }
    fn concat(self) -> Self::Item
    where
        Self: Sized,
        Self::Item: Extend<<Self::Item as IntoIterator>::Item> + IntoIterator + Default,
    { ... }
    fn collect_vec(self) -> Vec<Self::Item>
    where
        Self: Sized,
    { ... }
    fn set_from<'a, A: 'a, J>(&mut self, from: J) -> usize
    where
        Self: Iterator<Item = &'a mut A>,
        J: IntoIterator<Item = A>,
    { ... }
    fn join(&mut self, sep: &str) -> String
    where
        Self::Item: Display,
    { ... }
    fn format(self, sep: &str) -> Format<Self>
    where
        Self: Sized,
    { ... }
    fn format_with<F>(self, sep: &str, format: F) -> FormatWith<Self, F>
    where
        Self: Sized,
        F: FnMut(Self::Item, &mut dyn FnMut(&dyn Display) -> Result) -> Result,
    { ... }
    fn fold_results<A, E, B, F>(&mut self, start: B, f: F) -> Result<B, E>
    where
        Self: Iterator<Item = Result<A, E>>,
        F: FnMut(B, A) -> B,
    { ... }
    fn fold_options<A, B, F>(&mut self, start: B, f: F) -> Option<B>
    where
        Self: Iterator<Item = Option<A>>,
        F: FnMut(B, A) -> B,
    { ... }
    fn fold1<F>(self, f: F) -> Option<Self::Item>
    where
        F: FnMut(Self::Item, Self::Item) -> Self::Item,
        Self: Sized,
    { ... }
    fn tree_fold1<F>(self, f: F) -> Option<Self::Item>
    where
        F: FnMut(Self::Item, Self::Item) -> Self::Item,
        Self: Sized,
    { ... }
    fn fold_while<B, F>(&mut self, init: B, f: F) -> FoldWhile<B>
    where
        Self: Sized,
        F: FnMut(B, Self::Item) -> FoldWhile<B>,
    { ... }
    fn sorted(self) -> IntoIter<Self::Item>
    where
        Self: Sized,
        Self::Item: Ord,
    { ... }
    fn sorted_by<F>(self, cmp: F) -> IntoIter<Self::Item>
    where
        Self: Sized,
        F: FnMut(&Self::Item, &Self::Item) -> Ordering,
    { ... }
    fn sorted_by_key<K, F>(self, f: F) -> IntoIter<Self::Item>
    where
        Self: Sized,
        K: Ord,
        F: FnMut(&Self::Item) -> K,
    { ... }
    fn partition_map<A, B, F, L, R>(self, predicate: F) -> (A, B)
    where
        Self: Sized,
        F: Fn(Self::Item) -> Either<L, R>,
        A: Default + Extend<L>,
        B: Default + Extend<R>,
    { ... }
    fn into_group_map<K, V>(self) -> HashMap<K, Vec<V>>
    where
        Self: Iterator<Item = (K, V)> + Sized,
        K: Hash + Eq,
    { ... }
    fn minmax(self) -> MinMaxResult<Self::Item>
    where
        Self: Sized,
        Self::Item: PartialOrd,
    { ... }
    fn minmax_by_key<K, F>(self, key: F) -> MinMaxResult<Self::Item>
    where
        Self: Sized,
        K: PartialOrd,
        F: FnMut(&Self::Item) -> K,
    { ... }
    fn minmax_by<F>(self, compare: F) -> MinMaxResult<Self::Item>
    where
        Self: Sized,
        F: FnMut(&Self::Item, &Self::Item) -> Ordering,
    { ... }
}

An Iterator blanket implementation that provides extra adaptors and methods.

This trait defines a number of methods. They are divided into two groups:

• Adaptors take an iterator and parameter as input, and return a new iterator value. These are listed first in the trait. An example of an adaptor is .interleave()

• Regular methods are those that don't return iterators and instead return a regular value of some other kind. .next_tuple() is an example and the first regular method in the list.

Provided methods

Important traits for Interleave<I, J>

impl<I, J> Iterator for Interleave<I, J> where
    I: Iterator,
    J: Iterator<Item = I::Item>,  type Item = I::Item;

fn interleave<J>(self, other: J) -> Interleave<Self, J::IntoIter> where
    J: IntoIterator<Item = Self::Item>,
    Self: Sized, 

Alternate elements from two iterators until both have run out.

Iterator element type is Self::Item.

This iterator is fused.

use itertools::Itertools;

let it = (1..7).interleave(vec![-1, -2]);
itertools::assert_equal(it, vec![1, -1, 2, -2, 3, 4, 5, 6]);

Important traits for InterleaveShortest<I, J>

impl<I, J> Iterator for InterleaveShortest<I, J> where
    I: Iterator,
    J: Iterator<Item = I::Item>,  type Item = I::Item;

fn interleave_shortest<J>(
    self,
    other: J
) -> InterleaveShortest<Self, J::IntoIter> where
    J: IntoIterator<Item = Self::Item>,
    Self: Sized, 

Alternate elements from two iterators until at least one of them has run out.

Iterator element type is Self::Item.

use itertools::Itertools;

let it = (1..7).interleave_shortest(vec![-1, -2]);
itertools::assert_equal(it, vec![1, -1, 2, -2, 3]);

Important traits for Intersperse<I>

impl<I> Iterator for Intersperse<I> where
    I: Iterator,
    I::Item: Clone,  type Item = I::Item;

fn intersperse(self, element: Self::Item) -> Intersperse<Self> where
    Self: Sized,
    Self::Item: Clone, 

An iterator adaptor to insert a particular value between each element of the adapted iterator.

Iterator element type is Self::Item.

This iterator is fused.

use itertools::Itertools;

itertools::assert_equal((0..3).intersperse(8), vec![0, 8, 1, 8, 2]);

Important traits for ZipLongest<T, U>

impl<T, U> Iterator for ZipLongest<T, U> where
    T: Iterator,
    U: Iterator,  type Item = EitherOrBoth<T::Item, U::Item>;

fn zip_longest<J>(self, other: J) -> ZipLongest<Self, J::IntoIter> where
    J: IntoIterator,
    Self: Sized, 

Create an iterator which iterates over both this and the specified iterator simultaneously, yielding pairs of two optional elements.

This iterator is fused.

As long as neither input iterator is exhausted yet, it yields two values via EitherOrBoth::Both.

When the parameter iterator is exhausted, it only yields a value from the self iterator via EitherOrBoth::Left.

When the self iterator is exhausted, it only yields a value from the parameter iterator via EitherOrBoth::Right.

When both iterators return None, all further invocations of .next() will return None.

Iterator element type is EitherOrBoth<Self::Item, J::Item>.

use itertools::EitherOrBoth::{Both, Right};
use itertools::Itertools;
let it = (0..1).zip_longest(1..3);
itertools::assert_equal(it, vec![Both(0, 1), Right(2)]);

Important traits for ZipEq<I, J>

impl<I, J> Iterator for ZipEq<I, J> where
    I: Iterator,
    J: Iterator,  type Item = (I::Item, J::Item);

fn zip_eq<J>(self, other: J) -> ZipEq<Self, J::IntoIter> where
    J: IntoIterator,
    Self: Sized, 

Create an iterator which iterates over both this and the specified iterator simultaneously, yielding pairs of elements.

Panics if the iterators reach an end and they are not of equal lengths.

Important traits for Batching<I, F>

impl<B, F, I> Iterator for Batching<I, F> where
    I: Iterator,
    F: FnMut(&mut I) -> Option<B>,  type Item = B;

fn batching<B, F>(self, f: F) -> Batching<Self, F> where
    F: FnMut(&mut Self) -> Option<B>,
    Self: Sized, 

A “meta iterator adaptor”. Its closure receives a reference to the iterator and may pick off as many elements as it likes, to produce the next iterator element.

Iterator element type is B.

use itertools::Itertools;

// An adaptor that gathers elements in pairs
let pit = (0..4).batching(|it| {
           match it.next() {
               None => None,
               Some(x) => match it.next() {
                   None => None,
                   Some(y) => Some((x, y)),
               }
           }
       });

itertools::assert_equal(pit, vec![(0, 1), (2, 3)]);

fn group_by<K, F>(self, key: F) -> GroupBy<K, Self, F> where
    Self: Sized,
    F: FnMut(&Self::Item) -> K,
    K: PartialEq, 

Return an iterable that can group iterator elements. Consecutive elements that map to the same key (“runs”), are assigned to the same group.

GroupBy is the storage for the lazy grouping operation.

