Double "!!" in Ruby

Question

Possible Duplicate:
What does !! mean in ruby?
Ruby, !! operator (a/k/a the double-bang)

Sometimes I see a Ruby code like this

def sent?
  !!@sent_at
end

It seems to be not logical. Is it necessary to use here double !? As far as I'm concerned, it might be just

def sent?
  @sent_at
end

UPDATE: then what is the difference between these

def sent?
  !!@sent_at
end

def sent?
  @sent_at.nil?
end

def sent?
  @sent_at == nil
end

Hey man I updated my answer. (plz delete this reply after seeing it) — wecing, Sep 08 '12 at 23:42

wecing · Accepted Answer · 2012-09-08T23:36:58.730

4

I'm not a Ruby guy, but I guess it's for converting @sent_at to a boolean.

UPDATE: Ah ha I saw your update. Notice that false and nil are different; false.nil? == false, while nil.nil? == true. In ruby only false and nil will be treated as "false"; even an empty string, '', is evaluated as true (it is False in python).

edited Sep 08 '12 at 23:36

answered Sep 08 '12 at 06:20

wecing

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Yes you are right. Nice programmer instinct. – SwiftMango Sep 08 '12 at 06:23

score 2 · Answer 2 · answered Sep 08 '12 at 06:22

2

Double Bang is use to convert anything into a boolean in Ruby.

http://rubyquicktips.com/post/583755021/convert-anything-to-boolean

answered Sep 08 '12 at 06:22

manojlds

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score 1 · Answer 3 · 2012-09-08T15:55:49.043

It forces a truthy-value to true and a falsy-value to false. In Ruby the only falsy-values are nil and false and everything other value is a truthy-value.

!falsyExpr   # given -> true
!true        # and -> false
!!falsyExpr  # then -> true -> false

!truthyExpr  # given -> false
!false       # and -> true
!!truthyExpr # then -> false -> true

The reason to use !!expr over expr, when returned from a method like this, is that the !!expr form only yields true or false. This avoids leaking details, avoids accidental strong-references, reduces confusion when debugging, prevents cases of false/nil confusion, etc. It is also shorter than expr ? true : false, which has identical semantics.

The forms expr == nil and expr.nil? only evaluate to true when expr evaluates to nil. They are both false if expr evaluates to any other value, including false. This sets them apart from the !expr which evaluates to true for both false and nil values.

Of course the all of the above is expected behavior. Due to the open nature of Ruby and operator overloading it is possible (and very misguided) to create new methods that change this expected behavior (barring some implementation optimization quirks).

score 1 · Answer 4 · answered Sep 08 '12 at 06:22

1

Using !! makes sure either true or false is returned. So, yes, it makes sense.

@sent_at = 'Foo'
!!@sent_at # => true

@sent_at = nil
!!@sent_at # => false

answered Sep 08 '12 at 06:22

Mischa

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score 1 · Answer 5 · edited May 23 '17 at 12:23

1

It converts non-boolean types to boolean (true or false).

It is a shortcut way of doing this, instead of trying to cast it explicitly (which may take more characters). The ! operator negates the truthness of its argument. Hence, two of them are used.

Also see Use of double negation (!!) for perl (or for most other languages).

edited May 23 '17 at 12:23

Community

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answered Sep 08 '12 at 06:23

ronalchn

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Double "!!" in Ruby

5 Answers5