% Containers and iterators

# Containers

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

## Unique and managed vectors

Vectors have `O(1)` indexing and removal from the end, along with `O(1)`
amortized insertion. 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
`std::vec::bsearch` function, 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::deque` 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.

## 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::iterator` 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)
        }
    }
}
~~~

## Container iterators

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

* `vector.iter()`, for immutable references to the elements
* `vector.mut_iter()`, for mutable references to the elements
* `vector.rev_iter()`, for immutable references to the elements in reverse order
* `vector.mut_rev_iter()`, for mutable references to the elements in reverse 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 `IteratorUtil` trait implements common algorithms as methods extending
every `Iterator` implementation. 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);
~~~

Some 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);
~~~

Note that some adaptors like the `chain_` method above use a trailing
underscore to work around an issue with method resolve. The underscores will be
dropped when they become unnecessary.

## For loops

The `for` loop syntax is currently in transition, and will switch from the old
closure-based iteration protocol to iterator objects. For now, the `advance`
adaptor is required as a compatibility shim to use iterators with for loops.

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

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

// print out all but the first 3 elements in the vector
for xs.iter().skip(3).advance |x| {
    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 it.advance |(x, y)| {
    println(fmt!("%d %s", *x, *y));

    if *x == 3 {
        break;
    }
}

// yield and print the last pair from the iterator
println(fmt!("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).transform(|&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:

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

~~~
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(fmt!("%?", it.next())); // prints `Some(&1)`
println(fmt!("%?", it.next())); // prints `Some(&2)`
println(fmt!("%?", it.next_back())); // prints `Some(&6)`

// prints `5`, `4` and `3`
for it.invert().advance |&x| {
    println(fmt!("%?", x))
}
~~~

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