[ruby-core:68083] [Ruby trunk - Feature #5663] Combined map/select method

Issue #5663 has been updated by Andrew Ledvina.


~~~
The only caveat is that it would be impossible to intentionally return nil here
~~~

I don't see why you need that, just have the block return a pair `(keep_if_true, mapped_value)`. I also would second the name `filter_map`. Ignoring all edge-cases you could just do something like:

```
module Array
  def filter_map
    result = []
    self.each do |elem|
      keep, value = yield elem
      result << value if keep
    end
    result
  end
end

> [1,32,9,33,2,13].filter_map{|v| [v < 20, v%2==1 ? v*2 : nil]]}
 => [2, 18, nil, 26]
```

Although to be fair, `filter_map` is just `reduce` with a more complicated block

```
> [1,32,9,33,2,13].reduce([]){|acc, v| v<20 ? v%2==1 ? acc << v*2 : acc << nil : nil; acc}
 => [2, 18, nil, 26]
```

----------------------------------------
Feature #5663: Combined map/select method
https://bugs.ruby-lang.org/issues/5663#change-51466

* Author: Yehuda Katz
* Status: Assigned
* Priority: Normal
* Assignee: Yukihiro Matsumoto
----------------------------------------
It is pretty common to want to map over an Enumerable, but only include the elements that match a particular filter. A common idiom is:

enum.map { |i| i + 1 if i.even? }.compact

It is of course also possible to do this with two calls:

enum.select { |i| i.even? }.map { |i| i + 1 }

Both cases are clumsy and require two iterations through the loop. I'd like to propose a combined method:

enum.map_select { |i| i + 1 if i.even? }

The only caveat is that it would be impossible to intentionally return nil here; suggestions welcome. The naming is also a strawman; feel free to propose something better.



-- 
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