ruby - Why does .all? return true on an empty array?

Question

Using Ruby I want to evaluate all items in an array, and return true if they all pass a conditional test.

I can do this using e.g. array.all? { |value| value == 2 }

So:

> array=[2,2]
> array.all? { |value| value == 2 }
=> true
> array=[2,3]
> array.all? { |value| value == 2 }
=> false

Great!

But, why does an empty array pass this test?

> array=[]
> array.all? { |value| value == 2 }
=> true

Shouldn't this return false?

And if I need it to return false, how should I modify the method?

Answer 1

This is a vacuous truth. It's the standard interpretation of a universal quantification, i.e. a

collection.all? { |x| some_predicate(x) }

over an empty collection, but it's known to strike people as counter-intuitive when they first see it in a formal setting. One nice way to think about why this is the preferred semantics is to think about how you would implement all?.

To make your test require that the array is non-empty, just do

array.any? && array.all? { |x| x == 2 }

Note that array.any? is fast no matter how large the array, whereas array.all? { |x| x == 2 } can be slow, depending on how big array is and how rare 2 is in it. So put the array.any? first.

Also note, there are degenerate cases where this won't work, for instance if array is [nil] or [false]. If cases like this might come up, replace array.any? with array.any? { true }.