From: Simon Hicks <ruby@...>
Date: 2009-09-12T13:49:51+09:00
Subject: Re: Nooby question : multidimensional arrays.

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The first method simply creates an Array of size width and places an  
Array of size height in each index, so

Array.new(2){Array.new(3)}
=>  [ [ nil , nil , nil ] , [ nil , nil , nil] ]

The second method is merely a way of accessing a coordinate within the  
2d array, so for
a = Array2D.new 2, 2
a[0,1]

it would first find @data[0]  which would return the object in index 0  
which is an array [ nil, nil ] and then @data[0][1] would return the  
object in index 1 of the sub-array which in this case is nil. In fact  
the logic here is no more complex than the logic for a simple Array...  
it only looks more complex, because we are using the same method  
twice... perhaps it'd help to think about it like this

def [](x, y)
   sub_array = @data[x]
   return sub_array[y]
end

The third def is exactly the same as the second def, except instead of  
simply looking to see what's stored at a coordinate, you're changing  
the object that's stored at that coordinate. Again the logic is the  
same as with simple arrays, except the object in each index is also an  
array.

I don't know exactly what's causing the error you're seeing, but it  
looks like you're calling [x,y]= using a value of x that is higher  
than the maximum index in the outer array, so instead of returning a  
sub_array it returns nil, and then tries to call [y]= on nil...

hth...

On 12 Sep 2009, at 11:06, Shawn W_ wrote:

> 01 class Array2D
> 02
> 03   def initialize(width, height)
> 04   @data = Array.new(width) { Array.new(height) }
> 05   end
> 06
> 07  def [](x, y)
> 08    @data[x][y]
> 09  end
> 10
> 11  def []=(x, y, value)
> 12    @data[x][y] = value
> 13  end
> 14
> 15 end
>
> Can someone explain what's going on in this code for a newbie.
>
> I'm getting lost in the looping logic of the second def; and the 3rd  
> def
> I'm guessing has something do do with hashes?
>
> I'm using this code in a simple 2d array. It was working but then I
> changed all of my ranges from (0...value) to (1..value) [I used ranges
> to populate my 2D array with hashes] I now get the error message:
>
> 11:in `[]=': undefined method `[]=' for nil:NilClass (NoMethodError)
> -- 
> Posted via http://www.ruby-forum.com/.
>


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