Question

I am trying to create a hash with values as arrays. I am adding elements to those arrays, but for some reason, the hash becomes empty after it runs. I’m not sure why at all. Here is my code

def function(words)
  hash = Hash.new([])  # default value of empty list
  words.each do |word|
    sorted = word.chars.sort.join  # sort the string
    hash[sorted] << word
    ## hash becomes empty here
  end
  return hash
end

puts function ['cars', 'for', 'potatoes', 'racs', 'four']

I’m new to Ruby and I have no idea why the hash is emptying it self. I’ve written a similar algorithm with the same logic in Python and it works absolutely fine. Any suggestions?

Answer 1

The problem is the default hash value you give is a mutable value (partially, at least—see below), you need to instead use Hash.new’s block parameter:

hash = Hash.new { |h, k| h[k] = [] }

You get the correct result:

function ['cars', 'for', 'potatoes', 'racs', 'four']
#=> {"acrs"=>["cars", "racs"],
#    "for"=>["for"],
#    "aeoopstt"=>["potatoes"],
#    "foru"=>["four"]}

The problem with what you have is that hash[sorted] returns [], but never actually assigns to it. So you change the array, but never put it in the hash. If you use += instead (leaving your Hash.new([]), you can that this also works:

hash[sorted] += [words]