CODEWITHSUNDEEP
rusthashmap
Hugo Trentesaux
Hugo Trentesaux

Reputation: 1989

How do I collect the values of a HashMap into a vector?

I can not find a way to collect the values of a HashMap into a Vec in the documentation. I have score_table: HashMap<Id, Score> and I want to get all the Scores into all_scores: Vec<Score>.

I was tempted to use the values method (all_scores = score_table.values()), but it does not work since values is not a Vec.

I know that Values implements the ExactSizeIterator trait, but I do not know how to collect all values of an iterator into a vector without manually writing a for loop and pushing the values in the vector one after one.

I also tried to use std::iter::FromIterator; but ended with something like:

all_scores = Vec::from_iter(score_table.values());

expected type `std::vec::Vec<Score>`
   found type `std::vec::Vec<&Score>`

Thanks to Hash map macro refuses to type-check, failing with a misleading (and seemingly buggy) error message?, I changed it to:

all_scores = Vec::from_iter(score_table.values().cloned());

and it does not produce errors to cargo check.

Is this a good way to do it?

Upvotes: 72

Views: 53188

Answers (3)

xy2_
xy2_

Reputation: 7162

There are three useful methods1 on HashMaps, which all return iterators:

  1. values() borrows the collection and returns references (&T).
  2. values_mut() gives mutable references &mut T which is useful to modify elements of the collection without destroying score_table.
  3. into_values() gives you the elements directly: T! The iterator takes ownership of all the elements. This means that score_table no longer owns them, so you can't use score_table anymore!

In your example, you call values() to get &T references, then convert them to owned values T via a clone(), which creates potentially expensive copies.

Instead, if we have an iterator of owned values, then we can convert it to a Vec using Iterator::collect():

let all_scores: Vec<Score> = score_table.into_values().collect();

Sometimes, you may need to specify the collecting type:

let all_scores = score_table.into_values().collect::<Vec<Score>>();

1 This is a common pattern in Rust. In the standard library, and popular crates, you'll often find functions with these (into)_thing_(mut) names, with the presence or absence of these functions letting you know what you can do with that object.

Upvotes: 29

Daniel
Daniel

Reputation: 1507

If you don't need score_table anymore, you can transfer the ownership of Score values to all_scores by:

let all_scores: Vec<Score> = score_table.into_iter()
                                        .map(|(_id, score)| score)
                                        .collect();

This approach will be faster and consume less memory than the clone approach by @apetranzilla. It also supports any struct, not only structs that implement Clone.

Upvotes: 31

apetranzilla
apetranzilla

Reputation: 5939

The method Iterator.collect is designed for this specific task. You're right in that you need .cloned() if you want a vector of actual values instead of references (unless the stored type implements Copy, like primitives), so the code looks like this:

all_scores = score_table.values().cloned().collect();

Internally, collect() just uses FromIterator, but it also infers the type of the output. Sometimes there isn't enough information to infer the type, so you may need to explicitly specify the type you want, like so:

all_scores = score_table.values().cloned().collect::<Vec<Score>>();

Upvotes: 70

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