In today's laboratory, you will experiment with association lists, structures that make it easy to look up information.
a. Make sure you know that the
assoc procedure does.
b. Start DrScheme
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|
assoc procedure to search the
association list for someone who is on the list and for someone who is not
on the list.
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
b. Many people find these results disappointing. To help alleviate this
disappointment, define and test a procedure similar to
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") ("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
a. What happens if you search by date instead of by person? For example, you
(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
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" "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.
look-up-science-chair, look up the chair of this department.
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 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?
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.
Tuesday, 19 September 2000
Wednesday, 20 September 2000
Wednesday, 21 February 2001
Thursday, 22 February 2001
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