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Summary: We consider a number of Scheme procedures
that can be used to look up information.
Contents:
Procedures covered in this reading:
-
(assoc key alist)
-
(assq key alist)
-
(assv key alist)
Consider the organization of a simple telephone directory for on-campus
telephones: a sequence of entries, each consisting of a name and a
four-digit telephone number. In Scheme, it is natural to use strings for
names; it turns out that telephone numbers should also be represented as
strings, since string operations make a useful kind of sense when applied
to telephone numbers and integer operations do not. (For instance,
(string-append "269-" extension) does something
useful if the value of extension is a string, but not if
it is an integer.)
To represent each individual entry in a telephone directory, we can use
a list, such as ("Henry Walker" "4208"),
("John Stone" "3181"), or
("Sam Rebelsky" "4410"), with the name as
the car of the entry and a list of the telephone number as the
cadr. An entire directory, then, would be a list of such entries:
;;; Value:
;;; science-chairs-directory
;;; Type:
;;; List of lists.
;;; Each sublist is of length three and contains a name and a phone number.
;;; Each of those values are strings.
;;; Contents:
;;; A list of the department and division chairs in the Science division
;;; in academic year 2006-2007.
(define science-chairs-directory
(list (list "Samuel A. Rebelsky" "4410")
(list "Vince Eckhart" "4354")
(list "Lee Sharpe" "3008")
(list "Henry Walker" "4208")
(list "Mark Chamberland" "4207")
(list "Bob Cadmus" "3016")
(list "Janet Gibson" "3168")))
In Scheme, a list of pairs or lists is called an association list or
alist.
As the telephone-directory example illustrates, a particularly common
application of association lists involves looking for a desired name or
first component of a list and retrieving another component of a pair.
Thus, the first component of each pair (the car of a pair)
often is called a key, and the cdr of the pair is
its associated data or value. For example, in the above
illustration, "Samuel A. Rebelsky",
"Mark Chamberland", and "Janet Gibson"
are some of the keys, and the lists that contain department and telephone
numbers are the associated data. Thus, an association list is a simple
way to implement a small table in a database.
Since such applications are very common, Scheme provides procedures to
retrieve from an association list the pair containing a specified key. The
most frequently used procedure of this kind is assoc. Given a
key and association list, assoc returns the first pair with
the given key. If the key does not occur in the association list, then
assoc returns #f. For example,
> (assoc "Samuel A. Rebelsky" science-chairs-directory)
("Samuel A. Rebelsky" "Science" "4410")
> (assoc "Russell King Osgood" science-chairs-directory)
#f
To find the telephone number corresponding to a given name, we could
apply the cadr procedure (or list-ref with
a position of 1). Here's an example.
;;; Procedure:
;;; lookup-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 lookup-telephone-number
(lambda (name directory)
(if (assoc name directory)
(cadr (assoc name directory))
"unlisted")))
For example,
> (lookup-telephone-number "Samuel A. Rebelsky" science-chairs-directory)
"4410"
> (lookup-telephone-number "Russell King Osgood" science-chairs-directory)
"unlisted"
Note that the result depends on the directory. For example,
> (lookup-telephone-number "Samuel A. Rebelsky" null)
"unlisted"
Some of you may recall asking why if might take a value other
than #t or #f as a parameter. This procedure
is one example of why.
One problem with the lookup-telephone-number procedure
given above is that it calls assoc twice, once to determine
whether the phone number is in the list and once to determine the
phone number. Rather than calling assoc twice, we
might call it once and send the result to a helper procedure.
(define lookup-telephone-number
(lambda (name directory)
(let ((assoc-result (assoc name directory)))
(if assoc-result
(cadr assoc-result)
"unlisted"))))
In the previous example, we have only one value (the phone number) associated
with a key. However, in practice, we often want to associate many values
with the same key. For example, we might want to note which department each
chair is associated with. Here's a new version of our previous list that
also includes that information.
;;; Value:
;;; science-chairs-directory
;;; Type:
;;; List of lists.
;;; Each sublist is of length three and contains a name, a phone number,
;;; and a division.
;;; All three of those values are strings.
;;; Contents:
;;; A list of department and other chairs in the Science division in
;;; academic year 2006-2007.
(define science-chairs-directory
(list (list "Samuel A. Rebelsky" "4410" "Science")
(list "Vince Eckhart" "4354" "Biology")
(list "Lee Sharpe" "3008" "Chemistry")
(list "Henry Walker" "4208" "Computer Science")
(list "Mark Chamberland" "4207" "Mathematics and Statistics")
(list "Bob Cadmus" "3016" "Physics")
(list "Janet Gibson" "3168" "Psychology")))
You should note a few things about this list. First, we've left the
phone number as element 1 so that lookup-telephone-number
still works. Second, we've taken advantage of Scheme's decision to ignore
spaces between values by using spaces to put stuff in more tabular form.
