# Lab: Association Lists

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

Contents:

Useful procedures: `assoc`, `list-ref`, `member`.

## Exercises

### Exercise 0: Preparation

a. Make sure you know what the `assoc` procedure does.

b. Start DrScheme

### Exercise 1: Science Chairs, Revisited

In the reading on association lists, we claimed that the `look-up-telephone-number` procedure worked correctly, even for the extended table that included not only phone number, but also department.

Verify that claim.

You can find the table and the code at the end of this lab.

### Exercise 2: Science Chairs' Departments

Write and document a procedure, ```(look-up-division name directory)```, that someone can use to look up the department associated with a chair.

### Exercise 3: Cartoon Sidekicks

Define an association list, `sidekicks`, that associates the names of some famous cartoon protagonists (as strings) with the names of their sidekicks (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:

Protagonist Sidekick
Peabody Sherman
Yogi Booboo
Secret Squirrel Morocco Mole
Tennessee Tuxedo Chumley
Quick Draw McGraw Baba Looey
Dick Dastardly Muttley
Bullwinkle Rocky
Bart Simpson Milhouse Van Houten
Asterix Obelix

### Exercise 4: Finding Sidekicks

Use the `assoc` procedure to search the `sidekicks` association list for someone who is on the list and for someone who is not on the list.

### Exercise 5: Duplicate Keys

a. Redefine `sidekicks` so that it includes two entries with the same protagonist and different sidekicks -- say Scooby Doo, who has both Shaggy and Scrappy Doo as sidekicks. What happens if you try to apply `assoc` to retrieve these entries, using the common key `"Scooby Doo"`?

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

### Exercise 6: 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")
("Tom" . "Moore")
("Pam" . "Ferguson")
("Gene" . "Herman")
("Chris" . "Hill")
("Marc" "Chamberland")
("Royce" . "Wolf")
("Chuck" . "Jepsen")
```

b. Confirm or refute your answers by experimentation. If you're not sure how to create that list, check the notes on this problem, which give instructions for creating that list.

c. Based on your experience, what preconditions should `assoc` have? (No, you may not read the online documentation for an answer.)

### Exercise 7: Reverse Associations

a. What happens if you search by sidekick instead of by protagonist? For example, you might try

`(assoc "Chumley" sidekicks)`

b. Define and test a procedure `lookup-by-second` 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 8: 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"          "Mark Schneider")
(list "Biochemistry"     "Bruce Voyles") ; Well ...
(list "Biology"          "Jackie Brown")
(list "Chemistry"        "Martin Minelli")
(list "Math/CS"          "Gene Herman")
(list "Mathematics"      "Gene Herman") ; For those who forget CS
(list "Computer Science" "Gene Herman") ; For other folks
(list "Physics"          "Charles Cunningham")
(list "Psychology"       "Ann Ellis")))
```

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

```;;; Procedure:
;;;   look-up-science-chair
;;; Parameters:
;;;   dept, the name of a science deparment (or simply "Science")
;;; Purpose:
;;;   Look up the chair of a science department.
;;; Produces:
;;;   chair, a string (or #f)
;;; Preconditions:
;;;   science-department-chairs must be defined appropriately
;;;   dept must be a string
;;; Postconditions:
;;;   If science-department-chairs specifies a chair for dept,
;;;     chair is that chair.
;;;   Otherwise, chair is false (#f)
(define look-up-science-chair
(let ((lusc-helper
(lambda (assoc-result)
(if assoc-result (cadr assoc-result) #f))))
(lambda (dept)
(lusc-helper (assoc dept science-department-chairs)))))
```

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.

## For Those with Extra Time

### Extra 1: Lists as Keys

a. Define and test a procedure, ```(weird-lookup sym weirdlists)```, 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. For example,

```> (define whatever
(list
(list (list 'a 'b 'c) 'ABCs)
(list (list 'a 'e 'i 'o 'u) 'vowels)
(list (list 'e 't 'a 'i 'o 'n) 'shrdlu)))
> (weird-lookup 'a whatever)
(abcs vowels shrdlu)
> (weird-lookup 'b whatever)
(abcs)
> (weird-lookup 'o whatever)
(vowels shrdlu)
> (weird-lookup 'q whatever)
()
```

Note that you might find the `member` procedure helpful.

b. Why might you want to use this procedure?

