Lab 3: Multiple cases

Important Note Whenever you write a function in this class, you need to follow the design recipe.

1 Conditionals

Conditionals are a feature of the Beginning Student Language and every other programming language. They choose which expression is evaluated based on which condition is true rather than false.

2 Data definitions

Every function you write in this class requires following the design recipe. According to the design recipe, we need to define our data (step 1) in order to write a function signature (step 2) and template (step 4).

Some data types are well-known and you don’t need to define them. For example, you don’t need to explicitly define Image, Number, String, or Boolean. You can also define your own data types using data definitions. The data definition can have any name that you want, but you should try to choose a meaningful name so that other people will understand it. And after your data definition, you can use your new data type in function signatures or even other data definitions.

3 Enumerations

Exercise 5. Put the data definition above into the Definitions Window of DrRacket. What happens when you run the program? What happens if you remove the semicolons and then run the program?

In this class, we put data definitions in comments because they are interpreted by other programmers, not by the computer. Some other programming languages (which you will learn in other classes) will read and enforce the data definitions, so they will not be in comments.

Exercise 6. A deck of cards has four suits: clubs, diamonds, hearts, and spades. Define an enumeration for a suit of cards.

Exercise 7. The colors of the rainbow are hot pink, red, orange, yellow, green, turquoise, indigo, and violet. Define an enumeration for the colors of the rainbow.

4 Examples of enumerations

With the data definition of a Department in hand, we can define the following three examples of Departments:

(define alices-department "computer science")

(define bobs-department "business")

(define charlies-department "English")

Exercise 9. Define two examples of a suit of cards.

Exercise 10. Define two examples of a color of the rainbow.

Exercise 11. Define two examples of an order of your food truck.

If you try to define an example which is not one of the possible values of the enumeration, you will not get an error message because the Beginning Student language does not enforce the data definition. But it will still be incorrect.

Exercise 13. Define a non-example of a color of the rainbow.

Exercise 14. Define a non-example of an order of your food truck.

5 Templates from enumerations

From the data definition, you can write a function template. A function template is written as a function definition with parts missing, which are replaced with ellipsis: ... (sometimes pronounced “dot dot dot”). The body of a function template typically includes crucial information which will guide the function definition.

The Beginning Student language can check the syntax of a template, but since a template is incomplete, the Beginning Student language cannot run it.

Exercise 17. Write a template for processing a suit of cards.

Exercise 18. Write a template for processing a color of the rainbow.

Exercise 19. Write a template for processing an order of your food truck.

6 Functions on enumerations

Finally, we can use the template to design a function by filling in the ellipses. Consider the following function which calculates the salary for each department:

; salary : Department -> Number

; returns the salary for each department.

(check-expect (salary "biology")          100000)

(check-expect (salary "business")         110000)

(check-expect (salary "computer science") 120000)

(check-expect (salary "English")          130000)

(define (salary d)

(cond [(string=? d "biology")          100000]

[(string=? d "business")         110000]

[(string=? d "computer science") 120000]

[(string=? d "English")          130000]))

Exercise 20. Examine the function designed above. How many examples (expressed as tests) does it have? How many template cases does it fill in? What do these numbers have in common? Why should they?

Exercise 21. Clubs are worth one point, diamonds are worth two points, hearts are worth three points, and spades are worth four points. Design a function suit-points which, given a suit of cards, will calculate how many points it is worth.

Exercise 22. Design a function next-color which, given a color of the rainbow, will return the next color. For example, red will return orange, and yellow will return green. Since violet is the last color, there is no color after it. So as a special case, violet should return hot pink, which is the first color.

Exercise 23. Do the rest of this lab in groups of at least 2 people. Also, feel free to do this exercise and the next exercise in parallel. However, it is very important that you use the same data definition for an Order that you wrote in Exercise 8.

Design a function price-order that takes an Order and returns its price (a number).

Exercise 24. Design a function draw-order that takes an Order and returns a crude image of the food.

7 Challenge: intervals are cuts

In this section of the lab, you’ll make the beginning of a movie. That movie is Star Wars.

Exercise 26. Define a constant image epigraph-text. It should be the blue text epigraph1 above epigraph2. A good size of the text might be 40.

Then define another constant image shot1. It should be the epigraph-text you just defined, centered over background.

Exercise 27. Design a function shot3 that produces the yellow text "STAR" above "WARS", centered over background. The function should take the text size (a number like 200 or 2) as input.

Exercise 29. Enjoy (animate star-wars-opening). Can you improve the movie?