330 lines
7.5 KiB
Markdown
330 lines
7.5 KiB
Markdown
# Iterator
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The iterator pattern allows us to perform some tasks on a sequence of items in turn. An iterator is responsible for the logic of iterating over each item and determining when the sequence has finished.
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## for and iterator
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```rust
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fn main() {
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let v = vec![1, 2, 3];
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for x in v {
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println!("{}",x)
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}
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}
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```
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In above code, You may consider `for` as a simple loop, but actually it is iterating over a iterator.
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By default `for` will apply the `into_iter` to the collection, and change it into a iterator. As a result, the following code is equivalent to previous one:
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```rust
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fn main() {
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let v = vec![1, 2, 3];
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for x in v.into_iter() {
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println!("{}",x)
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}
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}
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```
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1. π
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```rust,editable
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/* Refactoring the following code using iterators */
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fn main() {
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let arr = [0; 10];
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for i in 0..arr.len() {
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println!("{}",arr[i])
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}
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}
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```
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2. π One of the easiest ways to create an iterator is to use the range notion: `a..b`.
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```rust,editable
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/* Fill in the blank */
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fn main() {
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let mut v = Vec::new();
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for n in __ {
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v.push(n);
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}
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assert_eq!(v.len(), 100);
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}
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```
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## next method
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All iterators implement a trait named `Iterator` that is defined in the standard library:
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```rust
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pub trait Iterator {
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type Item;
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fn next(&mut self) -> Option<Self::Item>;
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// methods with default implementations elided
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}
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```
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And we can call the `next` method on iterators directly.
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3. ππ
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```rust,editable
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/* Fill the blanks and fix the errors.
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Using two ways if possible */
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fn main() {
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let v1 = vec![1, 2];
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assert_eq!(v1.next(), __);
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assert_eq!(v1.next(), __);
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assert_eq!(v1.next(), __);
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}
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```
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## into_iter, iter and iter_mut
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In the previous section, we have mentioned that `for` will apply the `into_iter` to the collection, and change it into a iterator.However, this is not the only way to convert collections into iterators.
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`into_iter`, `iter`, `iter_mut`, all of them can convert an collection into iterator, but in different ways.
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- `into_iter` cosumes the collection, once the collection has been comsumed, it is no longer available for reuse, because its ownership has been moved within the loop.
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- `iter`, this borrows each element of the collection through each iteration, thus leaving the collection untouched and available for reuse after the loop
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- `iter_mut`, this mutably borrows each element of the collection, allowing for the collection to be modified in place.
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4. π
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```rust,editable
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/* Make it work */
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fn main() {
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let arr = vec![0; 10];
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for i in arr {
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println!("{}", i)
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}
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println!("{:?}",arr);
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}
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```
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5. π
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```rust,editable
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/* Fill in the blank */
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fn main() {
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let mut names = vec!["Bob", "Frank", "Ferris"];
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for name in names.__{
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*name = match name {
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&mut "Ferris" => "There is a rustacean among us!",
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_ => "Hello",
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}
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}
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println!("names: {:?}", names);
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}
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```
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6. ππ
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```rust,editable
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/* Fill in the blank */
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fn main() {
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let mut values = vec![1, 2, 3];
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let mut values_iter = values.__;
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if let Some(v) = values_iter.__{
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__
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}
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assert_eq!(values, vec![0, 2, 3]);
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}
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```
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## Creating our own iterator
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We can not only create iterators from collections types, but also can create iterators by implementing the `Iterator` trait on our own types.
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**Example**
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```rust
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struct Counter {
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count: u32,
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}
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impl Counter {
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fn new() -> Counter {
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Counter { count: 0 }
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}
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}
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impl Iterator for Counter {
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type Item = u32;
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fn next(&mut self) -> Option<Self::Item> {
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if self.count < 5 {
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self.count += 1;
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Some(self.count)
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} else {
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None
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}
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}
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}
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fn main() {
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let mut counter = Counter::new();
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assert_eq!(counter.next(), Some(1));
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assert_eq!(counter.next(), Some(2));
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assert_eq!(counter.next(), Some(3));
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assert_eq!(counter.next(), Some(4));
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assert_eq!(counter.next(), Some(5));
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assert_eq!(counter.next(), None);
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}
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```
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7. πππ
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```rust,editable
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struct Fibonacci {
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curr: u32,
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next: u32,
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}
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// Implement `Iterator` for `Fibonacci`.