### Extra 2: Compound Keys

Consider the following list of faculty in Mathematics and Computer Science.

```(define mathcs
(list
(cons "Sam"    "Rebelsky"    "Associate"      "Computer Science")
(cons "Henry"  "Walker"      "Chaired"        "Computer Science")
(cons "John"   "Stone"       "Instructor"     "Computer Science")
(cons "Ben"    "Gum"         "Assistant"      "Computer Science")
(cons "David"  "Bishop"      "Visitor"        "Computer Science")
(cons "Emily"  "Moore"       "Full"           "Mathematics")
(cons "Tom"    "Moore"       "Full"           "Statistics")
(cons "Pam"    "Ferguson"    "Chaired"        "Mathematics")
(cons "Gene"   "Herman"      "Chaired"        "Mathematics")
(cons "Chris"  "Hill"        "Visitor"        "Mathematics")
(cons "Marc"   "Chamberland" "Assistant"      "Mathematics")
(cons "Royce"  "Wolf"        "Chair Apparent" "Mathematics")
(cons "Chuck"  "Jepsen"      "Full"           "Mathematics")
(cons "Arnie"  "Adelberg"    "Chaired"        "Mathematics")))
```

When looking up people, we might want to make sure that we match both first and last names. Write a procedure, `(lookup first last people)` that extracts the entry in which both first and last match.

## Notes

Here you will find notes on selected problems.

### Notes on Exercise 1

Here's the revised table, just in case you've forgotten.

```(define science-chairs-directory
(list (list "Mark Schneider"     "3018" "Science")
(list "Jackie Brown"       "3096" "Biology")
(list "Martin Minelli"     "3007" "Chemistry")
(list "Gene Herman"        "4202" "Math/CS")
(list "Charles Cunningham" "3182" "Physics")
(list "Ann Ellis"          "4865" "Psychology")))
```

And here's the code for `look-up-telephone-number`.

```;;; Procedure:
;;;   look-up-telephone-number
;;; Parameters:
;;;   name, a string
;;;   directory, a list of telephone book entries
;;; Purpose:
;;;   Looks up someone's telephone number in the directory.
;;; Produces:
;;;   number, a string
;;; Preconditions:
;;;   Each telephone book entry must be a list. [Unverified]
;;;   Element 0 of each telephone book entry must be a string which
;;;     represents a name. [Unverified]
;;;   Element 1 of each telephone book entry must be a string which
;;;     represents that person's phone number. [Unverified]
;;; Postconditions:
;;;   If an entry for name appears somewhere in the directory, returns
;;;     the corresponding phone number.
;;;   If multiple entries with the same name appear, returns one of them.
;;;   If no entries appear, returns the string "unlisted"
;;;   Does not affect the directory.
(define look-up-telephone-number
(let ((lutn-helper
(lambda (assoc-result)
(if assoc-result (cadr assoc-result) "unlisted"))))
(lambda (name directory)
(lutn-helper (assoc name directory)))))

```

### Notes on Exercise 3

Note: The value of `sidekicks` 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 `sidekicks` should take.

### Notes on Exercise 6

You can create that list with

```(define mathcs
(list
(cons "Sam"    "Rebelsky")
(cons "Henry"  "Walker")
(cons "John"   "Stone")
(cons "Ben"    "Gum")
(cons "David"  "Bishop")
(cons "Emily"  "Moore")
(cons "Tom"    "Moore")
(cons "Pam"    "Ferguson")
(cons "Gene"   "Herman")
(cons "Chris"  "Hill")
(cons "Marc"   "Chamberland")
(cons "Royce"  "Wolf")
(cons "Chuck"  "Jepsen")
```

## History

Tuesday, 19 September 2000 [Samuel A. Rebelsky]

Wednesday, 20 September 2000 [Samuel A. Rebelsky]

Wednesday, 21 February 2001 [Samuel A. Rebelsky]

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

Thursday, 22 February 2001 [Samuel A. Rebelsky]

Sunday, 29 September 2002 [Samuel A. Rebelsky]

• Added first two problems (on using the science chairs directory)
• Replaced problems on presidents' birthdays with problems on cartoon sidekicks. The sidekick problems are primarily taken from similar problems from Ben Gum.
• Rewrote the last problem to give sample usage.
• Reformatted.
• Added the list of useful procedures.

Wednesday, 2 October 2002 [Samuel A. Rebelsky]

Tuesday, 4 March 2003 [Samuel A. Rebelsky]

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 Tue May 6 09:28:36 2003.
The source to the document was last modified on Tue Mar 4 22:17:58 2003.
This document may be found at `http://www.cs.grinnell.edu/~rebelsky/Courses/CS151/2003S/Labs/alists.html`.

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Samuel A. Rebelsky, rebelsky@grinnell.edu