Fundamentals of Computer Science I (CS151.01 2006F)

Lab: Objects in Scheme

This lab is also available in PDF.

Summary: In this lab, we consider techniques for building objects, collections of data that support operations on those data.



Exercise 1: Testing Switches

a. Make a copy of the make-switch procedure from the reading.

b. Test the switches created by the make-switch procedure. Here are a few possible instructions.

> (define lamp-switch (make-switch))
> (define vacuum-cleaner-switch (make-switch))
> (lamp-switch ':show-position)
> (vacuum-cleaner-switch ':show-position)
> (lamp-switch ':toggle!)
> (lamp-switch ':show-position)
> (vacuum-cleaner-switch ':show-position)
> (lamp-switch ':toggle!)
> (vacuum-cleaner-switch ':toggle!)
> (lamp-switch ':show-position)
> (vacuum-cleaner-switch ':show-position)

Exercise 2: A Multi-Stage Switch

As you may know, some switches have more than two positions. For example, we might have a switch that switches between the values 0, 1, 2, and 3. (For a two-way light, 0 might represent off, 1 might represent light one on, 2 might represent light two on, and 3 might represent both lights on.) In general, you can only toggle to the next higher value (with 3 returning to 0 when toggled).

Implement four-stage-switch, which responds to the same messages as a switch, but with four possible stages.

Exercise 3: A Single Tally Object

Define a one-field object, tally, that responds to exactly four messages:

The initial value of the field should be 0.

For example,

> (tally ':set-contents-to-zero!)
> (tally ':show-contents)
> (tally ':increment!)
> (tally ':show-contents)
> (tally ':decrement!)
> (tally ':decrement!)
> (tally ':decrement!)
> (tally ':show-contents)

Note that you are creating a single object, not a procedure that creates objects.

Exercise 4: Making Generic Tallys

a. Define a make-tally procedure that constructs and returns objects similar to the tally object you defined in exercise 2.

b. Create two tally objects and demonstrate that they can be incremented and reset independently.

Exercise 5: Tallys with Initial Values

Write a new make-tally procedure that allows the client to create new tallys with a specied initial value. For example, I might say that a starting grade is 90 with

> (define grade (make-tally 90))

I would then increment and decrement it as students do good or bad work.

Exercise 6: Monitored Tallys

a. Define a constructor procedure, make-monitored-tally, for objects similar to the tally objects from exercise 2 above, except that each such object keeps track of the total number of messages that it has received.

Hint: For this exercise, you may want to make two vectors, one for the value of the tally and one for the count of operations. Alternately, you could make a two-element vector in which element 0 of that vector is the value of the tally and element 1 of that vector is the count of operations.

b. Test your procedure.




Disclaimer: I usually create these pages on the fly, which means that I rarely proofread them and they may contain bad grammar and incorrect details. It also means that I tend to update them regularly (see the history for more details). Feel free to contact me with any suggestions for changes.

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Samuel A. Rebelsky,

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