Setting the value of an element within a nested array

Question

I'm trying to create a simple 10x10 Array so that I can create Conway's Game of Life, but I'm running into some strange behavior. First I initialize the board:

@board = Array.new(10, Array.new(10))
(0..9).each do |row|
    (0..9).each do |column|
        @board[row][column] = rand(0..1)
    end
end

Which produces this:

Looks fishy but it's entirely possible that this could be generated randomly. The real problems start when I try to set the value of an individual cell. When I change the value, it sets the entire column to that value! For example, let's say I change the value of the first cell:

@board[0][0] = 0

What I get is:

What gives? Why does the value change for ALL cells in column 0 instead of just the cell at 0, 0?

Not directly relevant to your problem, but take a look at narray for multi-dimensional numerical arrays. — Neil Slater
– Neil Slater, Commented Apr 2, 2013 at 22:03

fbonetti · Accepted Answer · 2013-04-02 21:30:09Z

5

I figured it out. When I initialized @board with Array.new(10, Array.new(10)), it created an Array of 10 identical Arrays. That is, each Array had the same object_id.

@board[0].object_id
=> 22148328
@board[1].object_id
=> 22148328

I solved the issue by using the map method:

@board = Array.new(10).map{ |x| Array.new(10) }

answered Apr 2, 2013 at 21:30

fbonetti

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2 Comments

dbenhur Over a year ago

Note you could use the block form of Array::new to get distinct row objects as well. @board = Array.new(10) { Array.new(10) }

fbonetti Over a year ago

Thanks for the tip. That looks a lot cleaner than using map.

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