Lab 2

Outline

Pair Programming

As always, you will work with your homework partner during the lab. Switch between the pilot and co-pilot roles frequently. Be sure to have your blue pair programming booklet with you at labs, lectures, and homework meetings, and always have your next meeting written down.

Stepping through functions

Exercise 1: Design a function that consumes a quantity (number) and a unit (symbol) representing a measurement and produces a string describing the measurement. For instance, if given 3 and 'inches it should produce "3 inches". Hint: Look up number->string and symbol->string in Help Desk.

Exercise 2: Design a function that consumes two quantities (numbers) and a unit (symbol) and produces a string describing both measurements and their sum, all in the given unit. For instance, if given 3, 4, and 'pounds it should produce "3 pounds + 4 pounds = 7 pounds". Hint: Use the program you wrote for Exercise 1.

Exercise 3: The Stepper is a DrScheme tool that shows every step a program takes. Activate the Stepper with the button labeled "Step" with a picture of a foot, to the left of the Run button. Use the Stepper on your programs from Exercise 1 and 2. Figure out what happens in the Stepper when one program calls another program.

Switch Partners

Conditional functions

Exercise 4: Design a function that calculates late fees on a movie rental. The function consumes the number of days the movie has been rented. Up to 3 days is a regular rental, no fee. For the next week the fee goes up $3 each day. Starting on the 10th day the fee becomes $20 and never changes.

Exercise 5: Run the Stepper on your program from Exercise 4. Figure out what happens in the Stepper when the program makes a conditional decision.

Switch Partners

Exercise 6: Design a function that calculates sales tax. The function consumes the sale price and the tax percentage (as a fraction of the original price) and produces the final price. For instance, if given 20 and 5/100 it should compute 105% of $20, or 21.

Exercise 7: Design a function that calculates conditional sales tax: only prices $100 or more are taxed. The function consumes a sale price and a tax percentage and produces the final price. For instance, if given 20 and 5/100, it computes $20 (no tax). But if given 200 and 5/100, it computes $210 (5% tax). Hint: Use your program from Exercise 6.

Exercise 8: Run the Stepper on your program from Exercise 7. Figure out where the Stepper calls your program from Exercise 6. Try it with values both over and under $100.

Switch Partners

Mouse clicks

This part of the lab will develop an interactive animation. Add the "world.ss" teachpack in DrScheme. Open the Language menu and select "Add Teachpack...". Open the htdp folder. Select and open "world.ss" (or just "world"). 

  1. ;; A Posn is (make-posn Number Number)
(define-struct posn (x y))
    Do not copy this data definition into DrScheme. It is already provided for you. But you should be aware of how this data definition works and how to make and use Posn data.

    A Posn is a structure defining a position with x and y coordinates. DrScheme provides Posns with the definition above. Write down the Posn functions provided by this definition. How many should there be? Remember to include constructors and selectors.

  2. Design a function place-circle that consumes a Posn and produces a 300-by-300 scene with a red circle of radius 10 at the position represented by the Posn. For instance, if given (make-posn 10 290) it should produce a scene with a red circle in the lower left corner.

    3. Switch Partners 

  3. big-bang : Number Number Number World -> true
on-redraw : (World -> Image) -> true
on-tick-event : (World -> World) -> true
on-mouse-event : (World Number Number MouseEvent -> World) -> true
    You've seen the first three animation functions before. Look them up in Help Desk if you need to remind yourself how they work. Now we have one more function for reacting to the mouse pointer. Look up on-mouse-event in Help Desk. Just like on-redraw and on-tick-event, we have to give a function to on-mouse-event. Find out what this function's inputs and outputs mean, and what a MouseEvent is.

  4. Design a function mouse-click to react to clicks. It consumes a World, two Numbers (x and y coordinates), and a MouseEvent, as described in on-mouse-event. For our purposes, a World is a Posn (make sure to write this in your program). When the MouseEvent is 'button-down, this function produces the x and y coordinates of the mouse click as a Posn. Any other time, it produces the original World unchanged.

  5. Put these two functions together to make an animation. Add the following three lines to your Definitions window, hit Run, and try clicking in the new window that opens. 

    (big-bang 300 300 .1 (make-posn 0 0))
(on-redraw place-circle)
(on-mouse-event mouse-click)