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

Question

I'm trying to make a version of Conway's game of life using ruby. I've created a Grid class with @play_area as an instance variable. However, when I run my code, @play_area turns up nil after it had already been evaluated twice (when evaluated in the line if @play_area[x_mod][y_mod].alive ). Why is this happening?

EDIT

Here's the initialize function:

def initialize(sizex, sizey)
    @x_length = sizex
    @y_length = sizey
    @play_area = []
    #initialize dead cells
    @x_length.times do |x|
        @play_area[x] ||= []
        @y_length.times do |y|
          @play_area[x][y] = Cell.new(x, y, false)
          puts @play_area[x][y].inspect
        end
    end
end

Here's the function the error occurs in:

def neighbor_num_of_cell(pos_x,pos_y)
    c = @play_area[pos_x][pos_y]
    count = 0
    ((pos_x-1)..(pos_x+1)).each do |x|
        ((pos_y-1)..(pos_y+1)).each do |y|
            unless @play_area[x][y].eql?(c)
                x_mod = x % (@x_length + 1)
                y_mod = y % (@y_length + 1)
                puts x_mod
                puts y_mod
                if @play_area[x_mod][y_mod].alive
                    count += 1
                end
            end
        end
    end
    count
end

The inspect on each of the cells in @play_area shows that each one of the cells is initialized correctly, here is the output of the inspect:

jon@jon-MacBook:~/programs/ruby$ ruby main.rb 
#<Cell:0x00000000f919d8 @alive=false, @alive_next=false, @x_pos=0, @y_pos=0>
#<Cell:0x00000000f91848 @alive=false, @alive_next=false, @x_pos=0, @y_pos=1>
#<Cell:0x00000000f916e0 @alive=false, @alive_next=false, @x_pos=0, @y_pos=2>
#<Cell:0x00000000f915a0 @alive=false, @alive_next=false, @x_pos=0, @y_pos=3>
#<Cell:0x00000000f91460 @alive=false, @alive_next=false, @x_pos=0, @y_pos=4>
#<Cell:0x00000000f91320 @alive=false, @alive_next=false, @x_pos=0, @y_pos=5>
#<Cell:0x00000000f911e0 @alive=false, @alive_next=false, @x_pos=0, @y_pos=6>
#<Cell:0x00000000f910a0 @alive=false, @alive_next=false, @x_pos=0, @y_pos=7>
#<Cell:0x00000000f90f38 @alive=false, @alive_next=false, @x_pos=0, @y_pos=8>

...

#<Cell:0x00000000f1abf8 @alive=false, @alive_next=false, @x_pos=19, @y_pos=7>
#<Cell:0x00000000f1aa90 @alive=false, @alive_next=false, @x_pos=19, @y_pos=8>
#<Cell:0x00000000f1a900 @alive=false, @alive_next=false, @x_pos=19, @y_pos=9>
#<Cell:0x00000000f1a798 @alive=false, @alive_next=false, @x_pos=19, @y_pos=10>
#<Cell:0x00000000f1a658 @alive=false, @alive_next=false, @x_pos=19, @y_pos=11>
#<Cell:0x00000000f1a518 @alive=false, @alive_next=false, @x_pos=19, @y_pos=12>
#<Cell:0x00000000f1a3b0 @alive=false, @alive_next=false, @x_pos=19, @y_pos=13>
#<Cell:0x00000000f1a270 @alive=false, @alive_next=false, @x_pos=19, @y_pos=14>
#<Cell:0x00000000f1a130 @alive=false, @alive_next=false, @x_pos=19, @y_pos=15>
#<Cell:0x00000000f19ff0 @alive=false, @alive_next=false, @x_pos=19, @y_pos=16>
#<Cell:0x00000000f19e88 @alive=false, @alive_next=false, @x_pos=19, @y_pos=17>
#<Cell:0x00000000f19d20 @alive=false, @alive_next=false, @x_pos=19, @y_pos=18>
#<Cell:0x00000000f19be0 @alive=false, @alive_next=false, @x_pos=19, @y_pos=19>

Answer 1

The problem is with this line:

@play_area[x][y] = Cell.new(x, y, false)

@play_area[x] is nil. You've only initialized one dimension of your multidimensional array. You need to initialize each element of @play_area[x] to an array before trying to add an element to it.

@x_length.times do |x|
  @play_area[x] ||= []
  @y_length.times do |y|
    @play_area[x][y] = Cell.new(x, y, false)
  end
end