Iterating Collections
In this page:
Iterating by Reference with iter
.iter() produces an iterator of references to each element, leaving the original collection untouched and still usable afterward.
Example: Iterating by Reference with iter
fn main() {
let numbers = vec![1, 2, 3];
for n in numbers.iter() {
println!("Value: {}", n);
}
println!("Still usable: {:?}", numbers);
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Mutating in Place with iter_mut
.iter_mut() produces mutable references to each element, allowing you to modify the collection's contents directly during iteration.
Example: Mutating in Place with iter_mut
fn main() {
let mut numbers = vec![1, 2, 3];
for n in numbers.iter_mut() {
*n *= 10;
}
println!("{:?}", numbers);
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Taking Ownership with into_iter
Iterating a collection by value (with into_iter(), or a plain for x in collection) consumes the collection, moving each element out one at a time.
Example: Taking Ownership with into_iter
fn main() {
let words = vec![String::from("a"), String::from("b")];
for word in words.into_iter() {
println!("Owned word: {}", word);
}
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Chaining Iterator Adapters
Iterator methods like .map() and .filter() can be chained together to transform and select elements declaratively before finally collecting or summing the results.
Example: Chaining Iterator Adapters
fn main() {
let numbers = vec![1, 2, 3, 4, 5];
let sum_of_squares: i32 = numbers.iter().map(|n| n * n).filter(|n| n % 2 == 0).sum();
println!("Sum of even squares: {}", sum_of_squares);
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- Using
for item in collection(taking ownership) when you only meant to read the items, needing.iter()instead. - Forgetting
.iter_mut()exists for modifying every element of a collection in place during iteration. - Assuming
.iter()on aVec<T>yieldsTvalues directly, when it actually yields&Treferences.
.iter()yields immutable references to each element without taking ownership of the collection..iter_mut()yields mutable references, letting you modify elements in place during iteration.into_iter()(or a barefor x in collection) takes ownership of the collection and its elements.- Iterator adapters like
.map(),.filter(), and.sum()can be chained together to process collections declaratively.
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