More Object-Oriented Programming

• Exercise 0: Preparation
• Exercises
  • Exercise 1: A Simple Statistician
  • Exercise 2: Adding Statistics
  • Exercise 3: A Variant of map
  • Exercise 4: Repeated Observations
  • Exercise 5: An Alternate Statistician
• Notes
  • A Note on Problem 3: Side Effects

Exercise 0: Preparation

a. Start DrScheme.

b. Make a copy of statistician.scm

Exercises

Exercise 1: A Simple Statistician

The (make-statistician) procedure creates an object that supports two operations, :observe! val, and :average. The first adds a value to those the statistician has already observed. The second computes the average of all observed values.

a. Create a new statistician named tom.

b. Tell tom to average the numbers 1, 4, and 9.

Exercise 2: Adding Statistics

a. Extend make-statistician so that it also supports a :max operation that reports on the largest value observed so far. In addition to adding that operation, you will also need to add another entry to the vector and update it each time a value is observed.

b. Use the improved version of make-statistician to compute the largest of -4, -1, -9.

Exercise 3: A Variant of map

Once we start to use side-effecting procedures like display and observe!, it becomes less appropriate to use map to apply a procedure to each element in a list. Why? Because we don't really care about the result list.

a. Use map to display each element in the list (1 2 3), one per line. What do you observe about the result?

b. I've created apply-to-each as an alternative. Rewrite the expression for part a to use apply-to-each rather than map. What do you observe about the result?

Exercise 4: Repeated Observations

a. Use apply-to-each to observe each value in the list (4 1 2 6 3 1 5).

b. Verify that the statistician computes the correct average of those values.

c. Verify that the statistician computes the correct maximum of those values.

Exercise 5: An Alternate Statistician

The (make-new-statistician) procedure also returns a statistician object, albeit a slightly different one. The objects returned by make-new-statistician respond to two messages, :observer! and :average. The first returns a procedure that can be used to observe valeus. The second computes the average of the returned values.

a. Create a new statistician named heidi using make-new-statistician.

b. Tell Heidi to compute the average of the values 1, 4, and 9.

c. Use apply-to-each to tell Heidi to observe each element of the list (4 1 2 6 3 1 5).

d. Which kind of statistician do you prefer? Why?

Notes

A Note on Problem 3: Side Effects

A side-effecting procedure is a procedure that is called for its effects, rather than its result. Many such procedures return no value (or, more precisely, the special value #<void>).