Check if an array is subset of another array in Ruby

Question

How can I check whether one array is a subset of another array, regardless of the order of elements?

a1 = [3, 6, 4]
a2 = [1, 2, 3, 4, 5, 6, 7, 8, 9]

...?

a         


        

Answer 1

Use sets. Then you can use set.subset?. Example:

require 'set'

a1 = Set[3,6,4]
a2 = Set[1,2,3,4,5,6,7,8,9]

puts a1.subset?(a2)

Output:

true

See it working online: ideone

Answer 2

Easiest may be:

(a1 - a2).empty?

Answer 3

The data structure you already have is perfect, just check the intersection:

(a1 & a2) == a1

Update: The comment discussing permutations is interesting and creative, but quite incorrect as the Ruby implementors anticipated this concern and specified that the order of the result is the order of a1. So this does work, and will continue to work in the future. (Arrays are ordered data structures, not sets. You can't just permute the order of an array operation.)

I do rather like Dave Newton's answer for coolness, but this answer also works, and like Dave's, is also core Ruby.

Answer 4

Perhaps not fast, but quite readable

def subset?(a,b)
  a.all? {|x| b.include? x}
end