Could you explain what's !! do?

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What does !! mean in ruby?

I found !! in Paypal gem here: https://github.com/tc/paypal_adaptive/blob/master/lib/paypal_adaptive/config.rb like 59

but I don't understand what it does.

I know that ! means NOT, but !! doesn't make sense.

here's the screen: http://tinyurl.com/7acklhr

If you know what `!(foo)` means then you should be able to work out what `!(!(foo))` means :) It's not a different operator, just the same operator applied twice — Gareth, Mar 12 '12 at 13:44

score 4 · Answer 1 · answered Mar 12 '12 at 13:45

4

It forces any value to true or false depending on its "truthy" nature.

This is simply because, as you've noted, ! is the Boolean-not operator. For instance:

t = 1
puts !t  # => false
puts !!t # => true
f = nil
puts !f  # => true
puts !!f # => false

answered Mar 12 '12 at 13:45

Chowlett

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score 4 · Answer 2 · answered Mar 12 '12 at 13:47

The !! is used to return either true or false on something that returns anything :

In Ruby, everything other than nil and false is interpreted as true. But it will not return true, it will return the value. So if you use !, you get true or false but the opposite value of what is really is. If you use !!, you get the true or false corresponding value.

score 1 · Accepted Answer · answered Mar 12 '12 at 13:46

1

It is used to make sure its the boolean type.

Explanation more detailed

Eg:

!!active => true

active = false => false

!!active => false

active = nil => nil

!!active => false

answered Mar 12 '12 at 13:46

Pedro Ferreira

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score 0 · Answer 4 · answered Mar 12 '12 at 13:46

This forces a result to be true or false. As in ruby nil is not exactly false this can be useful. For instance:

def try x
  if x == 1
    return nil
  else
    return "non-nil"
  end
end

p "try1" if try(1) # here you get a string printed
p "try2" if !!try(1) # here you don't

Could you explain what's !! do?

4 Answers4