If the groups are consumed in order, or if each group's iterator is dropped without keeping it around, then GroupBy uses no allocations. It needs allocations only if several group iterators are alive at the same time.

This type implements IntoIterator (it is not an iterator itself), because the group iterators need to borrow from this value. It should be stored in a local variable or temporary and iterated.

Iterator element type is (K, Group): the group's key and the group iterator.

use itertools::Itertools;

// group data into runs of larger than zero or not.
let data = vec![1, 3, -2, -2, 1, 0, 1, 2];
// groups:     |---->|------>|--------->|

// Note: The `&` is significant here, `GroupBy` is iterable
// only by reference. You can also call `.into_iter()` explicitly.
for (key, group) in &data.into_iter().group_by(|elt| *elt >= 0) {
    // Check that the sum of each group is +/- 4.
    assert_eq!(4, group.sum::<i32>().abs());
}

fn chunks(self, size: usize) -> IntoChunks<Self> where
    Self: Sized, 

Return an iterable that can chunk the iterator.

Yield subiterators (chunks) that each yield a fixed number elements, determined by size. The last chunk will be shorter if there aren't enough elements.

IntoChunks is based on GroupBy: it is iterable (implements IntoIterator, not Iterator), and it only buffers if several chunk iterators are alive at the same time.

Iterator element type is Chunk, each chunk's iterator.

Panics if size is 0.

use itertools::Itertools;

let data = vec![1, 1, 2, -2, 6, 0, 3, 1];
//chunk size=3 |------->|-------->|--->|

// Note: The `&` is significant here, `IntoChunks` is iterable
// only by reference. You can also call `.into_iter()` explicitly.
for chunk in &data.into_iter().chunks(3) {
    // Check that the sum of each chunk is 4.
    assert_eq!(4, chunk.sum());
}

Important traits for TupleWindows<I, T>

impl<I, T> Iterator for TupleWindows<I, T> where
    I: Iterator<Item = T::Item>,
    T: TupleCollect + Clone,
    T::Item: Clone,  type Item = T;

fn tuple_windows<T>(self) -> TupleWindows<Self, T> where
    Self: Sized + Iterator<Item = T::Item>,
    T: TupleCollect,
    T::Item: Clone, 

Return an iterator over all contiguous windows producing tuples of a specific size (up to 4).

tuple_windows clones the iterator elements so that they can be part of successive windows, this makes it most suited for iterators of references and other values that are cheap to copy.

use itertools::Itertools;
let mut v = Vec::new();
for (a, b) in (1..5).tuple_windows() {
    v.push((a, b));
}
assert_eq!(v, vec![(1, 2), (2, 3), (3, 4)]);

let mut it = (1..5).tuple_windows();
assert_eq!(Some((1, 2, 3)), it.next());
assert_eq!(Some((2, 3, 4)), it.next());
assert_eq!(None, it.next());

// this requires a type hint
let it = (1..5).tuple_windows::<(_, _, _)>();
itertools::assert_equal(it, vec![(1, 2, 3), (2, 3, 4)]);

// you can also specify the complete type
use itertools::TupleWindows;
use std::ops::Range;

let it: TupleWindows<Range<u32>, (u32, u32, u32)> = (1..5).tuple_windows();
itertools::assert_equal(it, vec![(1, 2, 3), (2, 3, 4)]);

Important traits for Tuples<I, T>

impl<I, T> Iterator for Tuples<I, T> where
    I: Iterator<Item = T::Item>,
    T: TupleCollect,  type Item = T;

fn tuples<T>(self) -> Tuples<Self, T> where
    Self: Sized + Iterator<Item = T::Item>,
    T: TupleCollect, 

Return an iterator that groups the items in tuples of a specific size (up to 4).

See also the method .next_tuple().

use itertools::Itertools;
let mut v = Vec::new();
for (a, b) in (1..5).tuples() {
    v.push((a, b));
}
assert_eq!(v, vec![(1, 2), (3, 4)]);

let mut it = (1..7).tuples();
assert_eq!(Some((1, 2, 3)), it.next());
assert_eq!(Some((4, 5, 6)), it.next());
assert_eq!(None, it.next());

// this requires a type hint
let it = (1..7).tuples::<(_, _, _)>();
itertools::assert_equal(it, vec![(1, 2, 3), (4, 5, 6)]);

// you can also specify the complete type
use itertools::Tuples;
use std::ops::Range;

let it: Tuples<Range<u32>, (u32, u32, u32)> = (1..7).tuples();
itertools::assert_equal(it, vec![(1, 2, 3), (4, 5, 6)]);

See also Tuples::into_buffer.

fn tee(self) -> (Tee<Self>, Tee<Self>) where
    Self: Sized,
    Self::Item: Clone, 

Split into an iterator pair that both yield all elements from the original iterator.

Note: If the iterator is clonable, prefer using that instead of using this method. It is likely to be more efficient.

Iterator element type is Self::Item.

use itertools::Itertools;
let xs = vec![0, 1, 2, 3];

let (mut t1, t2) = xs.into_iter().tee();
itertools::assert_equal(t1.next(), Some(0));
itertools::assert_equal(t2, 0..4);
itertools::assert_equal(t1, 1..4);

Important traits for Step<I>

impl<I> Iterator for Step<I> where
    I: Iterator,  type Item = I::Item;

fn step(self, n: usize) -> Step<Self> where
    Self: Sized, 

Deprecated since 0.8:

Use std .step_by() instead

Return an iterator adaptor that steps n elements in the base iterator for each iteration.

The iterator steps by yielding the next element from the base iterator, then skipping forward n - 1 elements.

Iterator element type is Self::Item.

Panics if the step is 0.

use itertools::Itertools;

let it = (0..8).step(3);
itertools::assert_equal(it, vec![0, 3, 6]);

Important traits for MapInto<I, R>

impl<I, R> Iterator for MapInto<I, R> where
    I: Iterator,
    I::Item: Into<R>,  type Item = R;

fn map_into<R>(self) -> MapInto<Self, R> where
    Self: Sized,
    Self::Item: Into<R>, 

Convert each item of the iterator using the Into trait.

use itertools::Itertools;

(1i32..42i32).map_into::<f64>().collect_vec();

Important traits for MapResults<I, F>

impl<I, F, T, U, E> Iterator for MapResults<I, F> where
    I: Iterator<Item = Result<T, E>>,
    F: FnMut(T) -> U,  type Item = Result<U, E>;

fn map_results<F, T, U, E>(self, f: F) -> MapResults<Self, F> where
    Self: Iterator<Item = Result<T, E>> + Sized,
    F: FnMut(T) -> U, 

Return an iterator adaptor that applies the provided closure to every Result::Ok value. Result::Err values are unchanged.

use itertools::Itertools;

let input = vec![Ok(41), Err(false), Ok(11)];
let it = input.into_iter().map_results(|i| i + 1);
itertools::assert_equal(it, vec![Ok(42), Err(false), Ok(12)]);

Important traits for Merge<I, J>

impl<I, J> Iterator for Merge<I, J> where
    I: Iterator,
    J: Iterator<Item = I::Item>,
    I::Item: PartialOrd,  type Item = I::Item;

fn merge<J>(self, other: J) -> Merge<Self, J::IntoIter> where
    Self: Sized,
    Self::Item: PartialOrd,
    J: IntoIterator<Item = Self::Item>, 

Return an iterator adaptor that merges the two base iterators in ascending order. If both base iterators are sorted (ascending), the result is sorted.

Iterator element type is Self::Item.

use itertools::Itertools;

let a = (0..11).step(3);
let b = (0..11).step(5);
let it = a.merge(b);
itertools::assert_equal(it, vec![0, 0, 3, 5, 6, 9, 10]);

Important traits for MergeBy<I, J, F>

impl<I, J, F> Iterator for MergeBy<I, J, F> where
    I: Iterator,
    J: Iterator<Item = I::Item>,
    F: FnMut(&I::Item, &I::Item) -> bool,  type Item = I::Item;

fn merge_by<J, F>(self, other: J, is_first: F) -> MergeBy<Self, J::IntoIter, F> where
    Self: Sized,
    J: IntoIterator<Item = Self::Item>,
    F: FnMut(&Self::Item, &Self::Item) -> bool, 

Return an iterator adaptor that merges the two base iterators in order. This is much like .merge() but allows for a custom ordering.

This can be especially useful for sequences of tuples.

