CSC151.01 2009F Functional Problem Solving : Labs

Lab: Objects in Scheme

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 ':get-position)
> (vacuum-cleaner-switch ':get-position)
> (lamp-switch ':toggle!)
> (lamp-switch ':get-position)
> (vacuum-cleaner-switch ':get-position)
> (lamp-switch ':toggle!)
> (vacuum-cleaner-switch ':toggle!)
> (lamp-switch ':get-position)
> (vacuum-cleaner-switch ':get-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 a four-stage-switch object that 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:

  • ':get-value,
  • ':set-contents-to-zero!,
  • ':increment!, which has the effect of increasing the number stored in the contents field by 1.
  • ':decrement!, which has the effect of decreasing the number stored in the contents field by 1.

The initial value of the tally should be 0.

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

For example,

> (tally ':get-value)
> (tally ':increment!)
> (tally ':get-value)
> (tally ':decrement!)
> (tally ':decrement!)
> (tally ':decrement!)
> (tally ':get-value)

Exercise 4: Making Generic Tallys

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

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, a professor might say that a starting grade is 90 with

> (define grade (make-tally 90))

The professor 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 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.

Creative Commons License

Samuel A. Rebelsky,

Copyright (c) 2007-9 Janet Davis, Matthew Kluber, Samuel A. Rebelsky, and Jerod Weinman. (Selected materials copyright by John David Stone and Henry Walker and used by permission.)

This material is based upon work partially supported by the National Science Foundation under Grant No. CCLI-0633090. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

This work is licensed under a Creative Commons Attribution-NonCommercial 2.5 License. To view a copy of this license, visit or send a letter to Creative Commons, 543 Howard Street, 5th Floor, San Francisco, California, 94105, USA.