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:
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)
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.
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.
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.
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.
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.
;
