From: Pete Yandell <pete@...>
Date: 2006-12-05T06:08:49+09:00
Subject: Re: tk.rb warning in Turtle Graphics (#104)


On 04/12/2006, at 8:28 PM, Edwin Fine wrote:

> Here's my solution.

Ok, a couple of comments.

First, your home method doesn't raise the pen as it should.

Second, all that flipping between turtle space angles and normal  
angles is unnecessary. Just swap the x and y axes when doing the trig  
and you'll get the right result.

Third, Ruby has an atan2 method that does most of what you do in your  
toward method.

Fourth, your normalize_degrees method is overkill. Try '-10 % 360' in  
irb.

Here's my solution:


class Turtle
    include Math # turtles understand math methods
    DEG = Math::PI / 180.0

    attr_accessor :track
    alias run instance_eval

    def initialize
      clear
    end

    attr_reader :xy, :heading

    # Place the turtle at [x, y]. The turtle does not draw when it  
changes
    # position.
    def xy=(coords)
      raise ArgumentError unless is_point?(coords)
      @xy = coords
    end

    # Set the turtle's heading to <degrees>.
    def heading=(degrees)
      raise ArgumentError unless degrees.is_a?(Numeric)
      @heading = degrees % 360
    end

    # Raise the turtle's pen. If the pen is up, the turtle will not  
draw;
    # i.e., it will cease to lay a track until a pen_down command is  
given.
    def pen_up
      @pen_is_down = false
    end

    # Lower the turtle's pen. If the pen is down, the turtle will draw;
    # i.e., it will lay a track until a pen_up command is given.
    def pen_down
      @pen_is_down = true
      @track << [@xy]
    end

    # Is the pen up?
    def pen_up?
      !@pen_is_down
    end

    # Is the pen down?
    def pen_down?
      @pen_is_down
    end

    # Places the turtle at the origin, facing north, with its pen up.
    # The turtle does not draw when it goes home.
    def home
      @heading = 0.0
      @xy = [0.0, 0.0]
      @pen_is_down = false
    end

    # Homes the turtle and empties out it's track.
    def clear
      @track = []
      home
    end

    # Turn right through the angle <degrees>.
    def right(degrees)
      raise ArgumentError unless degrees.is_a?(Numeric)
      @heading += degrees
      @heading %= 360
    end

    # Turn left through the angle <degrees>.
    def left(degrees)
      right(-degrees)
    end

    # Move forward by <steps> turtle steps.
    def forward(steps)
      raise ArgumentError unless steps.is_a?(Numeric)
      @xy = [@xy.first + sin(@heading * DEG) * steps, @xy.last + cos 
(@heading * DEG) * steps]
      @track.last << @xy if @pen_is_down
    end

    # Move backward by <steps> turtle steps.
    def back(steps)
      forward(-steps)
    end

    # Move to the given point.
    def go(pt)
      raise ArgumentError unless is_point?(pt)
      @xy = pt
      @track.last << @xy if @pen_is_down
    end

    # Turn to face the given point.
    def toward(pt)
      raise ArgumentError unless is_point?(pt)
      @heading = (atan2(pt.first - @xy.first, pt.last  - @xy.last) /  
DEG) % 360
    end

    # Return the distance between the turtle and the given point.
    def distance(pt)
      raise ArgumentError unless is_point?(pt)
      return sqrt((pt.first - @xy.first) ** 2 + (pt.last  - @xy.last)  
** 2)
    end

    # Traditional abbreviations for turtle commands.
    alias fd forward
    alias bk back
    alias rt right
    alias lt left
    alias pu pen_up
    alias pd pen_down
    alias pu? pen_up?
    alias pd? pen_down?
    alias set_h heading=
    alias set_xy xy=
    alias face toward
    alias dist distance

private

   def is_point?(pt)
     pt.is_a?(Array) and pt.length == 2 and pt.first.is_a?(Numeric)  
and pt.last.is_a?(Numeric)
   end

end