1 % Containers and iterators
5 The container traits are defined in the `std::container` module.
7 ## Unique and managed vectors
9 Vectors have `O(1)` indexing and removal from the end, along with `O(1)`
10 amortized insertion. Vectors are the most common container in Rust, and are
11 flexible enough to fit many use cases.
13 Vectors can also be sorted and used as efficient lookup tables with the
14 `std::vec::bsearch` function, if all the elements are inserted at one time and
15 deletions are unnecessary.
19 Maps are collections of unique keys with corresponding values, and sets are
20 just unique keys without a corresponding value. The `Map` and `Set` traits in
21 `std::container` define the basic interface.
23 The standard library provides three owned map/set types:
25 * `std::hashmap::HashMap` and `std::hashmap::HashSet`, requiring the keys to
26 implement `Eq` and `Hash`
27 * `std::trie::TrieMap` and `std::trie::TrieSet`, requiring the keys to be `uint`
28 * `extra::treemap::TreeMap` and `extra::treemap::TreeSet`, requiring the keys
29 to implement `TotalOrd`
31 These maps do not use managed pointers so they can be sent between tasks as
32 long as the key and value types are sendable. Neither the key or value type has
35 The `TrieMap` and `TreeMap` maps are ordered, while `HashMap` uses an arbitrary
38 Each `HashMap` instance has a random 128-bit key to use with a keyed hash,
39 making the order of a set of keys in a given hash table randomized. Rust
40 provides a [SipHash](https://131002.net/siphash/) implementation for any type
41 implementing the `IterBytes` trait.
43 ## Double-ended queues
45 The `extra::deque` module implements a double-ended queue with `O(1)` amortized
46 inserts and removals from both ends of the container. It also has `O(1)`
47 indexing like a vector. The contained elements are not required to be copyable,
48 and the queue will be sendable if the contained type is sendable.
52 The `extra::priority_queue` module implements a queue ordered by a key. The
53 contained elements are not required to be copyable, and the queue will be
54 sendable if the contained type is sendable.
56 Insertions have `O(log n)` time complexity and checking or popping the largest
57 element is `O(1)`. Converting a vector to a priority queue can be done
58 in-place, and has `O(n)` complexity. A priority queue can also be converted to
59 a sorted vector in-place, allowing it to be used for an `O(n log n)` in-place
66 The iteration protocol is defined by the `Iterator` trait in the
67 `std::iterator` module. The minimal implementation of the trait is a `next`
68 method, yielding the next element from an iterator object:
71 /// An infinite stream of zeroes
74 impl Iterator<int> for ZeroStream {
75 fn next(&mut self) -> Option<int> {
81 Reaching the end of the iterator is signalled by returning `None` instead of
85 /// A stream of N zeroes
91 fn new(n: uint) -> ZeroStream {
92 ZeroStream { remaining: n }
96 impl Iterator<int> for ZeroStream {
97 fn next(&mut self) -> Option<int> {
98 if self.remaining == 0 {
108 ## Container iterators
110 Containers implement iteration over the contained elements by returning an
111 iterator object. For example, vector slices have four iterators available:
113 * `vector.iter()`, for immutable references to the elements
114 * `vector.mut_iter()`, for mutable references to the elements
115 * `vector.rev_iter()`, for immutable references to the elements in reverse order
116 * `vector.mut_rev_iter()`, for mutable references to the elements in reverse order
120 Unlike most other languages with external iterators, Rust has no *iterator
121 invalidation*. As long an iterator is still in scope, the compiler will prevent
122 modification of the container through another handle.
125 let mut xs = [1, 2, 3];
129 // the vector is frozen for this scope, the compiler will statically
130 // prevent modification
132 // the vector becomes unfrozen again at the end of the scope
135 These semantics are due to most container iterators being implemented with `&`
140 The `IteratorUtil` trait implements common algorithms as methods extending
141 every `Iterator` implementation. For example, the `fold` method will accumulate
142 the items yielded by an `Iterator` into a single value:
145 let xs = [1, 9, 2, 3, 14, 12];
146 let result = xs.iter().fold(0, |accumulator, item| accumulator - *item);
147 assert_eq!(result, -41);
150 Some adaptors return an adaptor object implementing the `Iterator` trait itself:
153 let xs = [1, 9, 2, 3, 14, 12];
154 let ys = [5, 2, 1, 8];
155 let sum = xs.iter().chain_(ys.iter()).fold(0, |a, b| a + *b);
159 Note that some adaptors like the `chain_` method above use a trailing
160 underscore to work around an issue with method resolve. The underscores will be
161 dropped when they become unnecessary.