Iterator element type is Self::Item.

use itertools::Itertools;

let a = (0..).zip("bc".chars());
let b = (0..).zip("ad".chars());
let it = a.merge_by(b, |x, y| x.1 <= y.1);
itertools::assert_equal(it, vec![(0, 'a'), (0, 'b'), (1, 'c'), (1, 'd')]);

Important traits for MergeJoinBy<I, J, F>

impl<I, J, F> Iterator for MergeJoinBy<I, J, F> where
    I: Iterator,
    J: Iterator,
    F: FnMut(&I::Item, &J::Item) -> Ordering,  type Item = EitherOrBoth<I::Item, J::Item>;

fn merge_join_by<J, F>(
    self,
    other: J,
    cmp_fn: F
) -> MergeJoinBy<Self, J::IntoIter, F> where
    J: IntoIterator,
    F: FnMut(&Self::Item, &J::Item) -> Ordering,
    Self: Sized, 

Create an iterator that merges items from both this and the specified iterator in ascending order.

It chooses whether to pair elements based on the Ordering returned by the specified compare function. At any point, inspecting the tip of the iterators I and J as items i of type I::Item and j of type J::Item respectively, the resulting iterator will:

• Emit EitherOrBoth::Left(i) when i < j, and remove i from its source iterator
• Emit EitherOrBoth::Right(j) when i > j, and remove j from its source iterator
• Emit EitherOrBoth::Both(i, j) when i == j, and remove both i and j from their respective source iterators

use itertools::Itertools;
use itertools::EitherOrBoth::{Left, Right, Both};

let ki = (0..10).step(3);
let ku = (0..10).step(5);
let ki_ku = ki.merge_join_by(ku, |i, j| i.cmp(j)).map(|either| {
    match either {
        Left(_) => "Ki",
        Right(_) => "Ku",
        Both(_, _) => "KiKu"
    }
});

itertools::assert_equal(ki_ku, vec!["KiKu", "Ki", "Ku", "Ki", "Ki"]);

Important traits for KMerge<I>

impl<I> Iterator for KMerge<I> where
    I: Iterator,
    I::Item: PartialOrd,  type Item = I::Item;

fn kmerge(self) -> KMerge<<Self::Item as IntoIterator>::IntoIter> where
    Self: Sized,
    Self::Item: IntoIterator,
    <Self::Item as IntoIterator>::Item: PartialOrd, 

Return an iterator adaptor that flattens an iterator of iterators by merging them in ascending order.

If all base iterators are sorted (ascending), the result is sorted.

Iterator element type is Self::Item.

use itertools::Itertools;

let a = (0..6).step(3);
let b = (1..6).step(3);
let c = (2..6).step(3);
let it = vec![a, b, c].into_iter().kmerge();
itertools::assert_equal(it, vec![0, 1, 2, 3, 4, 5]);

Important traits for KMergeBy<I, F>

impl<I, F> Iterator for KMergeBy<I, F> where
    I: Iterator,
    F: FnMut(&I::Item, &I::Item) -> bool,  type Item = I::Item;

fn kmerge_by<F>(
    self,
    first: F
) -> KMergeBy<<Self::Item as IntoIterator>::IntoIter, F> where
    Self: Sized,
    Self::Item: IntoIterator,
    F: FnMut(&<Self::Item as IntoIterator>::Item, &<Self::Item as IntoIterator>::Item) -> bool, 

Return an iterator adaptor that flattens an iterator of iterators by merging them according to the given closure.

The closure first is called with two elements a, b and should return true if a is ordered before b.

If all base iterators are sorted according to first, the result is sorted.

Iterator element type is Self::Item.

use itertools::Itertools;

let a = vec![-1f64, 2., 3., -5., 6., -7.];
let b = vec![0., 2., -4.];
let mut it = vec![a, b].into_iter().kmerge_by(|a, b| a.abs() < b.abs());
assert_eq!(it.next(), Some(0.));
assert_eq!(it.last(), Some(-7.));

Important traits for Product<I, J>

impl<I, J> Iterator for Product<I, J> where
    I: Iterator,
    J: Clone + Iterator,
    I::Item: Clone,  type Item = (I::Item, J::Item);

fn cartesian_product<J>(self, other: J) -> Product<Self, J::IntoIter> where
    Self: Sized,
    Self::Item: Clone,
    J: IntoIterator,
    J::IntoIter: Clone, 

Return an iterator adaptor that iterates over the cartesian product of the element sets of two iterators self and J.

Iterator element type is (Self::Item, J::Item).

use itertools::Itertools;

let it = (0..2).cartesian_product("αβ".chars());
itertools::assert_equal(it, vec![(0, 'α'), (0, 'β'), (1, 'α'), (1, 'β')]);

Important traits for MultiProduct<I>

impl<I> Iterator for MultiProduct<I> where
    I: Iterator + Clone,
    I::Item: Clone,  type Item = Vec<I::Item>;

fn multi_cartesian_product(
    self
) -> MultiProduct<<Self::Item as IntoIterator>::IntoIter> where
    Self: Iterator + Sized,
    Self::Item: IntoIterator,
    <Self::Item as IntoIterator>::IntoIter: Clone,
    <Self::Item as IntoIterator>::Item: Clone, 

Return an iterator adaptor that iterates over the cartesian product of all subiterators returned by meta-iterator self.

All provided iterators must yield the same Item type. To generate the product of iterators yielding multiple types, use the iproduct macro instead.

The iterator element type is Vec<T>, where T is the iterator element of the subiterators.

use itertools::Itertools;
let mut multi_prod = (0..3).map(|i| (i * 2)..(i * 2 + 2))
    .multi_cartesian_product();
assert_eq!(multi_prod.next(), Some(vec![0, 2, 4]));
assert_eq!(multi_prod.next(), Some(vec![0, 2, 5]));
assert_eq!(multi_prod.next(), Some(vec![0, 3, 4]));
assert_eq!(multi_prod.next(), Some(vec![0, 3, 5]));
assert_eq!(multi_prod.next(), Some(vec![1, 2, 4]));
assert_eq!(multi_prod.next(), Some(vec![1, 2, 5]));
assert_eq!(multi_prod.next(), Some(vec![1, 3, 4]));
assert_eq!(multi_prod.next(), Some(vec![1, 3, 5]));
assert_eq!(multi_prod.next(), None);

Important traits for Coalesce<I, F>

impl<I, F> Iterator for Coalesce<I, F> where
    I: Iterator,
    F: FnMut(I::Item, I::Item) -> Result<I::Item, (I::Item, I::Item)>,  type Item = I::Item;

fn coalesce<F>(self, f: F) -> Coalesce<Self, F> where
    Self: Sized,
    F: FnMut(Self::Item, Self::Item) -> Result<Self::Item, (Self::Item, Self::Item)>, 

Return an iterator adaptor that uses the passed-in closure to optionally merge together consecutive elements.

The closure f is passed two elements, previous and current and may return either (1) Ok(combined) to merge the two values or (2) Err((previous', current')) to indicate they can't be merged. In (2), the value previous' is emitted by the iterator. Either (1) combined or (2) current' becomes the previous value when coalesce continues with the next pair of elements to merge. The value that remains at the end is also emitted by the iterator.

Iterator element type is Self::Item.

This iterator is fused.

use itertools::Itertools;

// sum same-sign runs together
let data = vec![-1., -2., -3., 3., 1., 0., -1.];
itertools::assert_equal(data.into_iter().coalesce(|x, y|
        if (x >= 0.) == (y >= 0.) {
            Ok(x + y)
        } else {
            Err((x, y))
        }),
        vec![-6., 4., -1.]);

Important traits for Dedup<I>

impl<I> Iterator for Dedup<I> where
    I: Iterator,
    I::Item: PartialEq,  type Item = I::Item;

fn dedup(self) -> Dedup<Self> where
    Self: Sized,
    Self::Item: PartialEq, 

Remove duplicates from sections of consecutive identical elements. If the iterator is sorted, all elements will be unique.

Iterator element type is Self::Item.

This iterator is fused.

use itertools::Itertools;

let data = vec![1., 1., 2., 3., 3., 2., 2.];
itertools::assert_equal(data.into_iter().dedup(),
                        vec![1., 2., 3., 2.]);

Important traits for Unique<I>

impl<I> Iterator for Unique<I> where
    I: Iterator,
    I::Item: Eq + Hash + Clone,  type Item = I::Item;

fn unique(self) -> Unique<Self> where
    Self: Sized,
    Self::Item: Clone + Eq + Hash, 

Return an iterator adaptor that filters out elements that have already been produced once during the iteration. Duplicates are detected using hash and equality.

