% Containers and iterators

# Containers

The container traits are defined in the `std::container` module.

## Unique vectors

Vectors have `O(1)` indexing, push (to the end) and pop (from the end). Vectors
are the most common container in Rust, and are flexible enough to fit many use
cases.

Vectors can also be sorted and used as efficient lookup tables with the
`bsearch()` method, if all the elements are inserted at one time and
deletions are unnecessary.

## Maps and sets

Maps are collections of unique keys with corresponding values, and sets are
just unique keys without a corresponding value. The `Map` and `Set` traits in
`std::container` define the basic interface.

The standard library provides three owned map/set types:

* `std::hashmap::HashMap` and `std::hashmap::HashSet`, requiring the keys to
  implement `Eq` and `Hash`
* `std::trie::TrieMap` and `std::trie::TrieSet`, requiring the keys to be `uint`
* `extra::treemap::TreeMap` and `extra::treemap::TreeSet`, requiring the keys
  to implement `TotalOrd`

These maps do not use managed pointers so they can be sent between tasks as
long as the key and value types are sendable. Neither the key or value type has
to be copyable.

The `TrieMap` and `TreeMap` maps are ordered, while `HashMap` uses an arbitrary
order.

Each `HashMap` instance has a random 128-bit key to use with a keyed hash,
making the order of a set of keys in a given hash table randomized. Rust
provides a [SipHash](https://131002.net/siphash/) implementation for any type
implementing the `IterBytes` trait.

## Double-ended queues

The `extra::ringbuf` module implements a double-ended queue with `O(1)`
amortized inserts and removals from both ends of the container. It also has
`O(1)` indexing like a vector. The contained elements are not required to be
copyable, and the queue will be sendable if the contained type is sendable.
Its interface `Deque` is defined in `extra::collections`.

The `extra::dlist` module implements a double-ended linked list, also
implementing the `Deque` trait, with `O(1)` removals and inserts at either end,
and `O(1)` concatenation.

## Priority queues

The `extra::priority_queue` module implements a queue ordered by a key.  The
contained elements are not required to be copyable, and the queue will be
sendable if the contained type is sendable.

Insertions have `O(log n)` time complexity and checking or popping the largest
element is `O(1)`. Converting a vector to a priority queue can be done
in-place, and has `O(n)` complexity. A priority queue can also be converted to
a sorted vector in-place, allowing it to be used for an `O(n log n)` in-place
heapsort.

# Iterators

## Iteration protocol

The iteration protocol is defined by the `Iterator` trait in the
`std::iter` module. The minimal implementation of the trait is a `next`
method, yielding the next element from an iterator object:

~~~
/// An infinite stream of zeroes
struct ZeroStream;

impl Iterator<int> for ZeroStream {
    fn next(&mut self) -> Option<int> {
        Some(0)
    }
}
~~~~

Reaching the end of the iterator is signalled by returning `None` instead of
`Some(item)`:

~~~
/// A stream of N zeroes
struct ZeroStream {
    priv remaining: uint
}

impl ZeroStream {
    fn new(n: uint) -> ZeroStream {
        ZeroStream { remaining: n }
    }
}

impl Iterator<int> for ZeroStream {
    fn next(&mut self) -> Option<int> {
        if self.remaining == 0 {
            None
        } else {
            self.remaining -= 1;
            Some(0)
        }
    }
}
~~~

In general, you cannot rely on the behavior of the `next()` method after it has
returned `None`. Some iterators may return `None` forever. Others may behave
differently.

## Container iterators

Containers implement iteration over the contained elements by returning an
iterator object. For example, vector slices several iterators available:

* `iter()` and `rev_iter()`, for immutable references to the elements
* `mut_iter()` and `mut_rev_iter()`, for mutable references to the elements
* `move_iter()` and `move_rev_iter()`, to move the elements out by-value

A typical mutable container will implement at least `iter()`, `mut_iter()` and
`move_iter()` along with the reverse variants if it maintains an order.

