Question

I have the following arrays:

A = "cheddar".split(//)  # ["c", "h", "e", "d", "d", "a", "r"]
B = "cheddaar".split(//) # ["c", "h", "e", "d", "d", "a", "a", "r"]

The A array is a subset of the B array. If the A array had another “d” element, it wouldn’t be a subset.

I want to compare and find if one is a subset of the other even if they have duplicates. The A – B or the A & B doesn’t capture the duplicates it just compares them and find them matched. So I wrote the following which it captures the duplicates:

B.each do |letter|
    A.delete_at(A.index(letter)) rescue ""
end

p A.empty?

Is this the best way or can it be optimized?

Answer 1

A similar question was posted a few weeks ago and I got the accepted answer with something like:

def is_subset?(a,b)
  !a.find{|x| a.count(x) > b.count(x)}
end

Benchmark Update

require 'benchmark'

def random_char
  ('a'..'z').to_a.sample
end

A = 8.times.map{random_char}
B = 8.times.map{random_char}

def ary_subset?(a,b) # true iff a is subset of b
  a_counts = a.reduce(Hash.new(0)) { |m,v| m[v] += 1; m }
  b_counts = b.reduce(Hash.new(0)) { |m,v| m[v] += 1; m }
  a_counts.all? { |a_key,a_ct| a_ct <= b_counts[a_key] }
end

Benchmark.bm do |x|
  x.report('me')     {100000.times{is_subset?(A,B)}}
  x.report('dbenhur'){100000.times{ary_subset?(A,B)}}
end

---

       user     system      total        real
me  0.375000   0.000000   0.375000 (  0.384022)
dbenhur  2.558000   0.000000   2.558000 (  2.550146)