From: Shawn W_ <shawnw@...>
Date: 2010-07-16T06:45:46+09:00
Subject: Re: Return nothing when looking outside the bounds of 2D array?

Dave Howell wrote:
> I was going to suggest using the 'case' statement instead of all those 
> elsifs, but then I realized there was an even better way.
> 
> class Array2D
> 
>   Delta=[[0,0], [0,1], [1,1], [1,0], [1,-1], [0,-1], [-1,-1], [-1,0], 
> [-1,1]]
>   attr_reader :width, :height
> 
>   def initialize(width, height)
>     @width = width
>     @height = height
>     @data = Array.new(@width) { Array.new(@height) }
>   end
> 
>   def [](x, y, z)
>     deltaX, deltaY = *Delta[z]
>     x = x + deltaX % @width
>     y = y + deltaY
>     @data[x][y] unless y<0 or y>@height
>   end
> 
>   def []=(x, y, z, value)
>     deltaX, deltaY = *Delta[z]
>     x = (x + deltaX) % @width # modulus % allows wrapping
>     y = y + deltaY
>     @data[x][y] = value unless y<0 or y>(@height-1)
>   end
> end
> 
> 
> Obviously the Delta array takes the place of the elsif chains in both [] 
> and []=. Also, :width and :height are defined with attr_reader, not 
> attr_accessor, since it doesn't do any good at all to change those 
> values after the array has been created.

Just be aware I'm a complete newb at programing and Ruby, so the more 
elegant the code, the harder it is for me to read.

David, I tried your solution but it still falls over. Don't you need to 
specify what happens if y<0 or y>(@height-1) is the case? I tried...

    if y<0 or y>(@height-1)
        nil
      else
        @data[x][y] = value
    end

...but that didn't work? It falls over the moment it looks north from 
the top row, or looks south east from the bottom row.

Also, I understand the line...

deltaX, deltaY = *Delta[z]

...is allocating the Delta array values to the deltaX and deltaY 
variables based on the direction z given, but what is the * symbol 
doing?

By the way, my program uses a hex grid, not a square grid - I posted a 
square grid just for clarity. Adjacent cells directions change for odd 
and even rows on a hex grid, but I think I can just create two Delta 
arrays (Delta_even and Delta_odd) and modify the 1,0,-1 values for each, 
then check for odd/eveness in y.

I have yet to nut out the other solution but will go through it later 
today.
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