Strange behaviour in Ruby's 2D arrays

Question

I'm writing a radix sort implementation in Ruby as a self-teaching exercise, and something very odd is happening here. letters ought to be a 2D array of buckets, one for each letter of the alphabet (0 for space/nil, 1-26 for letters). For some reason, this code is not inserting my word at the one index of letters, however. It's inserting it at every index. There also seems to be some sort of infinite loop which prevents it from terminating, which is also odd.

What am I doing wrong? Here is my code.

def radix_sort(words)
    letters = Array.new(27, [])
    offset = 'a'.ord
    max_length = words.max_by { |word| word.length }.length

    (max_length-1).downto(0) do |i|
        words.each do |word|
            if word[i] != nil then 
                index = word[i].downcase.ord - offset
                letters[index + 1] << word
            else
                letters[0] << word
            end
        end
        words = letters.flatten
        letters = letters.map { |bucket| bucket.clear }
    end

    words
end

w = ["cat", "dog", "boar", "Fish", "antelope", "moose"]

p radix_sort(w)

Answer 1

First of all, there are no 2D arrays, just arrays-of-arrays. Secondly, this:

default_array = [ ]
letters       = Array.new(27, default_array)

simply copies the default_array reference to every one of the 27 automatically created values so letters looks like this:

letters = [
  default_array,
  default_array,
  ...
]

Have a look at letters[0].object_id and letters[1].object_id and you'll see that they're exactly the same. The fine manual even says as much:

new(size=0, obj=nil)
new(array)
new(size) {|index| block }

Returns a new array.

[...] When a size and an optional obj are sent, an array is created with size copies of obj. Take notice that all elements will reference the same object obj.

Emphasis mine in the last sentence.

However, if you say:

letters = Array.new(27) { [ ] }

then the block will be executed once for each of the 27 initial values and each execution of the block will create a brand new array. Check letters[0].object_id and letters[1].object_id you'll see that they really are different.