From: Shawn W_ <shawnw@...>
Date: 2010-07-19T12:47:24+09:00
Subject: Re: Return nothing when looking outside the bounds of 2D array?

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

Thx again - very instructive.

I have managed to get the capping working in isolation as per the 
following code:

---

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
  attr_accessor :cell_content

  def initialize(width, height, cell_content)
    @width = width
    @height = height
    @data = Array.new(@height) {Array.new(@width){|i| cell_content} }
  end

  def [](x, y, z)
    deltaX, deltaY = *Delta[z]
    x = (x + deltaX)  % @width
    y = (y + deltaY)
    @data[y][x] unless y<0 or y>(@height-1)
  end

  def []=(x, y, z, value)
    deltaX, deltaY = *Delta[z]
    x = (x + deltaX)   % @width # modulus % allows wrapping
    y = (y + deltaY)
    @data[y][x] = value unless y<0 or y>(@height-1)
  end

  def display
    @height.times {|rownum| print(@data[rownum].join(" ") + "\n")};nil
  end

end

a=Array2D.new(10,5,"X")
m = rand(10)
n = rand(5)
a[m,n,0] = "A"
a[m,n,1] = "H"
a[m,n,2] = "H"
a[m,n,3] = "H"
a[m,n,4] = "A"
a[m,n,5] = "A"
a[m,n,6] = "A"
a[m,n,7] = "A"
a[m,n,8] = "A"
puts a.display

---

However, it still falls over when I try to insert the above into my main 
program, as below:

---

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(@height) {Array.new(@width) {|i|"X"}}
  end

  def [](x, y, z)
    deltaX, deltaY = *Delta[z]
    x = (x + deltaX)  % @width
    y = (y + deltaY)
    @data[y][x] unless y<0 or y>(@height-1)
  end

  def []=(x, y, z, value)
    deltaX, deltaY = *Delta[z]
    x = (x + deltaX)   % @width # modulus % allows wrapping
    y = (y + deltaY)
    @data[y][x] = value unless y<0 or y>(@height-1)
  end

  def display
    @height.times {|rownum| print(@data[rownum].join(" ") + "\n")};nil
  end

end


class World < Array2D

  attr_reader :size

  def initialize(size)
    @rows = size
    @cols = @rows * 2
    @world = Array2D.new(@cols, @rows)
  end

  def populate_array_cells_with_hash
    (0...@rows).each do |b|
      (0...@cols).each do |a|
        @world[a,b,0] = {
        "hex_col_no" => a,
        "hex_row_no" => b,
        "hex_id_A" => "X ",
        "hex_id_B" => nil,
        "hex_id_C" => nil,
        "hex_id_D" => nil,
        "hex_id_E" => nil,
        }
      end
    end
  end

  def insert_teleporting_square
    m = rand(@cols)
    n = rand(@rows)
    @world[m,n,0]["hex_id_A"] = "A "
    @world[m,n,1]["hex_id_A"] = "H "
    @world[m,n,2]["hex_id_A"] = "H "
    @world[m,n,3]["hex_id_A"] = "H "
    @world[m,n,4]["hex_id_A"] = "A "
    @world[m,n,5]["hex_id_A"] = "A "
    @world[m,n,6]["hex_id_A"] = "A "
    @world[m,n,7]["hex_id_A"] = "A "
    @world[m,n,8]["hex_id_A"] = "A "
  end

  def print_world(type)
    (0...@rows).each do |b|
      puts
        (0...@cols).each do |a|
          print @world[a,b,0][type]
        end
    end
  end

end

first_world = World.new(5)
first_world.populate_array_cells_with_hash
first_world.insert_teleporting_square
first_world.print_world("hex_id_A")
puts

---

It falls over when it hits one of these two lines:
@world[m,n,1]["hex_id_A"] = "H "
@world[m,n,4]["hex_id_A"] = "A "

With error as before:
undefined method `[]=' for nil:NilClass (NoMethodError)

What my program does is generate an array, then populates each cell of 
that array with a hash. It looks like when it tries to access the hash 
values outside the array boundary it runs into problems? Do I need 
another method inside the Array2D class to deal with hash access? If so, 
how can this be done? If not, can anyone see any other reason for the 
error?

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