Today I Learned

Matching array subset in Ruby

Problem:

How do you evaluate whether one array is a subset of another? For example, are the elements [a,c] included in [a,b,c]?

First attempt:

I was hoping to find something like Array.include?([...]), but this only checks if the array includes the argument as one of its values.

Second attempt:

Another approach is to pass a block into Array.any?

!arr1.any? { |e| !arr2.include?(e) }

But the double negation is rather indirect and doesn't easily reveal the intent.

I considered extracting a method to name the functionality:

def subset?(arr1, arr2)
  !arr1.any? { |e| !arr2.include?(e) }
end

But it's still difficult to read, as it's not clear whether arr1 is a subset of arr2, or vice versa.

Final Solution:

The Enumerable module includes a to_set method to convert the array to set, and Set includes a subset? method.

arr1.to_set.subset?(arr2.to_set)

Technically, you need to require set.rb to get this method defined on Enumberable:

require "set"

arr1.to_set.subset?(arr2.to_set)

But you get this require for free in Rails.