Clones of visited elements are stored in a hash set in the iterator.

use itertools::Itertools;

let data = vec![10, 20, 30, 20, 40, 10, 50];
itertools::assert_equal(data.into_iter().unique(),
                        vec![10, 20, 30, 40, 50]);

Important traits for UniqueBy<I, V, F>

impl<I, V, F> Iterator for UniqueBy<I, V, F> where
    I: Iterator,
    V: Eq + Hash,
    F: FnMut(&I::Item) -> V,  type Item = I::Item;

fn unique_by<V, F>(self, f: F) -> UniqueBy<Self, V, F> where
    Self: Sized,
    V: Eq + Hash,
    F: FnMut(&Self::Item) -> V, 

Return an iterator adaptor that filters out elements that have already been produced once during the iteration.

Duplicates are detected by comparing the key they map to with the keying function f by hash and equality. The keys are stored in a hash set in the iterator.

use itertools::Itertools;

let data = vec!["a", "bb", "aa", "c", "ccc"];
itertools::assert_equal(data.into_iter().unique_by(|s| s.len()),
                        vec!["a", "bb", "ccc"]);

Important traits for PeekingTakeWhile<'a, I, F>

impl<'a, I, F> Iterator for PeekingTakeWhile<'a, I, F> where
    I: PeekingNext,
    F: FnMut(&I::Item) -> bool,  type Item = I::Item;

fn peeking_take_while<F>(&mut self, accept: F) -> PeekingTakeWhile<Self, F> where
    Self: Sized + PeekingNext,
    F: FnMut(&Self::Item) -> bool, 

Return an iterator adaptor that borrows from this iterator and takes items while the closure accept returns true.

This adaptor can only be used on iterators that implement PeekingNext like .peekable(), put_back and a few other collection iterators.

The last and rejected element (first false) is still available when peeking_take_while is done.

See also .take_while_ref() which is a similar adaptor.

Important traits for TakeWhileRef<'a, I, F>

impl<'a, I, F> Iterator for TakeWhileRef<'a, I, F> where
    I: Iterator + Clone,
    F: FnMut(&I::Item) -> bool,  type Item = I::Item;

fn take_while_ref<F>(&mut self, accept: F) -> TakeWhileRef<Self, F> where
    Self: Clone,
    F: FnMut(&Self::Item) -> bool, 

Return an iterator adaptor that borrows from a Clone-able iterator to only pick off elements while the predicate accept returns true.

It uses the Clone trait to restore the original iterator so that the last and rejected element (first false) is still available when take_while_ref is done.

use itertools::Itertools;

let mut hexadecimals = "0123456789abcdef".chars();

let decimals = hexadecimals.take_while_ref(|c| c.is_numeric())
                           .collect::<String>();
assert_eq!(decimals, "0123456789");
assert_eq!(hexadecimals.next(), Some('a'));

Important traits for WhileSome<I>

impl<I, A> Iterator for WhileSome<I> where
    I: Iterator<Item = Option<A>>,  type Item = A;

fn while_some<A>(self) -> WhileSome<Self> where
    Self: Sized + Iterator<Item = Option<A>>, 

Return an iterator adaptor that filters Option<A> iterator elements and produces A. Stops on the first None encountered.

Iterator element type is A, the unwrapped element.

use itertools::Itertools;

// List all hexadecimal digits
itertools::assert_equal(
    (0..).map(|i| std::char::from_digit(i, 16)).while_some(),
    "0123456789abcdef".chars());

Important traits for TupleCombinations<I, T>

impl<I, T> Iterator for TupleCombinations<I, T> where
    I: Iterator,
    T: HasCombination<I>,  type Item = T;

fn tuple_combinations<T>(self) -> TupleCombinations<Self, T> where
    Self: Sized + Clone,
    Self::Item: Clone,
    T: HasCombination<Self>, 

Return an iterator adaptor that iterates over the combinations of the elements from an iterator.

Iterator element can be any homogeneous tuple of type Self::Item with size up to 4.

use itertools::Itertools;

let mut v = Vec::new();
for (a, b) in (1..5).tuple_combinations() {
    v.push((a, b));
}
assert_eq!(v, vec![(1, 2), (1, 3), (1, 4), (2, 3), (2, 4), (3, 4)]);

let mut it = (1..5).tuple_combinations();
assert_eq!(Some((1, 2, 3)), it.next());
assert_eq!(Some((1, 2, 4)), it.next());
assert_eq!(Some((1, 3, 4)), it.next());
assert_eq!(Some((2, 3, 4)), it.next());
assert_eq!(None, it.next());

// this requires a type hint
let it = (1..5).tuple_combinations::<(_, _, _)>();
itertools::assert_equal(it, vec![(1, 2, 3), (1, 2, 4), (1, 3, 4), (2, 3, 4)]);

// you can also specify the complete type
use itertools::TupleCombinations;
use std::ops::Range;

let it: TupleCombinations<Range<u32>, (u32, u32, u32)> = (1..5).tuple_combinations();
itertools::assert_equal(it, vec![(1, 2, 3), (1, 2, 4), (1, 3, 4), (2, 3, 4)]);

Important traits for Combinations<I>

impl<I> Iterator for Combinations<I> where
    I: Iterator,
    I::Item: Clone,  type Item = Vec<I::Item>;

fn combinations(self, n: usize) -> Combinations<Self> where
    Self: Sized,
    Self::Item: Clone, 

Return an iterator adaptor that iterates over the n-length combinations of the elements from an iterator.

Iterator element type is Vec<Self::Item>. The iterator produces a new Vec per iteration, and clones the iterator elements.

use itertools::Itertools;

let it = (1..5).combinations(3);
itertools::assert_equal(it, vec![
    vec![1, 2, 3],
    vec![1, 2, 4],
    vec![1, 3, 4],
    vec![2, 3, 4],
    ]);

Important traits for PadUsing<I, F>

impl<I, F> Iterator for PadUsing<I, F> where
    I: Iterator,
    F: FnMut(usize) -> I::Item,  type Item = I::Item;

fn pad_using<F>(self, min: usize, f: F) -> PadUsing<Self, F> where
    Self: Sized,
    F: FnMut(usize) -> Self::Item, 

Return an iterator adaptor that pads the sequence to a minimum length of min by filling missing elements using a closure f.

Iterator element type is Self::Item.

use itertools::Itertools;

let it = (0..5).pad_using(10, |i| 2*i);
itertools::assert_equal(it, vec![0, 1, 2, 3, 4, 10, 12, 14, 16, 18]);

let it = (0..10).pad_using(5, |i| 2*i);
itertools::assert_equal(it, vec![0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);

let it = (0..5).pad_using(10, |i| 2*i).rev();
itertools::assert_equal(it, vec![18, 16, 14, 12, 10, 4, 3, 2, 1, 0]);

Important traits for WithPosition<I>

impl<I: Iterator> Iterator for WithPosition<I> type Item = Position<I::Item>;

fn with_position(self) -> WithPosition<Self> where
    Self: Sized, 

Return an iterator adaptor that wraps each element in a Position to ease special-case handling of the first or last elements.

Iterator element type is Position<Self::Item>

use itertools::{Itertools, Position};

let it = (0..4).with_position();
itertools::assert_equal(it,
                        vec![Position::First(0),
                             Position::Middle(1),
                             Position::Middle(2),
                             Position::Last(3)]);

let it = (0..1).with_position();
itertools::assert_equal(it, vec![Position::Only(0)]);

Important traits for Positions<I, F>

impl<I, F> Iterator for Positions<I, F> where
    I: Iterator,
    F: FnMut(I::Item) -> bool,  type Item = usize;

fn positions<P>(self, predicate: P) -> Positions<Self, P> where
    Self: Sized,
    P: FnMut(Self::Item) -> bool, 

Return an iterator adaptor that yields the indices of all elements satisfying a predicate, counted from the start of the iterator.

Equivalent to iter.enumerate().filter(|(_, v)| predicate(v)).map(|(i, _)| i).

use itertools::Itertools;

let data = vec![1, 2, 3, 3, 4, 6, 7, 9];
itertools::assert_equal(data.iter().positions(|v| v % 2 == 0), vec![1, 4, 5]);

itertools::assert_equal(data.iter().positions(|v| v % 2 == 1).rev(), vec![7, 6, 3, 2, 0]);

Important traits for Update<I, F>

impl<I, F> Iterator for Update<I, F> where
    I: Iterator,
    F: FnMut(&mut I::Item),  type Item = I::Item;

fn update<F>(self, updater: F) -> Update<Self, F> where
    Self: Sized,
    F: FnMut(&mut Self::Item), 

Return an iterator adaptor that applies a mutating function to each element before yielding it.

use itertools::Itertools;

let input = vec![vec![1], vec![3, 2, 1]];
let it = input.into_iter().update(|mut v| v.push(0));
itertools::assert_equal(it, vec![vec![1, 0], vec![3, 2, 1, 0]]);

fn next_tuple<T>(&mut self) -> Option<T> where
    Self: Sized + Iterator<Item = T::Item>,
    T: TupleCollect, 

Advances the iterator and returns the next items grouped in a tuple of a specific size (up to 4).