### Freezing

Unlike most other languages with external iterators, Rust has no *iterator
invalidation*. As long an iterator is still in scope, the compiler will prevent
modification of the container through another handle.

~~~
let mut xs = [1, 2, 3];
{
    let _it = xs.iter();

    // the vector is frozen for this scope, the compiler will statically
    // prevent modification
}
// the vector becomes unfrozen again at the end of the scope
~~~

These semantics are due to most container iterators being implemented with `&`
and `&mut`.

## Iterator adaptors

The `Iterator` trait provides many common algorithms as default methods. For
example, the `fold` method will accumulate the items yielded by an `Iterator`
into a single value:

~~~
let xs = [1, 9, 2, 3, 14, 12];
let result = xs.iter().fold(0, |accumulator, item| accumulator - *item);
assert_eq!(result, -41);
~~~

Most adaptors return an adaptor object implementing the `Iterator` trait itself:

~~~
let xs = [1, 9, 2, 3, 14, 12];
let ys = [5, 2, 1, 8];
let sum = xs.iter().chain(ys.iter()).fold(0, |a, b| a + *b);
assert_eq!(sum, 57);
~~~

Some iterator adaptors may return `None` before exhausting the underlying
iterator. Additionally, if these iterator adaptors are called again after
returning `None`, they may call their underlying iterator again even if the
adaptor will continue to return `None` forever. This may not be desired if the
underlying iterator has side-effects.

In order to provide a guarantee about behavior once `None` has been returned, an
iterator adaptor named `fuse()` is provided. This returns an iterator that will
never call its underlying iterator again once `None` has been returned:

~~~
let xs = [1,2,3,4,5];
let mut calls = 0;
let it = xs.iter().scan((), |_, x| {
    calls += 1;
    if *x < 3 { Some(x) } else { None }});
// the iterator will only yield 1 and 2 before returning None
// If we were to call it 5 times, calls would end up as 5, despite only 2 values
// being yielded (and therefore 3 unique calls being made). The fuse() adaptor
// can fix this.
let mut it = it.fuse();
it.next();
it.next();
it.next();
it.next();
it.next();
assert_eq!(calls, 3);
~~~

## For loops

The function `range` (or `range_inclusive`) allows to simply iterate through a given range:

~~~
for i in range(0, 5) {
  print!("{} ", i) // prints "0 1 2 3 4"
}

for i in std::iter::range_inclusive(0, 5) { // needs explicit import
  print!("{} ", i) // prints "0 1 2 3 4 5"
}
~~~

The `for` keyword can be used as sugar for iterating through any iterator:

~~~
let xs = [2u, 3, 5, 7, 11, 13, 17];

// print out all the elements in the vector
for x in xs.iter() {
    println(x.to_str())
}

// print out all but the first 3 elements in the vector
for x in xs.iter().skip(3) {
    println(x.to_str())
}
~~~

For loops are *often* used with a temporary iterator object, as above. They can
also advance the state of an iterator in a mutable location:

~~~
let xs = [1, 2, 3, 4, 5];
let ys = ["foo", "bar", "baz", "foobar"];

// create an iterator yielding tuples of elements from both vectors
let mut it = xs.iter().zip(ys.iter());

// print out the pairs of elements up to (&3, &"baz")
for (x, y) in it {
    println!("{} {}", *x, *y);

    if *x == 3 {
        break;
    }
}

// yield and print the last pair from the iterator
println!("last: {:?}", it.next());

// the iterator is now fully consumed
assert!(it.next().is_none());
~~~

## Conversion

Iterators offer generic conversion to containers with the `collect` adaptor:

~~~
let xs = [0, 1, 1, 2, 3, 5, 8];
let ys = xs.rev_iter().skip(1).map(|&x| x * 2).collect::<~[int]>();
assert_eq!(ys, ~[10, 6, 4, 2, 2, 0]);
~~~

The method requires a type hint for the container type, if the surrounding code
does not provide sufficient information.

