From: Ruby Quiz Date: 2006-12-01T22:45:45+09:00 Subject: [QUIZ] Turtle Graphics (#104) The three rules of Ruby Quiz: 1. Please do not post any solutions or spoiler discussion for this quiz until 48 hours have passed from the time on this message. 2. Support Ruby Quiz by submitting ideas as often as you can: http://www.rubyquiz.com/ 3. Enjoy! Suggestion: A [QUIZ] in the subject of emails about the problem helps everyone on Ruby Talk follow the discussion. Please reply to the original quiz message, if you can. -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= by Morton Goldberg [Editor's Note: You can download the files for this quiz at: http://rubyquiz.com/turtle.zip --JEG2] Turtle Graphics =============== Turtle graphics is a form of computer graphics based on the ideas of turtle geometry, a formulation of local (coordinate-free) geometry. As a brief introduction to turtle graphics, I quote from [1]: Imagine that you have control of a little creature called a turtle that exists in a mathematical plane or, better yet, on a computer display screen. The turtle can respond to a few simple commands: FORWARD moves the turtle in the direction it is facing some number of units. RIGHT rotates it clockwise in its place some number of degrees. BACK and LEFT cause the opposite movements. ... The turtle can leave a trace of the places it has been: [its movements] can cause lines to appear on the screen. This is controlled by the commands PENUP and PENDOWN. When the pen is down, the turtle draws lines. For example, the turtle commands to draw a square, 100 units on a side, can be written (in a Ruby-ized form) as: pen_down 4.times { forward 100; right 90 } For more information, see [2] and [3]. This quiz is a bit different from most. If the usual Ruby quiz can be likened to an essay exam, this one is a fill-in-the-blanks test. I'm supplying you with a complete turtle graphics package, except -- to give you something to do -- I've removed the method bodies from the key file, lib/turtle.rb. Your job is to repair the damage I've done and make the package work again. Turtle Commands =============== There are quite a few turtle commands, but that doesn't mean you have to write a lot of code to solve this quiz. Most of the commands can be implemented in a couple of lines. It took me a lot longer to write a description of the commands than it did for me to implement and test all of them. I use the following format to describe turtle commands: long_name | short_name description ... Example: ... All turtle commands take either one argument or none, and not all turtle commands have both a long name and a short name. Required Commands ----------------- These commands are required in the sense that they are needed to reproduce the sample designs. Actually, you could get away without implementing 'back' and 'left', but implementing them is far easier than trying to write turtle code without them. pen_up | pu Raises the turtle's pen. The turtle doesn't draw (lay down a visible track) when its pen is up. pen_down | pd Lowers the turtle's pen. The turtle draws (lays down a visible track) when its pen is down. forward | fd Moves the turtle forwards in the direction it is facing. Example: forward(100) advances the turtle by 100 steps. back | bk Moves the turtle backwards along its line of motion. back == forward - Example: back(100) backs up the turtle by 100 steps. right | rt Turns the turtle clockwise by degrees. Example: right(90) turns the turtle clockwise by a right angle. left | lt Turns the turtle counterclockwise by degrees. left == right - Example: left(45) turns the turtle counterclockwise by 45 degrees. Traditional Commands -------------------- These commands are not needed to reproduce any of the sample designs, but they are found in all implementations of turtle graphics that I know of. home Places the turtle at the origin, facing north, with its pen up. The turtle does not draw when it goes home. clear Homes the turtle and empties out it's track. Sending a turtle a clear message essentially reinitializes it. xy Reports the turtle's location. Example: Suppose the turtle is 10 turtle steps north and 15 turtle steps west of the origin, then xy will return [-15.0, 10.0]. set_xy | xy= Places the turtle at . The turtle does not draw when this command is executed, not even if its pen is down. Returns . Example: Suppose the turtle is at [10.0, 20.0], then self.xy = [50, 80] moves the turtle to [50.0, 80.0], but no line will drawn between the [10, 20] and [50, 80]. heading Reports the direction in which the turtle is facing. Heading is measured in degrees, clockwise from north. Example: Suppose the turtle is at the origin facing the point [100, 200], then heading will return 26.565 (approximately). heading= | set_h Sets the turtle's heading to . should be given in degrees, measured clockwise from north. Returns . Example: After self.heading = 135 (or set_h(135) which is easier to write), the turtle will be facing southeast. pen_up? | pu? Reports true if the turtle's pen is up and false otherwise. pen_down? | pd? Reports true if the turtle's pen is down and false otherwise. Optional Commands ----------------- These commands are only found in some implementations of turtle graphics. When they are implemented, they make the turtle capable of doing global (coordinate) geometry in addition to local (coordinate-free) geometry. I used one of these commands, go, to draw the mandala design (see designs/mandala.tiff and samples/mandala.rb). If you choose not to implement the optional commands, you might try writing a turtle program for drawing the mandala design without using go. But, believe me, it is much easier to implement go than to write such a program. go Moves the turtle to . Example: Suppose the turtle is home (at the origin facing north). After go([100, 200]), the turtle will be located at [100.0, 200.0] but will still be facing north. If its pen was down, it will have drawn a line from [0, 0] to [100, 200]. toward | face Turns the turtle to face . Example: Suppose the turtle is at the origin. After toward([100, 200]), its heading will be 26.565 (approximately). distance | dist Reports the distance between the turtle and . Example: Suppose the turtle is at the origin, then distance([400, 300]) will return 500.0 (approximately). Interfacing to the Turtle Graphics Viewer ========================================= Implementing turtle graphics without being able to view what the turtle draws isn't much fun, so I'm providing a simple turtle graphics viewer. To interface with the viewer, turtle instances must respond to the message track by returning an array which the viewer can use to generate a line drawing. The viewer expects the array returned by track to take the following form: track ::= [segment, segment, ...] # drawing data segment ::= [point, point, ...] # points to be joined by line segments point ::= [x, y] # pair of floats Example: [[[0.0, 0.0], [200.0, 200.0]], [[200.0, 0.0], [0.0, 200.0]]] This represents an X located in the upper-right quadrant of the viewer; i.e., two line segments, one running from the center of the viewer up to its upper-right corner and the other running from the center of the top edge down to the center of the right edge. [Editor's Note: I added a script to dump your turtle graphics output to PPM image files, for those that don't have TK up and running. It works identically to Morton's turtle_viewer.rb, save that it writes output to a PPM image file in the current directory. For example, to output the included tree image, use `ruby turtle_ppm_writer.rb samples/tree.rb`. --JEG2] Unit Tests ========== I'm including the unit tests which I developed to test turtle commands. For the purposes of the quiz, you can ignore tests/turtle_view_test.rb. But I hope you will find the other test suite, tests/turtle_test.rb, helpful. It tests every one of the turtle commands described above as well as argument checking by the commands. Don't hesitate to modify any of the unit tests to meet the needs of your quiz solution. References ========== [1] Abelson, H. & A. diSessa, "Turtle Geometry", MIT Press, 1981. [2] Harvey, B., "Computer Science Logo Style", Chapter 10. http://www.cs.berkeley.edu/~bh/pdf/v1ch10.pdf [3] Wikipedia, http://en.wikipedia.org/wiki/LOGO_programming_language