If there are enough elements to be grouped in a tuple, then the tuple is returned inside Some, otherwise None is returned.

use itertools::Itertools;

let mut iter = 1..5;

assert_eq!(Some((1, 2)), iter.next_tuple());

fn collect_tuple<T>(self) -> Option<T> where
    Self: Sized + Iterator<Item = T::Item>,
    T: TupleCollect, 

Collects all items from the iterator into a tuple of a specific size (up to 4).

If the number of elements inside the iterator is exactly equal to the tuple size, then the tuple is returned inside Some, otherwise None is returned.

use itertools::Itertools;

let iter = 1..3;

if let Some((x, y)) = iter.collect_tuple() {
    assert_eq!((x, y), (1, 2))
} else {
    panic!("Expected two elements")
}

fn find_position<P>(&mut self, pred: P) -> Option<(usize, Self::Item)> where
    P: FnMut(&Self::Item) -> bool, 

Find the position and value of the first element satisfying a predicate.

The iterator is not advanced past the first element found.

use itertools::Itertools;

let text = "Hα";
assert_eq!(text.chars().find_position(|ch| ch.is_lowercase()), Some((1, 'α')));

fn all_equal(&mut self) -> bool where
    Self::Item: PartialEq, 

Check whether all elements compare equal.

Empty iterators are considered to have equal elements:

use itertools::Itertools;

let data = vec![1, 1, 1, 2, 2, 3, 3, 3, 4, 5, 5];
assert!(!data.iter().all_equal());
assert!(data[0..3].iter().all_equal());
assert!(data[3..5].iter().all_equal());
assert!(data[5..8].iter().all_equal());

let data : Option<usize> = None;
assert!(data.into_iter().all_equal());

fn dropping(self, n: usize) -> Self where
    Self: Sized, 

Consume the first n elements from the iterator eagerly, and return the same iterator again.

It works similarly to .skip( n ) except it is eager and preserves the iterator type.

use itertools::Itertools;

let mut iter = "αβγ".chars().dropping(2);
itertools::assert_equal(iter, "γ".chars());

Fusing notes: if the iterator is exhausted by dropping, the result of calling .next() again depends on the iterator implementation.

fn dropping_back(self, n: usize) -> Self where
    Self: Sized,
    Self: DoubleEndedIterator, 

Consume the last n elements from the iterator eagerly, and return the same iterator again.

This is only possible on double ended iterators. n may be larger than the number of elements.

Note: This method is eager, dropping the back elements immediately and preserves the iterator type.

use itertools::Itertools;

let init = vec![0, 3, 6, 9].into_iter().dropping_back(1);
itertools::assert_equal(init, vec![0, 3, 6]);

fn foreach<F>(self, f: F) where
    F: FnMut(Self::Item),
    Self: Sized, 

Deprecated since 0.8:

Use .for_each() instead

Run the closure f eagerly on each element of the iterator.

Consumes the iterator until its end.

use std::sync::mpsc::channel;
use itertools::Itertools;

let (tx, rx) = channel();

// use .foreach() to apply a function to each value -- sending it
(0..5).map(|x| x * 2 + 1).foreach(|x| { tx.send(x).unwrap(); } );

drop(tx);

itertools::assert_equal(rx.iter(), vec![1, 3, 5, 7, 9]);

fn concat(self) -> Self::Item where
    Self: Sized,
    Self::Item: Extend<<Self::Item as IntoIterator>::Item> + IntoIterator + Default, 

Combine all an iterator's elements into one element by using Extend.

This combinator will extend the first item with each of the rest of the items of the iterator. If the iterator is empty, the default value of I::Item is returned.

use itertools::Itertools;

let input = vec![vec![1], vec![2, 3], vec![4, 5, 6]];
assert_eq!(input.into_iter().concat(),
           vec![1, 2, 3, 4, 5, 6]);

fn collect_vec(self) -> Vec<Self::Item> where
    Self: Sized, 

.collect_vec() is simply a type specialization of .collect(), for convenience.

fn set_from<'a, A: 'a, J>(&mut self, from: J) -> usize where
    Self: Iterator<Item = &'a mut A>,
    J: IntoIterator<Item = A>, 

Assign to each reference in self from the from iterator, stopping at the shortest of the two iterators.

The from iterator is queried for its next element before the self iterator, and if either is exhausted the method is done.

Return the number of elements written.

use itertools::Itertools;

let mut xs = [0; 4];
xs.iter_mut().set_from(1..);
assert_eq!(xs, [1, 2, 3, 4]);

fn join(&mut self, sep: &str) -> String where
    Self::Item: Display, 

Combine all iterator elements into one String, seperated by sep.

Use the Display implementation of each element.

use itertools::Itertools;

assert_eq!(["a", "b", "c"].iter().join(", "), "a, b, c");
assert_eq!([1, 2, 3].iter().join(", "), "1, 2, 3");

fn format(self, sep: &str) -> Format<Self> where
    Self: Sized, 

Format all iterator elements, separated by sep.

All elements are formatted (any formatting trait) with sep inserted between each element.

Panics if the formatter helper is formatted more than once.

use itertools::Itertools;

let data = [1.1, 2.71828, -3.];
assert_eq!(
    format!("{:.2}", data.iter().format(", ")),
           "1.10, 2.72, -3.00");

fn format_with<F>(self, sep: &str, format: F) -> FormatWith<Self, F> where
    Self: Sized,
    F: FnMut(Self::Item, &mut dyn FnMut(&dyn Display) -> Result) -> Result, 

Format all iterator elements, separated by sep.

This is a customizable version of .format().

The supplied closure format is called once per iterator element, with two arguments: the element and a callback that takes a &Display value, i.e. any reference to type that implements Display.

Using &format_args!(...) is the most versatile way to apply custom element formatting. The callback can be called multiple times if needed.

Panics if the formatter helper is formatted more than once.

use itertools::Itertools;

let data = [1.1, 2.71828, -3.];
let data_formatter = data.iter().format_with(", ", |elt, f| f(&format_args!("{:.2}", elt)));
assert_eq!(format!("{}", data_formatter),
           "1.10, 2.72, -3.00");

// .format_with() is recursively composable
let matrix = [[1., 2., 3.],
              [4., 5., 6.]];
let matrix_formatter = matrix.iter().format_with("\n", |row, f| {
                                f(&row.iter().format_with(", ", |elt, g| g(&elt)))
                             });
assert_eq!(format!("{}", matrix_formatter),
           "1, 2, 3\n4, 5, 6");

fn fold_results<A, E, B, F>(&mut self, start: B, f: F) -> Result<B, E> where
    Self: Iterator<Item = Result<A, E>>,
    F: FnMut(B, A) -> B, 

Fold Result values from an iterator.

Only Ok values are folded. If no error is encountered, the folded value is returned inside Ok. Otherwise, the operation terminates and returns the first Err value it encounters. No iterator elements are consumed after the first error.

The first accumulator value is the start parameter. Each iteration passes the accumulator value and the next value inside Ok to the fold function f and its return value becomes the new accumulator value.

For example the sequence Ok(1), Ok(2), Ok(3) will result in a computation like this:

let mut accum = start;
accum = f(accum, 1);
accum = f(accum, 2);
accum = f(accum, 3);

With a start value of 0 and an addition as folding function, this effetively results in ((0 + 1) + 2) + 3

use std::ops::Add;
use itertools::Itertools;

let values = [1, 2, -2, -1, 2, 1];
assert_eq!(
    values.iter()
          .map(Ok::<_, ()>)
          .fold_results(0, Add::add),
    Ok(3)
);
assert!(
    values.iter()
          .map(|&x| if x >= 0 { Ok(x) } else { Err("Negative number") })
          .fold_results(0, Add::add)
          .is_err()
);

fn fold_options<A, B, F>(&mut self, start: B, f: F) -> Option<B> where
    Self: Iterator<Item = Option<A>>,
    F: FnMut(B, A) -> B, 

Fold Option values from an iterator.

Only Some values are folded. If no None is encountered, the folded value is returned inside Some. Otherwise, the operation terminates and returns None. No iterator elements are consumed after the None.

