Lab 4: Structures

Note: Whenever you design or write a function, you need to follow the design recipe.

1 Data definitions of structures

This week we have studied a new kind of data definition: structures. A structure packages several values into a single new value. Consider the following Vehicle structure, which contains three fields: company, model, and year.

; A Vehicle is (make-vehicle String String Number)

(define-struct vehicle (company model year))

Exercise 1. Which line above is a data definition? Which line above is a structure definition? Which line above has the types of each field? Which line above has the names of each field? Don’t get these backwards!

Exercise 2. Put the data definition above into the Definitions Window of DrRacket. What happens when you run the program? What happens if you remove the semicolon then run the program?

The data definition on the first line is a comment, so the Beginning Student language does not enforce the types of the fields. But the structure definition on the second line is not a comment, so the Beginning Student language does enforce how many fields and what they are called.

Exercise 3. A book has a title, author, and number of pages. Develop a data and structure definition for a book.

Exercise 4. (Come back to this exercise later if your food-truck colleagues are waiting.) An instructor has a name, department, and salary. Develop a data and structure definition for an instructor. Does your data definition for an instructor refer to the data definition for a department? It should.

Exercise 5. (Come back to this exercise later if your food-truck colleagues are waiting.) Develop a data and structure definition for a School, which holds three departments. Does your data definition for a School refer to the data definition for a department? It should.

2 Examples of structures

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

(define brians-vehicle (make-vehicle "Toyota" "Camry" 2005))

(define marks-vehicle (make-vehicle "Ford" "F-150 Flareside" 1998))

(define rachaels-vehicle (make-vehicle "Chrysler" "PT Cruiser" 2005))

In the Interactions Window, we can retrieve the examples of Vehicles that we defined:

A department is only one of the values: biology, business, computer science, or English. But a vehicle is all of the fields: company, model, and year.

Exercise 6. Is the following an example of a Vehicle? What happens when DrRacket runs this definition? Why?

(define kens-vehicle (make-vehicle "Bicycle" 1982))

Exercise 7. Is the following an example of a Vehicle? What happens when DrRacket runs this definition? Why?

(define elons-vehicle (make-vehicle 2018 "SpaceX" "Falcon-9 Heavy"))

Exercise 8. Define two examples of books.

Exercise 9. Do this exercise in groups of at least 2 people.

Define two examples of Orders.

Exercise 10. Define two examples of instructors.

Exercise 11. Define two examples of Schools.

3 Courtesy functions for structures

The constructor make-vehicle makes a new Vehicle, as we just saw.

Exercise 12. List the signatures for each of the courtesy functions for the book structure.

Exercise 13. List the signatures for each of the courtesy functions for Order.

Exercise 14. List the signatures for each of the courtesy functions for the instructor structure.

Exercise 15. List the signatures for each of the courtesy functions for School.

4 Templates from structures

The template for processing a structure reminds us to use each field.

Exercise 16. Does the template for processing Departments have a conditional? Does the template for processing Vehicles have a conditional? Why the difference?

Exercise 17. Write a template for processing a book.

Exercise 18. Write a template for processing an order.

Exercise 19. Write a template for processing an instructor.

Exercise 20. Point out where the data definition for an instructor refers to the data definition for a department. Does your template for processing an instructor refer to the template for processing a department in the corresponding place? It should.

Exercise 21. Write a template for processing a School.

Exercise 22. Point out where the data definition for a School refers to the data definition for a department. Does your template for processing a School refer to the template for processing a department in the corresponding places? It should.

5 Functions on structures

We can fill in the same template differently to design another function upgrade-vehicle which upgrades the vehicle by three years:

; upgrade-vehicle : Vehicle -> Vehicle

; Upgrades the given vehicle by three years.

(define (upgrade-vehicle v)

(make-vehicle (vehicle-company v)

(vehicle-model v)

(+ (vehicle-year v) 3)))

(check-expect (upgrade-vehicle brians-vehicle) (make-vehicle "Toyota" "Camry" 2008))

Exercise 23. Design a function tome? which takes a book and returns whether it has more than 300 pages.

Exercise 24. 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 9.

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

Exercise 25. Design a function draw-order that takes an Order and returns a crude image of the food.

Exercise 27. Design a function equalize-salary which takes an instructor and returns another instructor with their salary changed to the salary for their department. Use the salary function provided above.

Exercise 28. Design a function average-salary which takes a School and produces the average salary of its three departments. Use the salary function provided above.

6 Challenge problem

Design an interactive animation of a rocket launch as follows. When the animation starts, the rocket rests motionless, centered at the bottom of the scene, ready to launch. When the user/player hits any key, the rocket launches and flies straight upwards. Once the rocket is flying, it should ignore any further launch requests (keypresses); but it doesn’t. Unfortunately, a keypress while the rocket is in-flight in fact causes it to malfunction and explode. The user should then see an explosion of some sort (say, a brightly colored circle) at the last position of the rocket.

The key challenge in this problem is to come up with a good data definition for the state of the rocket, which can be used by big-bang as a World. If you’re having trouble getting started, feel free to consult section 4.5 of the textbook (which discusses a rocket launch program in detail) and lecture 7 of the course (which introduces big-bang).