In today's laboratory, you will experiment with association lists, structures that make it easy to look up information.
a. Make sure you know what the
assoc procedure does.
b. Start DrScheme
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.
Write and document a procedure,
directory), that someone can use to look up the
department associated with a chair.
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:
|Secret Squirrel||Morocco Mole|
|Quick Draw McGraw||Baba Looey|
|Bart Simpson||Milhouse Van Houten|
assoc procedure to search the
association list for someone who is on the list and for someone who is not
on the list.
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.
if you try to apply
assoc to retrieve these entries, using the
b. Many people find these results disappointing. To help alleviate this
disappointment, define and test a procedure,
(multi-assoc key alist),
except that it returns a list of all the pairs with the
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") ("Arnie" . "Adelberg"))
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.)
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
reverse-lookup that takes two arguments,
an association list
alist and an associated datum
val, and returns
valas its second component, if such an element exists
#fif there is no such element.
c. Define and test a procedure that takes two parameters, an association
alist, and an associated datum,
returns a list of all elements that have
val as the second
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 (lambda (dept) (lusc-helper (assoc dept science-department-chairs)))) (define lusc-helper (lambda (assoc-result) (if assoc-result (cadr assoc-result) #f)))
The strategy of using a specific database in a procedure is often called hard-coding the database.
look-up-science-chair, look up the chair of
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.
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?
Here you will find notes on selected problems.
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
;;; 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 (lambda (name directory) (lutn-helper (assoc name directory)))) (define lutn-helper (lambda (assoc-result) (if assoc-result (cadr assoc-result) "unlisted")))
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.
You can create that list with
(define mathcs (list (cons "Sam" "Rebelsky") (cons "Henry" "Walker") (cons "John" "Stone") (cons "Ben" "Gum") (cons "Emily" "Moore") (cons "Tom" "Moore") (cons "Pam" "Ferguson") (cons "Gene" "Herman") (cons "Chris" "Hill") (cons "Marc" "Chamberland") (cons "Royce" "Wolf") (cons "Chuck" "Jepsen") (cons "Arnie" "Adelberg")))
Tuesday, 19 September 2000 [Samuel A. Rebelsky]
Wednesday, 20 September 2000 [Samuel A. Rebelsky]
Wednesday, 21 February 2001 [Samuel A. Rebelsky]
Thursday, 22 February 2001 [Samuel A. Rebelsky]
Sunday, 29 September 2002 [Samuel A. Rebelsky]
Wednesday, 2 October 2002 [Samuel A. Rebelsky]
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