[ruby-core:97595] [Ruby master Feature#16739] Allow Hash#keys and Hash#values to accept a block for filtering output

Issue #16739 has been updated by mame (Yusuke Endoh).


I doubt if it is obvious.  See the following code.  I believe that many people expect `.map`.

```ruby
hash.keys {|k| k.to_s }
```

If you want to avoid an intermediate array, you may want to use `.each_key` or `.filter_map`.

```ruby
hash.each_key.select {|k| k.nil? || k.even? }
hash.filter_map {|k, v| k if valid_key?(k) && valid_value?(v) }
```

Personally I like the following explicit code, though.

```ruby
keys = []
hash.each do |k, v|
  keys << k if valid_key?(k) && valid_value?(v)
end
```

----------------------------------------
Feature #16739: Allow Hash#keys and Hash#values to accept a block for filtering output
https://bugs.ruby-lang.org/issues/16739#change-84779

* Author: jacobevelyn (Jacob Evelyn)
* Status: Open
* Priority: Normal
----------------------------------------
I often see code like the following:

``` ruby
hash.select { |_, v| v == :some_value }.keys
hash.keys.select { |k| k.nil? || k.even? }
hash.select { |k, v| valid_key?(k) && valid_value?(v) }.values
```

Each of these code snippets must allocate an intermediate data structure. I propose allowing `Hash#keys` and `Hash#values` to accept optional block parameters that *take both key and value*. For example, the above code could be rewritten as:

```ruby
hash.keys { |_, v| v == :some_value }
hash.keys { |k, _| k.nil? || k.even? }
hash.values { |k, v| valid_key?(k) && valid_value?(v) }
```

This behavior:

1. Does not break any existing code (since `Hash#keys` and `Hash#values` do not currently accept blocks).
2. Is very readable—it's obvious what it does at a glance.
3. Is more efficient than current alternatives.
4. Is more concise than current alternatives.
5. Is flexible and useful in a variety of scenarios, because the block has access to both key and value (unlike the behavior proposed in #14788).



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