collect

The collect method lets you build a collection from an Iterator.

// Copyright 2024 Google LLC
// SPDX-License-Identifier: Apache-2.0

fn main() {
    let primes = vec![2, 3, 5, 7];
    let prime_squares = primes.into_iter().map(|p| p * p).collect::<Vec<_>>();
    println!("prime_squares: {prime_squares:?}");
}
This slide should take about 5 minutes.
  • Any iterator can be collected in to a Vec, VecDeque, or HashSet. Iterators that produce key-value pairs (i.e. a two-element tuple) can also be collected into HashMap and BTreeMap.

Show the students the definition for collect in the standard library docs. There are two ways to specify the generic type B for this method:

  • With the “turbofish”: some_iterator.collect::<COLLECTION_TYPE>(), as shown. The _ shorthand used here lets Rust infer the type of the Vec elements.
  • With type inference: let prime_squares: Vec<_> = some_iterator.collect(). Rewrite the example to use this form.

More to Explore

  • If students are curious about how this works, you can bring up the FromIterator trait, which defines how each type of collection gets built from an iterator.
  • In addition to the basic implementations of FromIterator for Vec, HashMap, etc., there are also more specialized implementations which let you do cool things like convert an Iterator<Item = Result<V, E>> into a Result<Vec<V>, E>.
  • The reason type annotations are often needed with collect is because it’s generic over its return type. This makes it harder for the compiler to infer the correct type in many cases.