This lab has two purposes:
To give you hands-on experience writing Universe (big-bang) programs that interact with the mouse.
To give you practice using structures (definition by parts) and state machines (definition by choice) to build Universe programs.
Both parts of the lab will prepare you for the interactive user-interface experiment that you will build for your homework.
As in most labs, you will work with a partner. Tom Williams will have your partner assignment.
The lab will begin with a demonstration of two programs: a circle-dropping program and a line-drawing program. You will have time to build the full circle-dropping program and to start a prototype of the line-drawing program.
In any big-bang program, your key to success is the world-state. The world-state determines what you can draw, and it also determines how the mouse and the keyboard are supposed to behave.
Working with your partner, think about how you want to represent the state of the circle-dropping program. How will you represent the state of the circle-dropping world? If there is more than one state, draw a state diagram like the one we used for the traffic light.
Write down examples of the world state.
Working with your partner, think about how you want to represent the state of the line-drawing program. How will you represent the state of the line-drawing world? If there is more than one state, draw a state diagram like the one we used for the traffic light.
Write down examples of the world state.
Show your state diagrams and your examples to the lab staff.
These programs don’t have any animations that run by themselves. Instead the movement of lines or other things on the screen is driven by interaction with the mouse and/or keyboard. In addition to the to-draw method of big-bang, which is needed in every big-bang program, you will begin with one or two other methods on your wish list:
The on-mouse method is a mouse event handler. It reacts both to mouse movement and to mouse clicks. You will need to look up the documentation of mouse events in the 2htdp/universe teachpack and to analyze the conditions under which you think a mouse event is interesting. In different programs you may well be interested in different conditions.
All mouse-event handlers have the same signature. Here it is, along with a very generic purpose statement and a simple header:
; meh : world-state number number mouse-event -> world-state
; based on the given state, to react appropriately to the given
; mouse event at the given (x, y) coordinates, and to return
; the new world state that results
(define (meh world x y event)
...)The line-drawing program stops when the q key is pressed. To implement this behavior you will need to implement the on-key method.
Here is a generic key-event handler:
; keh : world-state key-event -> world-state
; based on the given state, to react appropriately to the given
; key event, and to return the new world state that results
(define (keh world a-key)
...)To understand when the program is over you will have to learn about the stop-when method, and you will have to design a data representation that includes the idea of a “final” or “stop” state.
Your wish list will include a rendering function (passed to to-draw), a mouse-event handler (passed to on-mouse), and possibly a keyboard-event handler. Your wish list should also include whatever functions you think you might need to change the world state in response to mouse events and keyboard events. Every wish-list function needs a signature and a purpose statement.
Write down your wish list!
Now you’re ready to tackle the “bricks”:
Begin with your data descriptions and data examples. You should already have a data definition for the world state, but you may not others. Use structure definitions where needed.
Write data examples!
Have the lab staff check your data descriptions.
Make sure that every function on your wish list has a signature, purpose statement, and header.
Have the lab staff check your signatures, purpose statements, and headers.
Using the signatures, purpose statements, and headers of the functions on your wish list, start devising functional examples.
Create function templates. Use the template for conditionals and the template for structures.
Code
Test
Run
Ten minutes before the end of the lab, put the following text at the beginning of a DrRacket file. You may use an empty file or the source code you have been working on:
#|
What I did during this lab:
(you fill in this part)
What I learned during this lab:
(you fill in this part)
|#The lab staff will help you articulate what you learned.
Finally, submit this file through the handin server as lab2. You will need to submit it using two usernames connected by a + sign, as in
Jane.Doe+Richard.RoeYou submit using Jane Doe’s password.