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 0..@array.length
    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 0..@array.length-1
        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

Answer 2

Matt's comment on your question is totally correct. However, based on the fact that you've tagged this "conways-game-of-life", it looks like you are trying to initialize a two dimensional array and then use this in calculations for the game. If you wanted to do this in Ruby, one way to do this would be:

a = Array.new(my_x_size) { |i| Array.new(my_y_size) { |i| 0 }}

which will create a my_x_size * my_y_size array filled with zeros.

What this code does is to create a new Array of your x size, then initialize each element of that array to be another Array of your y size, and initialize each element of each second array with 0's.

Answer 3

Ruby's Array doesn't give you this functionality.

Either you manually do it:

(@array[x] ||= [])[y] = 42

Or you use hashes:

@hash = Hash.new{|h, k| h[k] = []}
@hash[42][3] = 42
@hash # => {42 => [nil, nil, nil, 42]}