Lab #2 Solution

Goals







Relevant sections of HtDP/2e: Chapters 3 and 4.




This week’s lab will help you to practice:




conditionals, intervals, enumerations, simple itemizations



function composition: writing more complex functions by combining simpler functions



using the design recipe!



simple interactive programming using the universe library



Note: Exchange email addresses and phone numbers with your partner. Decide on your next meeting time this week in case you don't finish. Make sure both names are at the top of your lab submission!




Batch Program Problems







Note: Follow the Design Recipe! For each problem, consider if you need a Data




Definition more precise than just Number, String, Boolean, or Image. What is the




function Signature? The Purpose? Write at least one Example / Unit Test for every




different kind of data consumed and produced. Create the function Header with a body




stub that is an atomic value of the correct output type. Only now do you need to




consider how to actually write the function body (and if the inputs are enumerations,




then the body should say, case-by-case, what to do for each possible value using

cond). Don't be scared of a blank Definitions Window, and be aware that we grade

each of these steps separately.







Body Mass Index







Problem 1: Body Mass Index (BMI) is a number calculated from a person's weight and height that is a fairly reliable indicator of body fatness for most people. [http:// www.cdc.gov/healthyweight/assessing/bmi/adult_bmi/index.html]




Design a function bmi that consumes a person’s weight in pounds and height in inches,




and computes that person’s BMI. Make sure to carefully document your intended interpretation of the function parameters.









Follow the Design Recipe! We expect EVERY function you write, from now on, with few exceptions, to include the signature, purpose statement, and examples turned into unit tests, in addition to the necessary function header and body.




Problem 2: For adults 20 years old and older, BMI is interpreted using standard weight status categories that are the same for all ages and for both men and women. For children and teens, on the other hand, the interpretation of BMI is both age- and sex-specific. The standard weight status categories are underweight, normal, overweight,




and obese. Write a data definition for WeightStatusCat as an enumeration.







Problem 3: The standard weight status categories associated with BMI ranges for adults are shown in the following table:













BMI
Weight Status
Below 18.5
Underweight




18.5 up to but not including 25
Normal




25 up to but not including 30
Overweight




30 and above
Obese










Design a function bmi-wsc to convert a BMI value to a standard weight status category.




Traffic Light: Enumeration




Problem 4: A basic 3-bulb traffic light has three states, “red”, “yellow” and “green”.




Write a data definition for a TrafficLightState (an enumeration).







Problem 5: Design a function render-traffic-light that consumes a TrafficLightState and produces an Image of the corresponding light. For example,




(render-traffic-light "green")




might display something similar to





WARNING: you must write your unit test(s) to use actual drawing functions, NOT an inserted image when unit testing functions that produce images. For example:




;; Good




(check-expect (my-hat "blue")




(above (circle 10 "solid" "blue")




(rectangle 30 5 "solid" "blue")))




;; Bad




(check-expect (my-hat "blue") );don't paste images in unit tests!







Traffic Light: Intervals







Problem 6: A basic traffic light timer takes on values in the natural numbers (the “counting numbers” starting at 0) from 0 to 100, inclusive. The interpretation is as follows: times 0 to 50 indicate a green light; times 51 to 65 a yellow light, and 66 to 100 a red light. Write a data definition for a TrafficTimerValue (an itemization of




intervals).







Problem 7: Design a function increment-timer that consumes a TrafficTimerValue and produces the next TrafficTimerValue. Recall that the timer




resets to 0 after reaching 100. There are two possible solutions to this problem (one using cond, and one using modulo or remainder); use either solution.




Problem 8: Design a function render-traffic-timer that consumes a TrafficTimerValue and produces an image of the attached traffic light with the

correct bulb lit. Hint: you may want to use some of the functions that you already wrote and tested earlier in the lab.




