Question

I’m having some trouble figuring out the right way to do this:

I have an array and a separate array of arrays that I want to compare to the first array. The first array is a special Enumerable object that happens to contain an array.

Logic tells me that I should be able to do this:

[1,2,3].delete_if do |n|
  [[2,4,5], [3,6,7]].each do |m|
    ! m.include?(n)
  end
end

Which I would expect to return
=> [2,3]

But it returns [] instead.

This idea works if I do this:

[1,2,3].delete_if do |n|
  ! [2,4,5].include?(n)
end

It will return

 => [2]

I can’t assign the values to another object, as the [1,2,3] array must stay its special Enumerable object. I’m sure there is a much simpler explanation to this than what I’m trying. Anybody have any ideas?

Answer 1

The problem is that the return value of each is the array being iterated over, not the boolean (which is lost). Since the array is truthy, the value returned back to delete_if is always true, so all elements are deleted. You should instead use any?:

[1,2,3].delete_if do |n|
  ![[2,4,5], [3,6,7]].any? do |m|
    m.include?(n)
  end
end
#=> [2, 3]