k-subset generation example

ksubsets.rb

def subsets(s, k)
  max = 1 << s.length
  allsubsets = []
  (0..max).each do |i|
    subset = []
    ii = i
    index = 0
    while ii > 0
      if ii & 1 > 0
        subset << s[index]
      end
      ii >>= 1
      index += 1
    end
    allsubsets << subset if subset.length == k
  end
  allsubsets
end

Pretty straight forward, generate all binary strings of s.length and iteratively check each number of k elements.