Inject double ampersand operator

Question

I have an inject call

[2,4,6].inject(true) { |res, val| res && val % 2 == 0 }

and want to send the && operator to inject as in inject(0, :+). How can I do that?

Answer 1

You can't because && and ||, unlike other operators, are not syntacic sugar for methods (i.e. there is no method called && or ||), so you can't reference them using a symbol.

However you can avoid using inject to compute the logical conjunction or disjunction of an array of boolean values, replacing it with all? or any? respectively, because for any array the following conditions hold:

ary.inject(true) { |res, b| res && b } == ary.all?
ary.inject(false) { |res, b| res || b } == ary.any?

So, for example, the code you posted could be rewritten as:

[2,4,6].map(&:even?).all?
# => true

Update: obviously my latter example is not the right way to express this computation, falsetru's answer is much faster:

require 'fruity'

compare(
  -> { (0..1000).map(&:even?).all? },
  -> { (0..1000).all?(&:even?) }
)

Running each test 1024 times. Test will take about 2 seconds.
Code 2 is faster than Code 1 by 111x ± 10.0   

Answer 2

How about using Enumerable#all?

[2,4,6].all? &:even?
# => true
[2,4,6,5].all? &:even?
# => false

If you want to use inject, you need to define an instance method.

class Object
  def is_even(val)
    self && val % 2 == 0
  end
end

[2,4,6].inject(true, :is_even) # => true
[1,2,4,6,5].inject(true, :is_even) # => false

Answer 3

'&&' is not a method, hence you can't inject it. However you can inject & method.

[2,4,6,5].map(&:even?).inject(true, :&)

Which will do the same

NOTE: This however should not be done, as it is extremely risky and might cause unexpected consequences (if run on collection containing at least one non-boolean (true, false, nil) value). You should always use any? or all? methods instead.