Translating function for finding all partitions of a set from Python to Ruby

Question

I have the following python function to recursively find all partitions of a set:

def partitions(set_):
    if not set_:
        yield []
        return
             


        

Answer 1

You'll have to think of Ruby's yield like a call to a user-defined operation.

def twice
    yield
    yield
end

twice { puts "Hello" } 

So whenever your code yields a value, a processing function for this element will be called.

partitions([1, 2, 3, 4].to_set) { |result| 
    # process result
}

This code doesn't generate a list at all.

Answer 2

#!/usr/bin/ruby1.8

def partitions(set)
  yield [] if set.empty?
  (0 ... 2 ** set.size / 2).each do |i|
    parts = [[], []]
    set.each do |item|
      parts[i & 1] << item
      i >>= 1
    end
    partitions(parts[1]) do |b|
      result = [parts[0]] + b
      result = result.reject do |e|
        e.empty?
      end
      yield result
    end
  end
end

partitions([1, 2, 3, 4]) do |e|
  p e
end

# => [[1, 2, 3, 4]]
# => [[2, 3, 4], [1]]
# => [[1, 3, 4], [2]]
# => [[3, 4], [1, 2]]
# => [[3, 4], [2], [1]]
# => [[1, 2, 4], [3]]
# => [[2, 4], [1, 3]]
# => [[2, 4], [3], [1]]
# => [[1, 4], [2, 3]]
# => [[1, 4], [3], [2]]
# => [[4], [1, 2, 3]]
# => [[4], [2, 3], [1]]
# => [[4], [1, 3], [2]]
# => [[4], [3], [1, 2]]
# => [[4], [3], [2], [1]]

What's different:

• The guard calls set.empty? instead of (implicitly) testing for set.nil?
• Leave out the .each when calling partitions
• Use Array instead of Set
• Filter empty sets out of the yielded result