Answer to the question: how to initialise a multi-dimensional array, filled with nils, in ruby?

nil-array.rb

# My answer at the time was to do something like this
[[nil]*3]*4
#=> [[nil, nil, nil], [nil, nil, nil], [nil, nil, nil], [nil, nil, nil]]

# The next question was whether I knew some initializer to do that for us.
# Apparently, there is:
Array.new(4) { Array.new(3) }
#=> [[nil, nil, nil], [nil, nil, nil], [nil, nil, nil], [nil, nil, nil]]

# But that wasn't the brainwave my brain came up with just now.
# There's a Matrix class!

require 'matrix' # it's in the standard library, so no gems required

Matrix.build(3,4) {nil}.to_a
#=> [[nil, nil, nil, nil], [nil, nil, nil, nil], [nil, nil, nil, nil]]

# Of course, that's build(rows, columns). If you wanted x,y you could use:
Matrix.build(3,4) {nil}.to_a.transpose
#=> [[nil, nil, nil], [nil, nil, nil], [nil, nil, nil], [nil, nil, nil]]

# or just switch them around yourself.
# My original answer was more succinct I think :)