Ruby, !! Operator (A/K/A the Double-Bang)

Ruby, !! operator (a/k/a the double-bang)

In most programming languages, including Ruby, ! will return the opposite of the boolean value of the operand. So when you chain two exclamation marks together, it converts the value to a boolean.

!! (double bang) meaning in Ruby

It casts a variable into type boolean and determine its truthy or falsy value

For example:-

# Numbers...
!!1 # => true
!!0 # => true

# Numbers as strings...
!!'1' # => true
!!'0' # => false

# Truthy strings (case insensitive)...
!!'true'  # => true  (alias: 't')
!!'false' # => false (alias: 'f')
!!'yes'   # => false (alias: 'y')
!!'no'    # => false (alias: 'n')

# Booleans...
!!true  # => true
!!false # => false

# Nil...
!!nil # => false

What does !! mean in ruby?

Not not.

It's used to convert a value to a boolean:

!!nil   #=> false
!!"abc" #=> true
!!false #=> false

It's usually not necessary to use though since the only false values to Ruby are nil and false, so it's usually best to let that convention stand.

Think of it as

!(!some_val)

One thing that is it used for legitimately is preventing a huge chunk of data from being returned. For example you probably don't want to return 3MB of image data in your has_image? method, or you may not want to return your entire user object in the logged_in? method. Using !! converts these objects to a simple true/false.

Double !! in Ruby

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).

How to implicitly convert Ruby object to boolean

== is a syntax sugar for :== method. You can define object's own :== method, where you can specify, based on internal object's state, when the result of the comparision should be true and when false:

class SomeClass
  def ==(val)
    # specyfy the comparision behaviour
  end
end

And then use it like:

sc = SomeClass.new
sc == true
=> true # or false, depending on :== method's implementation

Keyword for exclusive or in ruby?

Firstly, I don't think shortcircuiting can sensibly apply to XOR: whatever the value of the first operand, the second needs to be examined.

Secondly, and, &&, or and || use shortcircuiting in all cases; the only difference between the "word" and "symbol" versions is precedence. I believe that and and or are present to provide the same function as perl has in lines like

process_without_error or die

I think the reason for not having a xor named function is probably that there's no point in a low-precedence operator in this case and that it's already a confusing enough situation!

Is there a Ruby, or Ruby-ism for not_nil? opposite of nil? method?

when you're using ActiveSupport, there's user.present? http://api.rubyonrails.org/classes/Object.html#method-i-present%3F, to check just for non-nil, why not use

def logged_in?
  user # or !!user if you really want boolean's
end

Should a method ending in ? (question mark) return only a boolean?

A method ending with ? should return a value which can be evaluated to true or false. If you want to ensure a boolean return, you can do so by adding a double bang to the finder.

def is_subscribed?(feed_url)
  !!Subscription.find_by_user_id_and_feed_id(self[ :id ], Feed.find_by_feed_url(feed_url))
end

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