165 The `for` loop syntax is currently in transition, and will switch from the old
166 closure-based iteration protocol to iterator objects. For now, the `advance`
167 adaptor is required as a compatibility shim to use iterators with for loops.
170 let xs = [2, 3, 5, 7, 11, 13, 17];
172 // print out all the elements in the vector
173 for xs.iter().advance |x| {
177 // print out all but the first 3 elements in the vector
178 for xs.iter().skip(3).advance |x| {
183 For loops are *often* used with a temporary iterator object, as above. They can
184 also advance the state of an iterator in a mutable location:
187 let xs = [1, 2, 3, 4, 5];
188 let ys = ["foo", "bar", "baz", "foobar"];
190 // create an iterator yielding tuples of elements from both vectors
191 let mut it = xs.iter().zip(ys.iter());
193 // print out the pairs of elements up to (&3, &"baz")
194 for it.advance |(x, y)| {
195 println(fmt!("%d %s", *x, *y));
202 // yield and print the last pair from the iterator
203 println(fmt!("last: %?", it.next()));
205 // the iterator is now fully consumed
206 assert!(it.next().is_none());
211 Iterators offer generic conversion to containers with the `collect` adaptor:
214 let xs = [0, 1, 1, 2, 3, 5, 8];
215 let ys = xs.rev_iter().skip(1).transform(|&x| x * 2).collect::<~[int]>();
216 assert_eq!(ys, ~[10, 6, 4, 2, 2, 0]);
219 The method requires a type hint for the container type, if the surrounding code
220 does not provide sufficient information.
222 Containers can provide conversion from iterators through `collect` by
223 implementing the `FromIterator` trait. For example, the implementation for
224 vectors is as follows:
227 impl<A, T: Iterator<A>> FromIterator<A, T> for ~[A] {
228 pub fn from_iterator(iterator: &mut T) -> ~[A] {
229 let (lower, _) = iterator.size_hint();
230 let mut xs = with_capacity(lower);
231 for iterator.advance |x| {
241 The `Iterator` trait provides a `size_hint` default method, returning a lower
242 bound and optionally on upper bound on the length of the iterator:
245 fn size_hint(&self) -> (uint, Option<uint>) { (0, None) }
248 The vector implementation of `FromIterator` from above uses the lower bound
249 to pre-allocate enough space to hold the minimum number of elements the
252 The default implementation is always correct, but it should be overridden if
253 the iterator can provide better information.
255 The `ZeroStream` from earlier can provide an exact lower and upper bound:
258 /// A stream of N zeroes
264 fn new(n: uint) -> ZeroStream {
265 ZeroStream { remaining: n }
268 fn size_hint(&self) -> (uint, Option<uint>) {
269 (self.remaining, Some(self.remaining))
273 impl Iterator<int> for ZeroStream {
274 fn next(&mut self) -> Option<int> {
275 if self.remaining == 0 {
285 ## Double-ended iterators
287 The `DoubleEndedIterator` trait represents an iterator able to yield elements
288 from either end of a range. It inherits from the `Iterator` trait and extends
289 it with the `next_back` function.
291 A `DoubleEndedIterator` can be flipped with the `invert` adaptor, returning
292 another `DoubleEndedIterator` with `next` and `next_back` exchanged.
295 let xs = [1, 2, 3, 4, 5, 6];
296 let mut it = xs.iter();
297 println(fmt!("%?", it.next())); // prints `Some(&1)`
298 println(fmt!("%?", it.next())); // prints `Some(&2)`
299 println(fmt!("%?", it.next_back())); // prints `Some(&6)`
301 // prints `5`, `4` and `3`
302 for it.invert().advance |&x| {
303 println(fmt!("%?", x))
307 The `rev_iter` and `mut_rev_iter` methods on vectors just return an inverted
308 version of the standard immutable and mutable vector iterators.