This is the Option equivalent to fold_results.

use std::ops::Add;
use itertools::Itertools;

let mut values = vec![Some(1), Some(2), Some(-2)].into_iter();
assert_eq!(values.fold_options(5, Add::add), Some(5 + 1 + 2 - 2));

let mut more_values = vec![Some(2), None, Some(0)].into_iter();
assert!(more_values.fold_options(0, Add::add).is_none());
assert_eq!(more_values.next().unwrap(), Some(0));

fn fold1<F>(self, f: F) -> Option<Self::Item> where
    F: FnMut(Self::Item, Self::Item) -> Self::Item,
    Self: Sized, 

Accumulator of the elements in the iterator.

Like .fold(), without a base case. If the iterator is empty, return None. With just one element, return it. Otherwise elements are accumulated in sequence using the closure f.

use itertools::Itertools;

assert_eq!((0..10).fold1(|x, y| x + y).unwrap_or(0), 45);
assert_eq!((0..0).fold1(|x, y| x * y), None);

fn tree_fold1<F>(self, f: F) -> Option<Self::Item> where
    F: FnMut(Self::Item, Self::Item) -> Self::Item,
    Self: Sized, 

Accumulate the elements in the iterator in a tree-like manner.

You can think of it as, while there's more than one item, repeatedly combining adjacent items. It does so in bottom-up-merge-sort order, however, so that it needs only logarithmic stack space.

This produces a call tree like the following (where the calls under an item are done after reading that item):

1 2 3 4 5 6 7
│ │ │ │ │ │ │
└─f └─f └─f │
  │   │   │ │
  └───f   └─f
      │     │
      └─────f

Which, for non-associative functions, will typically produce a different result than the linear call tree used by fold1:

1 2 3 4 5 6 7
│ │ │ │ │ │ │
└─f─f─f─f─f─f

If f is associative, prefer the normal fold1 instead.

use itertools::Itertools;

// The same tree as above
let num_strings = (1..8).map(|x| x.to_string());
assert_eq!(num_strings.tree_fold1(|x, y| format!("f({}, {})", x, y)),
    Some(String::from("f(f(f(1, 2), f(3, 4)), f(f(5, 6), 7))")));

// Like fold1, an empty iterator produces None
assert_eq!((0..0).tree_fold1(|x, y| x * y), None);

// tree_fold1 matches fold1 for associative operations...
assert_eq!((0..10).tree_fold1(|x, y| x + y),
    (0..10).fold1(|x, y| x + y));
// ...but not for non-associative ones
assert!((0..10).tree_fold1(|x, y| x - y)
    != (0..10).fold1(|x, y| x - y));

fn fold_while<B, F>(&mut self, init: B, f: F) -> FoldWhile<B> where
    Self: Sized,
    F: FnMut(B, Self::Item) -> FoldWhile<B>, 

Deprecated since 0.8:

Use .try_fold() instead

An iterator method that applies a function, producing a single, final value.

fold_while() is basically equivalent to fold() but with additional support for early exit via short-circuiting.

use itertools::Itertools;
use itertools::FoldWhile::{Continue, Done};

let numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

let mut result = 0;

// for loop:
for i in &numbers {
    if *i > 5 {
        break;
    }
    result = result + i;
}

// fold:
let result2 = numbers.iter().fold(0, |acc, x| {
    if *x > 5 { acc } else { acc + x }
});

// fold_while:
let result3 = numbers.iter().fold_while(0, |acc, x| {
    if *x > 5 { Done(acc) } else { Continue(acc + x) }
}).into_inner();

// they're the same
assert_eq!(result, result2);
assert_eq!(result2, result3);

The big difference between the computations of result2 and result3 is that while fold() called the provided closure for every item of the callee iterator, fold_while() actually stopped iterating as soon as it encountered Fold::Done(_).

fn sorted(self) -> IntoIter<Self::Item> where
    Self: Sized,
    Self::Item: Ord, 

Sort all iterator elements into a new iterator in ascending order.

Note: This consumes the entire iterator, uses the slice::sort() method and returns the result as a new iterator that owns its elements.

The sorted iterator, if directly collected to a Vec, is converted without any extra copying or allocation cost.

use itertools::Itertools;

// sort the letters of the text in ascending order
let text = "bdacfe";
itertools::assert_equal(text.chars().sorted(),
                        "abcdef".chars());

fn sorted_by<F>(self, cmp: F) -> IntoIter<Self::Item> where
    Self: Sized,
    F: FnMut(&Self::Item, &Self::Item) -> Ordering, 

Sort all iterator elements into a new iterator in ascending order.

Note: This consumes the entire iterator, uses the slice::sort_by() method and returns the result as a new iterator that owns its elements.

The sorted iterator, if directly collected to a Vec, is converted without any extra copying or allocation cost.

use itertools::Itertools;

// sort people in descending order by age
let people = vec![("Jane", 20), ("John", 18), ("Jill", 30), ("Jack", 27)];

let oldest_people_first = people
    .into_iter()
    .sorted_by(|a, b| Ord::cmp(&b.1, &a.1))
    .map(|(person, _age)| person);

itertools::assert_equal(oldest_people_first,
                        vec!["Jill", "Jack", "Jane", "John"]);

fn sorted_by_key<K, F>(self, f: F) -> IntoIter<Self::Item> where
    Self: Sized,
    K: Ord,
    F: FnMut(&Self::Item) -> K, 

Sort all iterator elements into a new iterator in ascending order.

Note: This consumes the entire iterator, uses the slice::sort_by_key() method and returns the result as a new iterator that owns its elements.

The sorted iterator, if directly collected to a Vec, is converted without any extra copying or allocation cost.

use itertools::Itertools;

// sort people in descending order by age
let people = vec![("Jane", 20), ("John", 18), ("Jill", 30), ("Jack", 27)];

let oldest_people_first = people
    .into_iter()
    .sorted_by_key(|x| -x.1)
    .map(|(person, _age)| person);

itertools::assert_equal(oldest_people_first,
                        vec!["Jill", "Jack", "Jane", "John"]);

fn partition_map<A, B, F, L, R>(self, predicate: F) -> (A, B) where
    Self: Sized,
    F: Fn(Self::Item) -> Either<L, R>,
    A: Default + Extend<L>,
    B: Default + Extend<R>, 

Collect all iterator elements into one of two partitions. Unlike Iterator::partition, each partition may have a distinct type.

use itertools::{Itertools, Either};

let successes_and_failures = vec![Ok(1), Err(false), Err(true), Ok(2)];

let (successes, failures): (Vec<_>, Vec<_>) = successes_and_failures
    .into_iter()
    .partition_map(|r| {
        match r {
            Ok(v) => Either::Left(v),
            Err(v) => Either::Right(v),
        }
    });

assert_eq!(successes, [1, 2]);
assert_eq!(failures, [false, true]);

fn into_group_map<K, V>(self) -> HashMap<K, Vec<V>> where
    Self: Iterator<Item = (K, V)> + Sized,
    K: Hash + Eq, 

Return a HashMap of keys mapped to Vecs of values. Keys and values are taken from (Key, Value) tuple pairs yielded by the input iterator.

use itertools::Itertools;
 
let data = vec![(0, 10), (2, 12), (3, 13), (0, 20), (3, 33), (2, 42)];
let lookup = data.into_iter().into_group_map();
 
assert_eq!(lookup[&0], vec![10, 20]);
assert_eq!(lookup.get(&1), None);
assert_eq!(lookup[&2], vec![12, 42]);
assert_eq!(lookup[&3], vec![13, 33]);

fn minmax(self) -> MinMaxResult<Self::Item> where
    Self: Sized,
    Self::Item: PartialOrd, 

Return the minimum and maximum elements in the iterator.

The return type MinMaxResult is an enum of three variants:

• NoElements if the iterator is empty.
• OneElement(x) if the iterator has exactly one element.
• MinMax(x, y) is returned otherwise, where x <= y. Two values are equal if and only if there is more than one element in the iterator and all elements are equal.

On an iterator of length n, minmax does 1.5 * n comparisons, and so is faster than calling min and max separately which does 2 * n comparisons.

Examples

use itertools::Itertools;
use itertools::MinMaxResult::{NoElements, OneElement, MinMax};

let a: [i32; 0] = [];
assert_eq!(a.iter().minmax(), NoElements);

let a = [1];
assert_eq!(a.iter().minmax(), OneElement(&1));

let a = [1, 2, 3, 4, 5];
assert_eq!(a.iter().minmax(), MinMax(&1, &5));

let a = [1, 1, 1, 1];
assert_eq!(a.iter().minmax(), MinMax(&1, &1));

The elements can be floats but no particular result is guaranteed if an element is NaN.

fn minmax_by_key<K, F>(self, key: F) -> MinMaxResult<Self::Item> where
    Self: Sized,
    K: PartialOrd,
    F: FnMut(&Self::Item) -> K, 

Return the minimum and maximum element of an iterator, as determined by the specified function.

