Inversion of &: in Ruby

Question

In Ruby I'm selecting some hash values by key names this way

new_values = my_hash.values_at(:value2, :value3, :value6, :value8).select {|a| !a.empty?}

and what about this? It's not working.

new_values = my_hash.values_at(:value2, :value3, :value6, :value8).select(&:!empty?)

you cannot directly use symbol-to-proc for that, but you can implement the logically negated `Object#present?` method (like active_support does) which will probably useful somewhere else. — tokland, Sep 04 '12 at 10:45

score 4 · Accepted Answer · answered Sep 04 '12 at 10:30

4

As Sergio explained there is no way to do this with standard Ruby since the expression you are using won't parse. One can hack something together so it works though (of course reject would be the way to go here):

['1', '', '11', ''].select(&fn(:empty?, :!))
#=> ["1", "11"]

Here's the implementation of fn as used in my extension library:

def fn(*funs)
  -> x do
    funs.inject(x) do |v,f|
      Proc === f ? f.call(v) : v.send(f)
    end
  end
end

answered Sep 04 '12 at 10:30

Michael Kohl

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I think, he wanted to do that to save a couple of characters. This approach does neither save space, nor is cleaner. But it's nice to have options :) – Sergio Tulentsev Sep 04 '12 at 10:41
Sure, but he said he wanted alternatives. :-) I upvoted your answer cause it makes the most sense to me. – Michael Kohl Sep 04 '12 at 12:05

score 3 · Answer 2 · answered Sep 04 '12 at 10:20

3

It doesn't work, because there's no method with name !empty?. And this name is illegal in ruby, AFAIK. You either have to use full lambda form (as in your first snippet) or use reject

['1', '', '11', ''].select(&:empty?) # => ["", ""]
['1', '', '11', ''].reject(&:empty?) # => ["1", "11"]

answered Sep 04 '12 at 10:20

Sergio Tulentsev

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No, I don't need to reject empty values. I need to make an inversion. – Sep 04 '12 at 10:22
1

@Grienders: why don't you want to reject empty values? Result is the same. – Sergio Tulentsev Sep 04 '12 at 10:24
I want to know an alternative way. – Sep 04 '12 at 10:29

Victor Moroz · Answer 3 · 2012-09-04T14:12:35.777

1

Talking about alternatives:

Not = 
  lambda do |x|
    p = x.to_proc 
    lambda do |*ys|
      !p[*ys]
    end
  end

['1', '', '11', ''].select(&Not[:empty?])

Can be easily extended to more interesting cases like compositions, fanouts.

edited Sep 04 '12 at 14:12

answered Sep 04 '12 at 13:54

Victor Moroz

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Why is it named with "N" but "n"? – Sep 05 '12 at 17:32
Because it's a constant in this particular case (I usually place it in a separate file and `require` it when I need it) and can be used everywhere. – Victor Moroz Sep 05 '12 at 20:53
then why isn't it named NOT? – Jun 08 '14 at 03:55

score 0 · Answer 4 · answered Sep 04 '12 at 10:44

0

The inverse method for empty? is any?. With that you can achieve your goal by using

new_values = my_hash.values_at(:value2, :value3, :value6, :value8).select(&:any?)

answered Sep 04 '12 at 10:44

Holger Just

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As @Sergio said above, only if those are arrays, there may be strings. – tokland Sep 04 '12 at 10:48
`present?` is only available with ActiveSupport. So if this is actually important (and the OP doesn't use Rails), it is probably a good idea to just use the long block form. – Holger Just Sep 04 '12 at 10:54
I agree, the present? was just a side suggestion. – tokland Sep 04 '12 at 11:11

score 0 · Answer 5 · edited May 23 '17 at 11:49

0

In the spirit of my answer here:

class Symbol
  def on
    ->(*args) { yield(*args).send(self) }
  end
end

['1', '', '11', ''].select(&:!.on(&:empty?))
# => ["1", "11"]

edited May 23 '17 at 11:49

Community

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answered Jun 03 '14 at 08:49

Uri Agassi

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Inversion of &: in Ruby

5 Answers5