CODEWITHSUNDEEP
arraysruby
davidhu
davidhu

Reputation: 10432

Ruby 2D array assignment

So I have a tic-tac-toe board class. See below: 

class Board
  attr_accessor :grid

  def initialize(grid = Array.new(3, Array.new(3, nil)))
    @grid = grid
  end

  def place_mark(position, symbol)
    @grid[position[0]][position[1]] = symbol
  end
end

Whenever I call the place_mark method, and tried to assign a symbol to an element in the 2D array, the entire column gets assigned. 

board = Board.new
board.place_mark([0,0], :x)

Would result in 

[[:X, nil, nil], 
 [:X, nil, nil], 
 [:X, nil, nil]]

Where the desired result is

[[:X , nil, nil], 
 [nil, nil, nil], 
 [nil, nil, nil]]

I found a solution to my problem, in the initialize method, I just need to assign the default value of grid like this: 

def initialize(grid = [[nil, nil, nil], [nil, nil, nil], [nil, nil, nil]])
  @grid = grid
end

Then the place_mark method works just fine.

So my question is how are the two different array declarations different that would make them behave this way?

Upvotes: 0

Views: 788

Answers (1)

user94559
user94559

Reputation: 60143

The issue is that Array.new(3, Array.new(3, nil)) gives you an array with the same array in it three times.

It's like doing this:

x = Array.new(3, nil)
grid = Array.new(3, x)

So you have an array containing x three times. You really want three separate arrays that can each have their own values.

Per http://ruby-doc.org/core-2.3.1/Array.html:

Note that the second argument populates the array with references to the same object. Therefore, it is only recommended in cases when you need to instantiate arrays with natively immutable objects such as Symbols, numbers, true or false.

To create an array with separate objects a block can be passed instead. This method is safe to use with mutable objects such as hashes, strings or other arrays:

Array.new(4) { Hash.new } #=> [{}, {}, {}, {}]

This is also a quick way to build up multi-dimensional arrays:

empty_table = Array.new(3) { Array.new(3) }
#=> [[nil, nil, nil], [nil, nil, nil], [nil, nil, nil]]

That last example is exactly what you're looking for.

Upvotes: 2

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