The return value is a variant of MinMaxResult like for minmax().

For the minimum, the first minimal element is returned. For the maximum, the last maximal element wins. This matches the behavior of the standard Iterator::min() and Iterator::max() methods.

The keys can be floats but no particular result is guaranteed if a key is NaN.

fn minmax_by<F>(self, compare: F) -> MinMaxResult<Self::Item> where
    Self: Sized,
    F: FnMut(&Self::Item, &Self::Item) -> Ordering, 

Return the minimum and maximum element of an iterator, as determined by the specified comparison function.

The return value is a variant of MinMaxResult like for minmax().

For the minimum, the first minimal element is returned. For the maximum, the last maximal element wins. This matches the behavior of the standard Iterator::min() and Iterator::max() methods.

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Implementors

impl<T: ?Sized> Itertools for T where
    T: Iterator, 

Important traits for Interleave<I, J>

impl<I, J> Iterator for Interleave<I, J> where
    I: Iterator,
    J: Iterator<Item = I::Item>,  type Item = I::Item;

fn interleave<J>(self, other: J) -> Interleave<Self, J::IntoIter> where
    J: IntoIterator<Item = Self::Item>,
    Self: Sized, 

Important traits for InterleaveShortest<I, J>

impl<I, J> Iterator for InterleaveShortest<I, J> where
    I: Iterator,
    J: Iterator<Item = I::Item>,  type Item = I::Item;

fn interleave_shortest<J>(
    self,
    other: J
) -> InterleaveShortest<Self, J::IntoIter> where
    J: IntoIterator<Item = Self::Item>,
    Self: Sized, 

Important traits for Intersperse<I>

impl<I> Iterator for Intersperse<I> where
    I: Iterator,
    I::Item: Clone,  type Item = I::Item;

fn intersperse(self, element: Self::Item) -> Intersperse<Self> where
    Self: Sized,
    Self::Item: Clone, 

Important traits for ZipLongest<T, U>

impl<T, U> Iterator for ZipLongest<T, U> where
    T: Iterator,
    U: Iterator,  type Item = EitherOrBoth<T::Item, U::Item>;

fn zip_longest<J>(self, other: J) -> ZipLongest<Self, J::IntoIter> where
    J: IntoIterator,
    Self: Sized, 

Important traits for ZipEq<I, J>

impl<I, J> Iterator for ZipEq<I, J> where
    I: Iterator,
    J: Iterator,  type Item = (I::Item, J::Item);

fn zip_eq<J>(self, other: J) -> ZipEq<Self, J::IntoIter> where
    J: IntoIterator,
    Self: Sized, 

Important traits for Batching<I, F>

impl<B, F, I> Iterator for Batching<I, F> where
    I: Iterator,
    F: FnMut(&mut I) -> Option<B>,  type Item = B;

fn batching<B, F>(self, f: F) -> Batching<Self, F> where
    F: FnMut(&mut Self) -> Option<B>,
    Self: Sized, 

fn group_by<K, F>(self, key: F) -> GroupBy<K, Self, F> where
    Self: Sized,
    F: FnMut(&Self::Item) -> K,
    K: PartialEq, 

fn chunks(self, size: usize) -> IntoChunks<Self> where
    Self: Sized, 

Important traits for TupleWindows<I, T>

impl<I, T> Iterator for TupleWindows<I, T> where
    I: Iterator<Item = T::Item>,
    T: TupleCollect + Clone,
    T::Item: Clone,  type Item = T;

fn tuple_windows<T>(self) -> TupleWindows<Self, T> where
    Self: Sized + Iterator<Item = T::Item>,
    T: TupleCollect,
    T::Item: Clone, 

Important traits for Tuples<I, T>

impl<I, T> Iterator for Tuples<I, T> where
    I: Iterator<Item = T::Item>,
    T: TupleCollect,  type Item = T;

fn tuples<T>(self) -> Tuples<Self, T> where
    Self: Sized + Iterator<Item = T::Item>,
    T: TupleCollect, 

fn tee(self) -> (Tee<Self>, Tee<Self>) where
    Self: Sized,
    Self::Item: Clone, 

Important traits for Step<I>

impl<I> Iterator for Step<I> where
    I: Iterator,  type Item = I::Item;

fn step(self, n: usize) -> Step<Self> where
    Self: Sized, 

Deprecated since 0.8:

Use std .step_by() instead

Important traits for MapInto<I, R>

impl<I, R> Iterator for MapInto<I, R> where
    I: Iterator,
    I::Item: Into<R>,  type Item = R;

fn map_into<R>(self) -> MapInto<Self, R> where
    Self: Sized,
    Self::Item: Into<R>, 

Important traits for MapResults<I, F>

impl<I, F, T, U, E> Iterator for MapResults<I, F> where
    I: Iterator<Item = Result<T, E>>,
    F: FnMut(T) -> U,  type Item = Result<U, E>;

fn map_results<F, T, U, E>(self, f: F) -> MapResults<Self, F> where
    Self: Iterator<Item = Result<T, E>> + Sized,
    F: FnMut(T) -> U, 

Important traits for Merge<I, J>

impl<I, J> Iterator for Merge<I, J> where
    I: Iterator,
    J: Iterator<Item = I::Item>,
    I::Item: PartialOrd,  type Item = I::Item;

fn merge<J>(self, other: J) -> Merge<Self, J::IntoIter> where
    Self: Sized,
    Self::Item: PartialOrd,
    J: IntoIterator<Item = Self::Item>, 

Important traits for MergeBy<I, J, F>

impl<I, J, F> Iterator for MergeBy<I, J, F> where
    I: Iterator,
    J: Iterator<Item = I::Item>,
    F: FnMut(&I::Item, &I::Item) -> bool,  type Item = I::Item;

fn merge_by<J, F>(self, other: J, is_first: F) -> MergeBy<Self, J::IntoIter, F> where
    Self: Sized,
    J: IntoIterator<Item = Self::Item>,
    F: FnMut(&Self::Item, &Self::Item) -> bool, 

Important traits for MergeJoinBy<I, J, F>

impl<I, J, F> Iterator for MergeJoinBy<I, J, F> where
    I: Iterator,
    J: Iterator,
    F: FnMut(&I::Item, &J::Item) -> Ordering,  type Item = EitherOrBoth<I::Item, J::Item>;

fn merge_join_by<J, F>(
    self,
    other: J,
    cmp_fn: F
) -> MergeJoinBy<Self, J::IntoIter, F> where
    J: IntoIterator,
    F: FnMut(&Self::Item, &J::Item) -> Ordering,
    Self: Sized, 

Important traits for KMerge<I>

impl<I> Iterator for KMerge<I> where
    I: Iterator,
    I::Item: PartialOrd,  type Item = I::Item;

fn kmerge(self) -> KMerge<<Self::Item as IntoIterator>::IntoIter> where
    Self: Sized,
    Self::Item: IntoIterator,
    <Self::Item as IntoIterator>::Item: PartialOrd, 

Important traits for KMergeBy<I, F>

impl<I, F> Iterator for KMergeBy<I, F> where
    I: Iterator,
    F: FnMut(&I::Item, &I::Item) -> bool,  type Item = I::Item;

fn kmerge_by<F>(
    self,
    first: F
) -> KMergeBy<<Self::Item as IntoIterator>::IntoIter, F> where
    Self: Sized,
    Self::Item: IntoIterator,
    F: FnMut(&<Self::Item as IntoIterator>::Item, &<Self::Item as IntoIterator>::Item) -> bool, 

Important traits for Product<I, J>

impl<I, J> Iterator for Product<I, J> where
    I: Iterator,
    J: Clone + Iterator,
    I::Item: Clone,  type Item = (I::Item, J::Item);

fn cartesian_product<J>(self, other: J) -> Product<Self, J::IntoIter> where
    Self: Sized,
    Self::Item: Clone,
    J: IntoIterator,
    J::IntoIter: Clone, 

Important traits for MultiProduct<I>

impl<I> Iterator for MultiProduct<I> where
    I: Iterator + Clone,
    I::Item: Clone,  type Item = Vec<I::Item>;

fn multi_cartesian_product(
    self
) -> MultiProduct<<Self::Item as IntoIterator>::IntoIter> where
    Self: Iterator + Sized,
    Self::Item: IntoIterator,
    <Self::Item as IntoIterator>::IntoIter: Clone,
    <Self::Item as IntoIterator>::Item: Clone, 

