[ruby-core:69535] [Ruby trunk - Feature #9108] Hash sub-selections

Issue #9108 has been updated by Yukihiro Matsumoto.


I prefer use of `Hash#select`, but in form of `hash.select([:foo, :bar])`, since it may consume too much stack region, besides that `hash.select(*arg)` could work differently when `arg=[]`. It would cause confusion.

Matz.


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Feature #9108: Hash sub-selections
https://bugs.ruby-lang.org/issues/9108#change-52863

* Author: Tom Wardrop
* Status: Open
* Priority: Normal
* Assignee: Yukihiro Matsumoto
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Hi,

I seem to regularly have the requirement to work on a sub-set of key/value pairs within a hash. Ruby doesn't seem to provide a concise means of selecting a sub-set of keys from a hash. To give an example of what I mean, including how I currently achieve this:

```ruby
    sounds = {dog: 'woof', cat: 'meow', mouse: 'squeak', horse: 'nay', cow: 'moo'}
    domestic_sounds = sounds.select { |k,v| [:dog, :cat].include? k } #=> {dog: 'woof', cat: 'meow'}
```

I think a more concise and graceful solution to this would be to allow the Hash#[] method to take multiple arguments, returning a sub-hash, e.g.

```ruby
    domestic_sounds = sounds[:dog, :cat] #=> {dog: 'woof', cat: 'meow'}
```

I had a requirement in the current project I'm working on to concatenate two values in a hash. If this proposed feature existed, I could of just done this...

```ruby
    sounds[:dog, :cat].values.join #=> 'woofmeow'
```

You could do something similar for the setter also...

```ruby
    sounds[:monkey, :bat] = 'screech'
    sounds #=> {dog: 'woof', cat: 'meow', mouse: 'squeak', horse: 'nay', cow: 'moo', monkey: 'screech', bat: 'screech'}
```

Concise, convenient and readable. Thoughts?




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