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

Question

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?

Answer 1

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>>();

Answer 2

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.

Answer 3

When using .clone(), you are explicitly duplicating the value, which is slower. This is fine if you plan to keep using the hashmap afterwards.

However, if you don't want to keep score_table afterwards, use HashMap.into_values. It will return an owning iterator over the values:

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

In contrast to other answers, HashMap.values's iterator is not owning and returns references (&Score) instead of values.