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// The `Iterator` trait only requires a method to be defined for the `next` element.
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impl Iterator for Fibonacci {
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// We can refer to this type using Self::Item
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type Item = u32;
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/* Implement next method */
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fn next(&mut self)
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}
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// Returns a Fibonacci sequence generator
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fn fibonacci() -> Fibonacci {
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Fibonacci { curr: 0, next: 1 }
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}
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fn main() {
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let mut fib = fibonacci();
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assert_eq!(fib.next(), Some(1));
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assert_eq!(fib.next(), Some(1));
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assert_eq!(fib.next(), Some(2));
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assert_eq!(fib.next(), Some(3));
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assert_eq!(fib.next(), Some(5));
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}
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```
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## Methods that Consume the Iterator
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The `Iterator` trait has a number of methods with default implementations provided by the standard library.
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### Consuming adaptors
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Some of these methods call the method `next`to use up the iterator, so they are called *consuming adaptors*.
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8. ππ
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```rust,edtiable
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/* Fill in the blank and fix the errors */
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fn main() {
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let v1 = vec![1, 2, 3];
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let v1_iter = v1.iter();
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// The sum method will take the ownership of the iterator and iterates through the items by repeatedly calling next method
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let total = v1_iter.sum();
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assert_eq!(total, __);
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println!("{:?}, {:?}",v1, v1_iter);
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}
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```
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#### collect
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Other than converting a collection into an iterator, we can also `collect` the result values into a collection, `collect` will cosume the iterator.
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9. ππ
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```rust,editable
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/* Make it work */
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use std::collections::HashMap;
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fn main() {
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let names = [("sunface",18), ("sunfei",18)];
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let folks: HashMap<_, _> = names.into_iter().collect();
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println!("{:?}",folks);
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let v1: Vec<i32> = vec![1, 2, 3];
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let v2 = v1.iter().collect();
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assert_eq!(v2, vec![1, 2, 3]);
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}
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```
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### Iterator adaptors
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Methods allowing you to change one iterator into another iterator are known as *iterator adaptors*. You can chain multiple iterator adaptors to perform complex actions in a readable way.
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But because **all iterators are lazy**, you have to call one of the consuming adapers to get results from calls to iterator adapters.
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10. ππ
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```rust,editable
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/* Fill in the blanks */
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fn main() {
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let v1: Vec<i32> = vec![1, 2, 3];
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let v2: Vec<_> = v1.iter().__.__;
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assert_eq!(v2, vec![2, 3, 4]);
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}
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```
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11. ππ
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```rust
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/* Fill in the blanks */
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use std::collections::HashMap;
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fn main() {
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let names = ["sunface", "sunfei"];
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let ages = [18, 18];
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let folks: HashMap<_, _> = names.into_iter().__.collect();
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println!("{:?}",folks);
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}
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```
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#### Using closures in iterator adaptors
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12. ππ
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```rust
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/* Fill in the blanks */
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#[derive(PartialEq, Debug)]
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struct Shoe {
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size: u32,
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style: String,
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}
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fn shoes_in_size(shoes: Vec<Shoe>, shoe_size: u32) -> Vec<Shoe> {
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shoes.into_iter().__.collect()
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}
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fn main() {
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let shoes = vec![
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Shoe {
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size: 10,
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style: String::from("sneaker"),
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},
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Shoe {
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size: 13,
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style: String::from("sandal"),
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},
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Shoe {
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size: 10,
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style: String::from("boot"),
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},
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];
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let in_my_size = shoes_in_size(shoes, 10);
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assert_eq!(
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in_my_size,
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vec![
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Shoe {
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size: 10,
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style: String::from("sneaker")
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},
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Shoe {
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size: 10,
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style: String::from("boot")
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},
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]
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);
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}
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```
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