Array#transpose that accepts a block for populating missing elements

transpose.rb

class Array

  # A more meaningful message when you have an insufficient number of columns
  # in a row for transposition.
  MSG_TRANSPOSE_INDEX_ERROR = "Row %s has only %s element(s), but transposition
  requires %s. Please specify a block to populate missing items or adjust
  your array to contain an equal number of elements in each row array."

  # Overrides the default transpose method to allow a block that can be used to return
  # elements required to populate the transposition.
  # @example Using Transpose
  #   > [[1, 2], [3, 4]].transpose 
  #   => [[1, 3], [2, 4]]
  #
  #   > [[1], [3, 4]].transpose
  #   IndexError: Row 0 has only 1 element(s), but transposition requires 2...
  #
  #   > [[1,], [3, 4]].transpose { |x, y| 2 } 
  #   => [[1, 3], [2, 4]]
  # @note
  #   When yielding to the block the column(x) and row(y) index for the 
  #   missing element are provided
  # @return [Array] The transposed array
  def transpose
    return clone if size == 0
    rows = size
    columns = map(&:size).max
    result = Array.new(columns) { Array.new(rows) }

    0...rows.times do |row_index|
      0...columns.times do |column_index|
        datum = if self[row_index].size > column_index
                  self[row_index][column_index]
                elsif block_given?
                  yield(column_index, row_index)
                else
                  raise IndexError, MSG_TRANSPOSE_INDEX_ERROR %
                                                  [row_index,
                                                  self[row_index].size,
                                                  columns]
                end
        result[column_index][row_index] = datum
      end
    end

    result
  end
end

require 'test/unit'

class TestArray < Test::Unit::TestCase

  def test_transpose_empty
    assert(Array.new.transpose.is_a?(Array))
  end

  def test_transpose_simple
    assert_equal([[1, 2], [3, 4]].transpose,
                 [[1, 3], [2, 4]])
  end

  def test_transpose_with_block
    filled = [[1], [3, 4]].transpose { |x, y| 2 }
    assert_equal([[1, 3],[2, 4]], filled)
  end

  def test_your_array_sucks
    assert_raise(IndexError) do
      [[1, 2], [1]].transpose
    end
  end
end