From: Dave Howell <groups.2009a@...>
Date: 2010-07-17T11:44:24+09:00
Subject: Re: Return nothing when looking outside the bounds of 2D array?

OK, let's try this from the top. {grin} Here's some fresh new code:

	class HexArray
	
		Delta=[[0,0], [0.5,-1], [1,0], [0.5,1], [-0.5,1], [-1,0], [-0.5,-1]]
		attr_reader :width, :height
	
		def initialize(width, height, default="-")
			@width = width
			@height = height
			@data = Array.new(@width){|row| [default] * @height}
		end
	
		def [](x, y, z)
			col, row = move(x,y,z)
			@data[row][col] unless (row<0 or row>(@height-1))
		end
		
		def []=(x, y, z, value)
			col, row = move(x,y,z)
			@data[row][col] = value unless (row<0 or row>(@height-1))
		end
	
		def inspect
			@height.times{|rownum| print  ("  " * (rownum % 2) + @data[rownum].join("   ") + "\n")};nil
		end

	private

		def move(x, y, z) # returns an array with the row and column of the new position
			deltaX, deltaY = Delta[z]
			[(x + deltaX + 0.25 + (0.5 * (y%2)) % @width).floor, y + deltaY]
		end
	
	end

Instead of repeating the math for figuring out the row and column, I moved it to the "move" method. I also created an 'inspect' method so we can see what we're doing. The math in the 'move' method is all packed together, but it's just what I talked about earlier, except with all the parentheses in the right places to make it work correctly. 

Note that @data is no longer defined as "Array.new(@width) { Array.new(@height) }". I think I copied that from your code? but discovered that it gave each row the SAME array, so that changing [1][3] caused every cell in that column to have the same value. Useless! Not the first time that feature of Ruby (and it is a feature, wacky though it sometimes seems) has surprised me. 

Let's take it for a spin.

	>> a=HexArray.new(5,5)
	-   -   -   -   -
	  -   -   -   -   -
	-   -   -   -   -
	  -   -   -   -   -
	-   -   -   -   -
	=> 
	>> a[0,0,0] = "A" # upper left
	=> "A"
	>> a[4,4,0] = "Z" # lower right
	=> "Z"
	>> a[0,2,0] = "J" # center left
	=> "J"
	>> a[2,0,0] = "S" # center top
	=> "S"
	>> a[0,2,2] = "K" # right of J
	=> "K"
	>> a[2,0,3] = "T" # below S and to the right
	=> "T"
	>> a[0,2,5] = "I" # left of J (should wrap)
	=> "I"
	>> a[2,0,1] = "R" # above & right of S (should be ignored)
	=> "R"
	>> a[4,4,4] = "X" # below and left of Z (should be ignored)
	=> "X"
	>> a
	A   -   S   -   -
	  -   -   T   -   -
	J   K   -   -   I
	  -   -   -   -   -
	-   -   -   -   Z
	=> 

	>> a[0,2,4] = "M" # left and below J (should wrap)
	=> "M"
	>> a
	A   -   S   -   -
	  -   -   T   -   -
	J   K   -   -   I
	  -   -   -   -   M
	-   -   -   -   Z
	=> 
	>> 
	?> a[4,2,0]    #should be position "I"
	=> "I"
	>> a[4,2,3] = "N" # right and below "I", should overwrite "M"
	=> "N"
	>> a
	A   -   S   -   -
	  -   -   T   -   -
	J   K   -   -   I
	  -   -   -   -   N
	-   -   -   -   Z
	=> 


Also, I set it up so that a new HexArray gets filled with dashes. If you want to fill it will asterisks, or nils, just do this:
	a = Array2D(5,5,"*")
or 
	a = Array2D(5,5,nil)