As is the case with many of the built-in Scheme procedures,
assoc is relatively easy to implement ourselves. In essence,
assoc recursively steps through the list until it finds a
match or runs out of elements.
(define assoc
(lambda (key alist)
(cond
; If there are no entries left in the association list,
; there are no entries with the given key.
((null? alist) #f)
; If the key we're looking for is the key of the first
; entry, then use that entry.
((equal? key (car (car alist))) (car alist))
; Otherwise, look in the rest of the association list.
(else (assoc key (cdr alist))))))
The DrScheme implementation of assoc also includes some
error checking that you'll learn how to do later this semester.
The assoc procedure works fine if the key is the first element
of a data item. But what if it's the second (or third, or fourth, or ...).
For example, what if we know someone's phone number and want to find his
or her name? Then we can't rely on assoc, because it only looks
at the first element of each list. Instead, we need to write our own
procedure. For example, to find someone with a particular phone number,
we might write:
;;; Procedure:
;;; lookup-by-number
;;; Parameters:
;;; number, a string
;;; directory, a list of telephone book entries
;;; Purpose:
;;; Looks up the entry for a particular phone number.
;;; Produces:
;;; entry, a telephone book entry.
;;; Preconditions:
;;; Each telephone book entry must be a list. [Unverified]
;;; Element 1 of each telephone book entry must be a string which
;;; represents that person's phone number. [Unverified]
;;; Postconditions:
;;; If an entry with phone number number appears in the directory,
;;; (1) number equals (list-ref entry 1);
;;; (2) entry is an element of directory
;;; If no entries with that phone number appear, entry is false (#f).
;;; Does not affect the directory.
(define lookup-by-number
(lambda (number directory)
(cond
; If there are no entries in the directory, our desired
; entry is not there.
((null? directory) #f)
; If the number we're looking for is in the initial entry,
; use that entry
((equal? number (list-ref (car directory) 1))
(car directory))
; Otherwise, look in the rest of the directory.
(else (lookup-by-number number (cdr directory))))))
It might even make sense to generalize this procedure, so that we can look
up by name, by telephone number, or by department.
;;; Procedure:
;;; lookup
;;; Parameters:
;;; pos, a nonnegative integer [unverified]
;;; key, a string [unverified]
;;; directory, a list of lists [unverified]
;;; Purpose:
;;; Looks up the entry that corresponds to a particular key.
;;; Produces:
;;; entry, one of the member lists.
;;; Preconditions:
;;; Each element of directory must be a list. [Unverified]
;;; Each element of directory must have at least (pos+1) values. [Unverified]
;;; Postconditions:
;;; If there is a list, lst, in directory such that key equals
;;; (list-ref lst pos), then entry is some such list.
;;; Otherwise, entry is #f.
;;; Does not affect the directory.
(define lookup
(lambda (pos key directory)
(cond
((null? directory) #f)
((equal? key (list-ref (car directory) pos))
(car directory))
(else (lookup pos key (cdr directory))))))
We can now define lookup-by-number (and even
lookup-by-department and lookup-by-name)
in terms of lookup.
(define lookup-by-number
(lambda (number directory)
(lookup 1 number directory)))
(define lookup-by-department
(lambda (department directory)
(lookup 2 department directory)))
(define lookup-by-name
(lambda (name department)
(lookup 0 name directory)))
And we can see that these work fine.
> (lookup-by-department "Biology" science-chairs-directory)
("Vince Eckhart" "4354" "Biology")
> (lookup-by-department "Chemistry" science-chairs-directory)
("Lee Sharpe" "3008" "Chemistry")
> (lookup-by-department "Economics" science-chairs-directory)
#f
> (lookup-by-number "4410" science-chairs-directory)
("Samuel A. Rebelsky" "4410" "Science")
> (lookup-by-number "3000" science-chairs-directory)
#f
The assoc procedure is actually one of three related built-in
procedures in Scheme; the other two are assq and
assv. Each of these procedures scan association lists for
keys. They differ only in the test used for determining when a key is found:
-
assoc uses the predicate equal? to compare
the key sought with the key components of the entries in the association
list.
-
assq uses the predicate eq? for those
comparisons.
-
assv uses the predicate eqv? for those
comparisons.
You may wish to refresh your memory on the purpose of these predicates
(hint hint).
;
;