Containers can provide conversion from iterators through `collect` by
implementing the `FromIterator` trait. For example, the implementation for
vectors is as follows:

~~~ {.xfail-test}
impl<A> FromIterator<A> for ~[A] {
    pub fn from_iterator<T: Iterator<A>>(iterator: &mut T) -> ~[A] {
        let (lower, _) = iterator.size_hint();
        let mut xs = with_capacity(lower);
        for x in iterator {
            xs.push(x);
        }
        xs
    }
}
~~~

### Size hints

The `Iterator` trait provides a `size_hint` default method, returning a lower
bound and optionally on upper bound on the length of the iterator:

~~~ {.xfail-test}
fn size_hint(&self) -> (uint, Option<uint>) { (0, None) }
~~~

The vector implementation of `FromIterator` from above uses the lower bound
to pre-allocate enough space to hold the minimum number of elements the
iterator will yield.

The default implementation is always correct, but it should be overridden if
the iterator can provide better information.

The `ZeroStream` from earlier can provide an exact lower and upper bound:

~~~
/// A stream of N zeroes
struct ZeroStream {
    priv remaining: uint
}

impl ZeroStream {
    fn new(n: uint) -> ZeroStream {
        ZeroStream { remaining: n }
    }

    fn size_hint(&self) -> (uint, Option<uint>) {
        (self.remaining, Some(self.remaining))
    }
}

impl Iterator<int> for ZeroStream {
    fn next(&mut self) -> Option<int> {
        if self.remaining == 0 {
            None
        } else {
            self.remaining -= 1;
            Some(0)
        }
    }
}
~~~

## Double-ended iterators

The `DoubleEndedIterator` trait represents an iterator able to yield elements
from either end of a range. It inherits from the `Iterator` trait and extends
it with the `next_back` function.

A `DoubleEndedIterator` can be flipped with the `invert` adaptor, returning
another `DoubleEndedIterator` with `next` and `next_back` exchanged.

~~~
let xs = [1, 2, 3, 4, 5, 6];
let mut it = xs.iter();
println!("{:?}", it.next()); // prints `Some(&1)`
println!("{:?}", it.next()); // prints `Some(&2)`
println!("{:?}", it.next_back()); // prints `Some(&6)`

// prints `5`, `4` and `3`
for &x in it.invert() {
    println!("{}", x)
}
~~~

The `rev_iter` and `mut_rev_iter` methods on vectors just return an inverted
version of the standard immutable and mutable vector iterators.

The `chain`, `map`, `filter`, `filter_map` and `inspect` adaptors are
`DoubleEndedIterator` implementations if the underlying iterators are.

~~~
let xs = [1, 2, 3, 4];
let ys = [5, 6, 7, 8];
let mut it = xs.iter().chain(ys.iter()).map(|&x| x * 2);

println!("{:?}", it.next()); // prints `Some(2)`

// prints `16`, `14`, `12`, `10`, `8`, `6`, `4`
for x in it.invert() {
    println!("{}", x);
}
~~~

The `reverse_` method is also available for any double-ended iterator yielding
mutable references. It can be used to reverse a container in-place. Note that
the trailing underscore is a workaround for issue #5898 and will be removed.

~~~
let mut ys = [1, 2, 3, 4, 5];
ys.mut_iter().reverse_();
assert_eq!(ys, [5, 4, 3, 2, 1]);
~~~

## Random-access iterators

The `RandomAccessIterator` trait represents an iterator offering random access
to the whole range. The `indexable` method retrieves the number of elements
accessible with the `idx` method.

The `chain` adaptor is an implementation of `RandomAccessIterator` if the
underlying iterators are.

~~~
let xs = [1, 2, 3, 4, 5];
let ys = ~[7, 9, 11];
let mut it = xs.iter().chain(ys.iter());
println!("{:?}", it.idx(0)); // prints `Some(&1)`
println!("{:?}", it.idx(5)); // prints `Some(&7)`
println!("{:?}", it.idx(7)); // prints `Some(&11)`
println!("{:?}", it.idx(8)); // prints `None`

// yield two elements from the beginning, and one from the end
it.next();
it.next();
it.next_back();

println!("{:?}", it.idx(0)); // prints `Some(&3)`
println!("{:?}", it.idx(4)); // prints `Some(&9)`
println!("{:?}", it.idx(6)); // prints `None`
~~~