How do I collect the values of a HashMap into a vector?
Solution 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>>();
Solution 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
.
Solution 3
HashMap.into_values
will return an owning iterator over the values:
let all_scores: Vec<Score> = score_table.into_values().collect();
In contrast, HashMap.values
's iterator is not owning and returns references (&Score
) instead of values.
![Hugo Trentesaux](https://lh5.googleusercontent.com/-syObEwk9Mds/AAAAAAAAAAI/AAAAAAAAARU/Qz5nNseD1Pc/photo.jpg?sz=256)
Comments
-
Hugo Trentesaux about 2 years
I can not find a way to collect the values of a
HashMap
into aVec
in the documentation. I havescore_table: HashMap<Id, Score>
and I want to get all theScore
s intoall_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 aVec
.I know that
Values
implements theExactSizeIterator
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?