Start DrRacket. Make sure that the language level is set to "Beginning Student" (Note: if you change the language level, it will not take effect until you hit the "Run" button).
Exercise 1: Experiment using DrRacket's Interactions Window as a calculator. See what operations you can use to calculate. Make sure to try big numbers and fractions. [3 min]
Exercise 2: Design a function that when given an integer that represents distance in miles will return the time (in hours) it takes to travel that distance when going 60 MPH. Write your function in the Definition Window, click Run, then use the Interactions Window to test your function. [5 mins]
Exercise 3: Design a function that given an integer representing time (in minutes) will return the distance traveled when going 70 MPH, rounding up to whole miles. [15 mins]
Hint: DrRacket comes with a help system called Help Desk. Open the Help Desk and click "How to Design Programs Languages", then click "Beginning Student". You will get a reference for the Beginning Student language. You can also highlight the name of a function and press F1, this will search Help Desk for the highlighted text.
Exercise 4: There are three kinds of errors you can make when writing a program in DrRacket. Come up with examples of each and try them out. Examine any error messages you get back. [5 mins]
Access the 2nd edition HTDP "universe" and "image" libraries in DrRacket. Put the following magic text at the top of your definitions window:
(require 2htdp/image)
(require 2htdp/universe)
Libraries come with documentation. In the Help Desk, search for "universe". Also have a look at the documentation of the image library titled "2htdp/image".
Exercise 5: Download an image off the web and insert it
into your Definition window. Use define to give
it a descriptive name.
Examples: cars, boats, planes etc.
Exercise 6: Use place-image and
empty-scene to create scenes with your image.
Exercise 7: Create rectangles with the following dimensions
Exercise 8: Design the function rec-sequence
that given an index will return the corresponding rectangle,
e.g., (rec-sequence 2) returns a rectangle of size 20
by 50.
Using the "universe" module we can create animations. The module allows you to model a world. Every time the clock ticks, the module uses one of your functions to update or create a new world, which becomes the current world. The module can use another of your functions to create an image of your world.
;; World is a positive number (current time)
;; World -> World
;; calculates the next world (increments the time)
(define (next-world w)
(add1 w))
;; World -> Image
;; draw the image for snapshot time w in the middle
;; of a 200x200 scene
(define (world-draw w)
(place-image (circle w "solid" "black")
100 100
(empty-scene 200 200)))
big-bang in Help Desk).
;; We start at time 1, use next-world to update the world at each
;; tick, and use world-draw to render the world as a scene:
(big-bang 1
(on-tick next-world)
(to-draw world-draw))
Exercise 9: Right now the circle grows forever, so
you might want to stop it from growing when it gets too big
for the canvas. Look up stop-when in the
Help Desk.
Exercise 10: Change the animation above so that instead of growing larger, the disk moves across the canvas. Try left to right (x), top to bottom (y) and diagonal (both x and y).
Exercise 11: 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 (ie - a week late) there is a flat $20 fee that never changes. Write tests for your function when you are done. Make sure to try out all the branches in your code.
Exercise 12: Run the Stepper on your program from the previous exercise. Figure out what happens in the Stepper when the program makes a conditional decision.
Exercise 13: Design a function that calculates sales tax. The
function consumes the sale price and the tax percentage (as a decimal or
a fraction) and produces the final price. For instance, if
given 20 and 5/100 it should compute 105% of
$20, or 21.
Exercise 14: 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 the previous exercise.
Did you write tests for all the branches of your code for the last two exercises?
Exercise 15: Run the Stepper on your program from the previous exercise. Figure out where the Stepper calls your program from Exercise 13. Try it with values both over and under $100.
In this part of the lab, we will create an interactive animation,
using the "universe" library in DrRacket. We'll use big-bang
again, but we'll add a new sub-form, on-mouse. You should
read the Help Desk documentation for on-mouse before
beginning. In particular, determine what the inputs of
on-mouse's argument.
Exercise 16: Design a function place-bar that
consumes the world, and produces a 300-by-300 scene with a black bar
of width 300 and height 20 at the position (150, world).
Exercise 17: 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. In this
exercise, a World is a Number (make sure to write this in your program).
When the MouseEvent is "button-down", your function should
produce the y coordinate of the mouse click. Any other time, it produces
the original World unchanged.
Exercise 18: Use your functions mouse-click and
place-bar 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 150
(to-draw place-bar)
(on-mouse mouse-click))
If you are done early, try creating an animation that places a circle at the x and y positions of the user's mouse clicks.