map, filter, and collect
map, filter, and collect let you transform, pick out, and finally gather items from a list, one clean step at a time.In this page:
let result: Vec<Type> = collection
.iter()
.map(|x| expression)
.filter(|x| condition)
.collect();Transforming with map
.map() applies a closure to every element of an iterator, producing a new iterator of the transformed results without modifying the original collection.
Example: Transforming with map
// Entry point: execution of the program starts here
fn main() {
// Declare an immutable variable `numbers`, set to `vec![1, 2, 3, 4]`
let numbers = vec![1, 2, 3, 4];
// Declare an immutable variable `squared` (a `Vec<i32>`), set to `numbers.iter().map(|n| n * n).collect()`
let squared: Vec<i32> = numbers.iter().map(|n| n * n).collect();
// Print "{:?}" to the console, substituting in `squared` (with a trailing newline)
println!("{:?}", squared);
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Selecting with filter
.filter() keeps only the elements for which the given closure returns true, discarding everything else from the resulting iterator.
Example: Selecting with filter
// Entry point: execution of the program starts here
fn main() {
// Declare an immutable variable `numbers`, set to `vec![1, 2, 3, 4, 5, 6]`
let numbers = vec![1, 2, 3, 4, 5, 6];
// Declare an immutable variable `evens` (a `Vec<&i32>`), set to `numbers.iter().filter(|&&n| n % 2 == 0).collect()`
let evens: Vec<&i32> = numbers.iter().filter(|&&n| n % 2 == 0).collect();
// Print "{:?}" to the console, substituting in `evens` (with a trailing newline)
println!("{:?}", evens);
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Gathering Results with collect
.collect() consumes an iterator and gathers its elements into a concrete collection type, such as Vec<T> or a String, inferred from context or an explicit type annotation.
Example: Gathering Results with collect
// Entry point: execution of the program starts here
fn main() {
// Declare an immutable variable `words`, set to `vec!["Rust", "is", "fun"]`
let words = vec!["Rust", "is", "fun"];
// Declare an immutable variable `joined` (a String), set to `words.iter().map(|w| w.to_uppercase()).collect::<Vec<String>>().join(" ")`
let joined: String = words.iter().map(|w| w.to_uppercase()).collect::<Vec<String>>().join(" ");
// Print "{}" to the console, substituting in `joined` (with a trailing newline)
println!("{}", joined);
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Chaining map and filter Together
.map() and .filter() can be chained in sequence to express a multi-step transformation clearly and efficiently, since both are lazy and only run once .collect() consumes the pipeline.
Example: Chaining map and filter Together
// Entry point: execution of the program starts here
fn main() {
// Declare an immutable variable `numbers`, set to `vec![1, 2, 3, 4, 5, 6, 7, 8]`
let numbers = vec![1, 2, 3, 4, 5, 6, 7, 8];
// Declare an immutable variable `result` (a `Vec<i32>`), set to `numbers.iter().filter(|&&n| n % 2 == 0).map(|n| n * 10).collect()`
let result: Vec<i32> = numbers.iter().filter(|&&n| n % 2 == 0).map(|n| n * 10).collect();
// Print "{:?}" to the console, substituting in `result` (with a trailing newline)
println!("{:?}", result);
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- Forgetting to call
.collect()(or another consuming method) at the end of a chain, since.map()/.filter()alone produce a lazy iterator, not a final collection. - Not annotating the target type for
.collect()when it cannot be inferred, leading to a compile error about an ambiguous type. - Chaining
.filter()after.map()when filtering first would avoid transforming elements that will just be thrown away.
.map(closure)transforms each element of an iterator, producing a new iterator of transformed values..filter(closure)keeps only the elements for which the closure returnstrue..collect()consumes an iterator and gathers its items into a concrete collection, likeVec<T>.- These three methods are commonly chained together to express a full data transformation pipeline concisely.
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