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 ins…

Accepted Answer

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
```

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

Solution

Problem