# 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.

Contents:

## Exercises

### 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:

• `':show-contents`,
• `':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 field should be 0.

For example,

```> (tally ':set-contents-to-zero!)
> (tally ':show-contents)
0
> (tally ':increment!)
> (tally ':show-contents)
1
> (tally ':decrement!)
> (tally ':decrement!)
> (tally ':decrement!)
> (tally ':show-contents)
-2
```

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.

## History

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.

This document was generated by Siteweaver on Thu Nov 30 21:43:06 2006.
The source to the document was last modified on Wed Nov 22 09:36:39 2006.
This document may be found at `http://www.cs.grinnell.edu/~rebelsky/Courses/CS151/2006F/Labs/objects.html`.

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Samuel A. Rebelsky, rebelsky@grinnell.edu

Copyright © 2006 Samuel A. Rebelsky. This work is licensed under a Creative Commons Attribution-NonCommercial 2.5 License. To view a copy of this license, visit `http://creativecommons.org/licenses/by-nc/2.5/` or send a letter to Creative Commons, 543 Howard Street, 5th Floor, San Francisco, California, 94105, USA.