How to determine if one array contains all elements of another array

Question

a = [5, 1, 6, 14, 2, 8]
b = [2, 6, 15]

a - b
# => [5, 1, 14, 8]

b - a
# => [15]

(b - a).empty?
# => false

Perhaps this is easier to read:

a2.all? { |e| a1.include?(e) }

You can also use array intersection:

(a1 & a2).size == a1.size

Note that size is used here just for speed, you can also do (slower):

(a1 & a2) == a1

But I guess the first is more readable. These 3 are plain ruby (not rails).

This can be achieved by doing

(a2 & a1) == a2

This creates the intersection of both arrays, returning all elements from a2 which are also in a1. If the result is the same as a2, you can be sure you have all elements included in a1.

This approach only works if all elements in a2 are different from each other in the first place. If there are doubles, this approach fails. The one from Tempos still works then, so I wholeheartedly recommend his approach (also it's probably faster).

If there are are no duplicate elements or you don't care about them, then you can use the Set class:

a1 = Set.new [5, 1, 6, 14, 2, 8]
a2 = Set.new [2, 6, 15]
a1.subset?(a2)
=> false

Behind the scenes this uses

all? { |o| set.include?(o) }

You can monkey-patch the Array class:

class Array
    def contains_all?(ary)
        ary.uniq.all? { |x| count(x) >= ary.count(x) }
    end
end

test

irb(main):131:0> %w[a b c c].contains_all? %w[a b c]
=> true
irb(main):132:0> %w[a b c c].contains_all? %w[a b c c]
=> true
irb(main):133:0> %w[a b c c].contains_all? %w[a b c c c]
=> false
irb(main):134:0> %w[a b c c].contains_all? %w[a]
=> true
irb(main):135:0> %w[a b c c].contains_all? %w[x]
=> false
irb(main):136:0> %w[a b c c].contains_all? %w[]
=> true
irb(main):137:0> %w[a b c d].contains_all? %w[d c h]
=> false
irb(main):138:0> %w[a b c d].contains_all? %w[d b c]
=> true

Of course the method can be written as a standard-alone method, eg

def contains_all?(a,b)
    b.uniq.all? { |x| a.count(x) >= b.count(x) }
end

and you can invoke it like

contains_all?(%w[a b c c], %w[c c c])

Indeed, after profiling, the following version is much faster, and the code is shorter.

def contains_all?(a,b)
    b.all? { |x| a.count(x) >= b.count(x) }
end