# 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

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:

President Date of birth
Clinton August 19, 1946
Bush June 12, 1924
Reagan February 6, 1911
Carter October 1, 1924
Ford July 14, 1913
Nixon January 9, 1913
Johnson August 27, 1908
Kennedy May 29, 1917
Eisenhower October 14, 1890

### 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")
```

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.

## History

Tuesday, 19 September 2000

Wednesday, 20 September 2000

• Fixed an internal link.
• Added problem on preconditions.
• Added more parts to problem on partial keys.

Wednesday, 21 February 2001

• Started to update. Some reformatting. Added skeletons of new questions.

Thursday, 22 February 2001

• Added problem 6 (skeleton yesterday).
• Removed parts of problem 7.

Disclaimer: I usually create these pages on the fly. This means that they are rarely proofread and may contain bad grammar and incorrect details. It also means that I may update them regularly (see the history for more details). Feel free to contact me with any suggestions for changes.

This page was generated by Siteweaver on Thu May 3 23:07:39 2001.
This page may be found at `http://www.cs.grinnell.edu/~rebelsky/Courses/CS151/2001S/alists.html`.