Important traits for Coalesce<I, F>

impl<I, F> Iterator for Coalesce<I, F> where
    I: Iterator,
    F: FnMut(I::Item, I::Item) -> Result<I::Item, (I::Item, I::Item)>,  type Item = I::Item;

fn coalesce<F>(self, f: F) -> Coalesce<Self, F> where
    Self: Sized,
    F: FnMut(Self::Item, Self::Item) -> Result<Self::Item, (Self::Item, Self::Item)>, 

Important traits for Dedup<I>

impl<I> Iterator for Dedup<I> where
    I: Iterator,
    I::Item: PartialEq,  type Item = I::Item;

fn dedup(self) -> Dedup<Self> where
    Self: Sized,
    Self::Item: PartialEq, 

Important traits for Unique<I>

impl<I> Iterator for Unique<I> where
    I: Iterator,
    I::Item: Eq + Hash + Clone,  type Item = I::Item;

fn unique(self) -> Unique<Self> where
    Self: Sized,
    Self::Item: Clone + Eq + Hash, 

Important traits for UniqueBy<I, V, F>

impl<I, V, F> Iterator for UniqueBy<I, V, F> where
    I: Iterator,
    V: Eq + Hash,
    F: FnMut(&I::Item) -> V,  type Item = I::Item;

fn unique_by<V, F>(self, f: F) -> UniqueBy<Self, V, F> where
    Self: Sized,
    V: Eq + Hash,
    F: FnMut(&Self::Item) -> V, 

Important traits for PeekingTakeWhile<'a, I, F>

impl<'a, I, F> Iterator for PeekingTakeWhile<'a, I, F> where
    I: PeekingNext,
    F: FnMut(&I::Item) -> bool,  type Item = I::Item;

fn peeking_take_while<F>(&mut self, accept: F) -> PeekingTakeWhile<Self, F> where
    Self: Sized + PeekingNext,
    F: FnMut(&Self::Item) -> bool, 

Important traits for TakeWhileRef<'a, I, F>

impl<'a, I, F> Iterator for TakeWhileRef<'a, I, F> where
    I: Iterator + Clone,
    F: FnMut(&I::Item) -> bool,  type Item = I::Item;

fn take_while_ref<F>(&mut self, accept: F) -> TakeWhileRef<Self, F> where
    Self: Clone,
    F: FnMut(&Self::Item) -> bool, 

Important traits for WhileSome<I>

impl<I, A> Iterator for WhileSome<I> where
    I: Iterator<Item = Option<A>>,  type Item = A;

fn while_some<A>(self) -> WhileSome<Self> where
    Self: Sized + Iterator<Item = Option<A>>, 

Important traits for TupleCombinations<I, T>

impl<I, T> Iterator for TupleCombinations<I, T> where
    I: Iterator,
    T: HasCombination<I>,  type Item = T;

fn tuple_combinations<T>(self) -> TupleCombinations<Self, T> where
    Self: Sized + Clone,
    Self::Item: Clone,
    T: HasCombination<Self>, 

Important traits for Combinations<I>

impl<I> Iterator for Combinations<I> where
    I: Iterator,
    I::Item: Clone,  type Item = Vec<I::Item>;

fn combinations(self, n: usize) -> Combinations<Self> where
    Self: Sized,
    Self::Item: Clone, 

Important traits for PadUsing<I, F>

impl<I, F> Iterator for PadUsing<I, F> where
    I: Iterator,
    F: FnMut(usize) -> I::Item,  type Item = I::Item;

fn pad_using<F>(self, min: usize, f: F) -> PadUsing<Self, F> where
    Self: Sized,
    F: FnMut(usize) -> Self::Item, 

Important traits for WithPosition<I>

impl<I: Iterator> Iterator for WithPosition<I> type Item = Position<I::Item>;

fn with_position(self) -> WithPosition<Self> where
    Self: Sized, 

Important traits for Positions<I, F>

impl<I, F> Iterator for Positions<I, F> where
    I: Iterator,
    F: FnMut(I::Item) -> bool,  type Item = usize;

fn positions<P>(self, predicate: P) -> Positions<Self, P> where
    Self: Sized,
    P: FnMut(Self::Item) -> bool, 

Important traits for Update<I, F>

impl<I, F> Iterator for Update<I, F> where
    I: Iterator,
    F: FnMut(&mut I::Item),  type Item = I::Item;

fn update<F>(self, updater: F) -> Update<Self, F> where
    Self: Sized,
    F: FnMut(&mut Self::Item), 

fn next_tuple<T>(&mut self) -> Option<T> where
    Self: Sized + Iterator<Item = T::Item>,
    T: TupleCollect, 

fn collect_tuple<T>(self) -> Option<T> where
    Self: Sized + Iterator<Item = T::Item>,
    T: TupleCollect, 

fn find_position<P>(&mut self, pred: P) -> Option<(usize, Self::Item)> where
    P: FnMut(&Self::Item) -> bool, 

fn all_equal(&mut self) -> bool where
    Self::Item: PartialEq, 

fn dropping(self, n: usize) -> Self where
    Self: Sized, 

fn dropping_back(self, n: usize) -> Self where
    Self: Sized,
    Self: DoubleEndedIterator, 

fn foreach<F>(self, f: F) where
    F: FnMut(Self::Item),
    Self: Sized, 

Deprecated since 0.8:

Use .for_each() instead

fn concat(self) -> Self::Item where
    Self: Sized,
    Self::Item: Extend<<Self::Item as IntoIterator>::Item> + IntoIterator + Default, 

fn collect_vec(self) -> Vec<Self::Item> where
    Self: Sized, 

fn set_from<'a, A: 'a, J>(&mut self, from: J) -> usize where
    Self: Iterator<Item = &'a mut A>,
    J: IntoIterator<Item = A>, 

fn join(&mut self, sep: &str) -> String where
    Self::Item: Display, 

fn format(self, sep: &str) -> Format<Self> where
    Self: Sized, 

fn format_with<F>(self, sep: &str, format: F) -> FormatWith<Self, F> where
    Self: Sized,
    F: FnMut(Self::Item, &mut dyn FnMut(&dyn Display) -> Result) -> Result, 

fn fold_results<A, E, B, F>(&mut self, start: B, f: F) -> Result<B, E> where
    Self: Iterator<Item = Result<A, E>>,
    F: FnMut(B, A) -> B, 

fn fold_options<A, B, F>(&mut self, start: B, f: F) -> Option<B> where
    Self: Iterator<Item = Option<A>>,
    F: FnMut(B, A) -> B, 

fn fold1<F>(self, f: F) -> Option<Self::Item> where
    F: FnMut(Self::Item, Self::Item) -> Self::Item,
    Self: Sized, 

fn tree_fold1<F>(self, f: F) -> Option<Self::Item> where
    F: FnMut(Self::Item, Self::Item) -> Self::Item,
    Self: Sized, 

fn fold_while<B, F>(&mut self, init: B, f: F) -> FoldWhile<B> where
    Self: Sized,
    F: FnMut(B, Self::Item) -> FoldWhile<B>, 

Deprecated since 0.8:

Use .try_fold() instead

fn sorted(self) -> IntoIter<Self::Item> where
    Self: Sized,
    Self::Item: Ord, 

fn sorted_by<F>(self, cmp: F) -> IntoIter<Self::Item> where
    Self: Sized,
    F: FnMut(&Self::Item, &Self::Item) -> Ordering, 

fn sorted_by_key<K, F>(self, f: F) -> IntoIter<Self::Item> where
    Self: Sized,
    K: Ord,
    F: FnMut(&Self::Item) -> K, 

fn partition_map<A, B, F, L, R>(self, predicate: F) -> (A, B) where
    Self: Sized,
    F: Fn(Self::Item) -> Either<L, R>,
    A: Default + Extend<L>,
    B: Default + Extend<R>, 

fn into_group_map<K, V>(self) -> HashMap<K, Vec<V>> where
    Self: Iterator<Item = (K, V)> + Sized,
    K: Hash + Eq, 

fn minmax(self) -> MinMaxResult<Self::Item> where
    Self: Sized,
    Self::Item: PartialOrd, 

fn minmax_by_key<K, F>(self, key: F) -> MinMaxResult<Self::Item> where
    Self: Sized,
    K: PartialOrd,
    F: FnMut(&Self::Item) -> K, 

fn minmax_by<F>(self, compare: F) -> MinMaxResult<Self::Item> where
    Self: Sized,
    F: FnMut(&Self::Item, &Self::Item) -> Ordering, 

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