Today’s Ruby Tuesday is on Set#disjoint?.

To start using Sets, we first need to require Set.

require 'set'

Set#disjoint? is useful when you want to make sure a group of items is not in another group of items.

Without using Set#disjoint? you would probably be writing something like the following:

[1, 3, 1, 5, 7].none? { |item| [2, 4, 6, 8].include? item }
# => true

While this works, it does not make the intent of what you are trying to do explicit, which is where Set#disjoint? comes in.

Set#disjoint? operates against a set, and takes another Set as it’s argument, and checks to see if there is no common element between the two sets.

[1, 3, 1, 5, 7].to_set.disjoint? [2, 4, 6, 8].to_set
# => true
[1, 2, 3].to_set.disjoint? [1, 3, 5, 7].to_set
# => false

If a nil is present in both sets, Set#disjoint? treats the nils as equal and returns false.

[2, nil].to_set.disjoint? [1, 3, 5, 7, nil].to_set
=> false

Set#disjoint? works against empty sets as well.

[].to_set.disjoint? [1, 2, 3].to_set
# => true
[1, 2, 3].to_set.disjoint? [].to_set
# => true

If two empty sets are passed to Set#disjoint?, it returns true, as both sets have no elements, and therefore no elements in common.

[].to_set.disjoint? [].to_set
# => true

–Proctor