Traffic Light: Simple Itemization







A traffic light can run on a timer, but many also have a manual override. A basic traffic light world state can be either a timer value (a natural number from 0 to 100 inclusive) OR a constant manual traffic light state indicating steady green, yellow, or red (a String enumeration). The complete data definition is given as follows [this should already be in your Solution File]:




A TrafficLightWorld is either



— a TrafficTimerValue [a Number interval], or



— a TrafficLightState [a String enumeration]



Now we are ready to build an interactive program: a simple traffic light simulator.


Interactive Program Problem







Let’s actually build the traffic light simulator using the Universe package. An interactive program consists of several parts: an underlying computational model of the world, a mapping from that model to a view that people can see (here, a graphical view), and controllers that interact with people (or other things, like the computer’s clock) to change the model.




At this point our model can only be a simple piece of atomic data or simple itemization (until next week!). The model must represent what changes; what the user manipulates. Here, our model is a TrafficLightWorld, which represents both a




traffic light on a changing timer, or one that is controlled manually with the keyboard. Interactive programs are created using the big-bang form, which in turn requires us to




write several different functions with pre-specified signatures. We’ll take these one at a time.




For this section, we use the following Data Definition (from above):




A TrafficLightWorld is either



— a TrafficTimerValue [a Number interval], or



— a TrafficLightState [a String enumeration]



Problem 9: Design a function render-traffic-world that consumes a TrafficLightWorld and produces an Image of the attached traffic light with the correct bulb lit. You may want to use some of the functions that you already wrote and tested. This function, render-traffic-world, maps the model to a view.




You may now define a function main that consumes an initial TrafficLightWorld and calls big-bang to create a new universe as follows:




main : TrafficLightWorld — TrafficLightWorld



The main function consumes the initial TrafficLightWorld



and calls big-bang to create the universe.



(define (main init)




(big-bang init ;the init parameter has type TrafficLightWorld (to-draw render-traffic-world))) ;TrafficLightWorld -- Image




If you load your definitions and call, for example, (main 0) or (main "red") you




should get the correct traffic light image. Now to add some interaction using the keyboard!




Note: DO NOT write unit tests for main.





When you press a key, a key-event is generated by the keyboard controller that can be used to “change” the world (the model). For example, we could set it up that pressing “g” changes the state to "green", pressing “r” changes it to "red", "y" changes it to "yellow", and "t" starts the timer at 0. Note that the key controller affects only the model, not the view (you already wrote a function that draws whatever the current model is on the screen: the model and the view are separate)!




Problem 10: Design a function handle-key that consumes a TrafficLightWorld and a KeyEvent, and produces a new TrafficLightWorld representing the new state. So for example




(check-expect (handle-key 55 “g”) "green")




(check-expect (handle-key "red" “t”) 0)




ignore any key that is not "g", "r", "y", or "t": (check-expect (handle-key 77 “w”) 77)



Note that you should compare keys using (key=? akeyevent "5"), and not string=? (see if you can figure out why).




Now modify main, adding a new clause for the key controller handler:




(define (main init)




(big-bang init ;TrafficLightWorld




(to-draw render-traffic-world) ;TrafficLightWorld -- Image




(on-key handle-key))) ;TrafficLightWorld KeyEvent —- TrafficLightWorld




Now if you run (main 0) in the interactions window, you should be able to change the Light by pressing keys.




Finally, let's get the timer working.







Problem 11: Design a function handle-tick the consumes a TrafficLightWorld and produces a new TrafficLightWorld. If the TrafficLightWorld is a TrafficTimerValue (a Number), then increment it appropriately. If the TrafficLightWorld is a TrafficLightState (a String, indicating that the light is being manually controlled) then return the same value, changing nothing.











Now modify main, adding a new clause for the tick handler:




(define (main init)




(big-bang init ;TrafficLightWorld

(to-draw
render-traffic-world) ;TrafficLightWorld -- Image
(on-tick
handle-tick)
;TrafficLightWorld -- TrafficLightWorld
(on-key
handle-key)))
;TrafficLightWorld KeyEvent —- TrafficLightWorld



Now if you run (main 0) in the interactions window, you should see the light change on its own timer AND be able to change the light by pressing keys.



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