How to declare a two-dimensional array in Ruby

Question

I want a twodimensional array in Ruby, that I can access for example like this:

if @array[x][y] == "1" then @array[x][y] = "0"

The problem is: I don't know the initial sizes of the array dimensions and i grow the array (with the << operator).

How do I declare it as an instance variable, so I get no error like this?

undefined method `[]' for nil:NilClass (NoMethodError)

---

QUESTION UPDATED:

@array = Array.new {Array.new} 

now works for me, so the comment from Matt below is correct!

I just found out the reason why I received the error was because I iterated over the array like this:

for i in [email protected]
    for j in 0..@array[0].length
        @array[i][j] ...

which was obviously wrong and produced the error. It has to be like this:

for i in [email protected]
        for j in 0..@array[0].length-1
            @array[i][j] ...

Answer 1

A simple implementation for a sparse 2-dimensional array using nested Hashes,

class SparseArray
  attr_reader :hash

  def initialize
    @hash = {}
  end

  def [](key)
    hash[key] ||= {}
  end

  def rows
    hash.length   
  end

  alias_method :length, :rows
end

Usage:

sparse_array = SparseArray.new
sparse_array[1][2] = 3
sparse_array[1][2] #=> 3

p sparse_array.hash
#=> {1=>{2=>3}}

#
# dimensions
#
sparse_array.length    #=> 1
sparse_array.rows      #=> 1

sparse_array[0].length #=> 0
sparse_array[1].length #=> 1