Lab: Association Lists

In today's laboratory, you will experiment with association lists, structures that make it easy to look up information.

• Exercises
  • Exercise 0: Preparation
  • Exercise 1: Birthdays
  • Exercise 2: Finding Birthdays
  • Exercise 3: Duplicate Keys
  • Exercise 4: Preconditions
  • Exercise 5: Reverse Associations
  • Exercise 6: Using a Specific Database
  • Exercise 7: Compound Keys
• Notes
  • Note on exercise 1

Exercises

Exercise 0: Preparation

a. Make sure you know that the assoc procedure does.

b. Start DrScheme

Exercise 1: Birthdays

Define an association list birth-dates that associates the surnames of recent presidents of the United States (as strings) with their birth-dates (again, as strings).

You may also want to look at the note on this exercise.

Here's a table containing information for your association list:

Exercise 2: Finding Birthdays

Use the assoc procedure to search the birth-dates association list for someone who is on the list and for someone who is not on the list.

Exercise 3: Duplicate Keys

a. Redefine birth-dates so that it includes two entries with the same key, for two people who have the same surname -- say, John Adams (born October 30, 1735) and John Quincy Adams (born July 11, 1767). What happens if you try to apply assoc to retrieve these entries, using the common key "Adams"?

b. Many people find these results disappointing. To help alleviate this disappointment, define and test a procedure similar to assoc, except that it returns a list of all the pairs with the given key.

Exercise 4: Preconditions

a. What do you think that assoc will do if it is given a list in which each element is a pair, rather than a list? For example, can we use assoc to search the following list to determine the last name of a faculty member?

(("Sam" . "Rebelsky") 
 ("Henry" . "Walker") 
 ("John" . "Stone")
 ("Ben" . "Gum")
 ("Emily" . "Moore")
 ("Pam" . "Ferguson")
 ("Gene" . "Herman")
 ("Royce" . "Wolf")
 ("Chuck" . "Jepsen")
 ("Arnie" . "Adelberg"))

b. Confirm or refute your answers by experimentation.

c. Based on your experience, what preconditions should assoc have?

Exercise 5: Reverse Associations

a. What happens if you search by date instead of by person? For example, you might try (assoc "October 1, 1924" birth-dates).

b. Define and test a procedure reverse-lookup that takes two arguments, an association list alist and an associated datum val, and returns

• an element from alist that has val as its second component, if such an element exists
• #f if there is no such element.

c. Define and test a procedure that takes two parameters, an association list, alist, and an associated datum, val, and returns a list of all elements that have val as the second component.

Exercise 6: Using a Specific Database

For some problems, it seems natural to always use a specific database, rather than to pass the database as a parameter. For example, suppose we'd set up a table of science department chairs (which may sound familliar from the reading, although we've expressed it differently here).

;;; Value:
;;;   science-department-chairs
;;; Type:
;;;   List of lists.
;;;   Each sublist is of length two and contains a department (or "science")
;;;   and a name.
;;;   Both of those values are strings.
;;; Contents:
;;;   A list of the department and division chairs in the Science division 
;;;   in academic year 2000-2001.
(define science-department-chairs
  (list (list "Science" "Bruce Voyles")
        (list "Biochemistry" "Bruce Voyles") ; Well ...
        (list "Biology" "Diane Robertson")
        (list "Chemistry" "Lee Sharpe")
        (list "Math/CS" "Emily Moore")
        (list "Mathematics" "Emily Moore") ; For those who forget CS
        (list "Computer Science" "Emily Moore") ; For other folks
        (list "Physics" "Paul Tjossem")
        (list "Psychology" "David Lopatto")))

We can write a procedure to look up a department chair as follows:

;;; Procedure:
;;;   look-up-science-chair
;;; Parameters:
;;;   dept, he name of a science deparment (or simply "Science")
;;; Purpose:
;;;   Look up the chair of a science department.
;;; Produces:
;;;   chair, a string, if the department has a chair
;;;   #f, otherwise
;;; Preconditions:
;;;   science-department-chairs must be defined appropriately
;;;   dept must be a string
;;; Postconditions:
;;;   If the procedure returns a string, chair is the chair of dept.
;;;   If the procedure fails to return a string, that department has
;;;     no chair (or isn't even a department).
(define look-up-science-chair
  (lambda (dept)
    (if (assoc dept science-department-chairs)
        (cadr (assoc dept science-department-chairs))
        #f)))

The strategy of using a specific database in a procedure is often called hard-coding the database.

a. Using look-up-science-chair, look up the chair of this department.

b. Using look-up-science-chair, look up the chair of Geology.

c. Suppose we wanted to write the converse procedure (one that given a name, tells which department he or she chairs). Can we still hard-code the database? If so, show how. If not, explain why not.

Exercise 7: Compound Keys

a. Define and test a procedure that takes two arguments, the first an atom and the second an association list whose keys are all lists of atoms. The procedure should return a list of all the values whose keys contain the atom.

b. Why might you want to use this procedure?

Notes

Note on exercise 1

Note: The value of birth-dates is not a procedure, so it is not necessary to use a lambda-expression in this exercise. Look at the definition of science-chairs-directory for an example of the form that your definition of birth-dates should take.