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Fortunately, the University was able to get "Chapter 11" status from the Commonwealth, which gives it more power to control its own destiny by raising tuition and selling off assets. The Board of Transients has decided the time for drastic measures has arrived, and they will conduct an auction to raise money. To avoid paying fees to a commercial auction service, they have decided to exploit free labor from cs150 students to develop an auction service for them.
The auction service will be built using a database. A database is just a way of storing and manipulating a large amount of structured data. For this problem set, you will implement a database and use it to implement an auction service. Our database is controlled using Scheme procedures that are similar to the SQL commands used to control nearly all commercial databases. In the final course project, you will build a dynamic web site that uses SQL commands to interact with a database.
Data in a database is stored in tables. The fields of a table describe the data and are not mutable. The entries of a table are the data, and can be changed. We will represent a table using a cons pair where the car is a list of the table fields and the cdr is a list of the table entries. These definitions are found in database.scm:
(define (make-new-table fieldlist) (cons fieldlist null))
(define (make-table fieldlist entries) (cons fieldlist entries))
(define (table-fields table) (car table))
(define (table-entries table) (cdr table))
For the Wahoo! Auctions service, we will need several different tables
to keep track of the bidders, the items for sale, and the bids on those
items. We use quoted symbols to describe the fields in a table. Those
tables are defined in auction.scm:
(define bidders (make-new-table (list 'name 'email)))
(define items (make-new-table (list 'item-name 'description)))
(define bids (make-new-table (list 'bidder-name 'item-name 'amount)))
Hint: See the lecture slides from Class 13 or Figure 10.1 from the Course Book for examples of drawn global environments.
(define t1 (make-new-table (list 'name 'email))) (set-car! (table-fields t1) 'nom) (set-cdr! t1 t1)b. Suppose we then evaluate:
(define (length lst) (if (null? lst) 0 (+ 1 (length (cdr lst))))) (length t1)Explain why the evaluation of (length t1) never terminates.
Try evaluating the expressions in Question 2 in DrScheme. Also evaluate t1 and see if you can figure out what the printed value means.
The table-insert! procedure inserts an entry into a table. We follow the Scheme (and Yahoo!?) convention of using an ! at the end of the names of procedures that mutate state. You should follow this convention also.
You shouldn't need to change the definition of table-insert!, but you should be able to understand it (defined in database.scm):
(define (table-insert! table entry)
(assert (= (length entry) (length (table-fields table))))
(if (null? (table-entries table))
(set-cdr! table (list entry))
(append! (table-entries table) (list entry)))
(void)) ;;; don't evaluate to a value
The expression (assert (= (length entry) (length (table-fields table))))
checks that the entry we are adding to the table has the right number of elements
— there must be one element in the entry corresponding to each
field in the table. The assert procedure will produce an error
if the passed parameter is false. It is defined:
(define (assert pred) (if (not pred) (error "Assertion failed!")))We use (void) at the end of the procedure body to prevent an application of table-insert! from evaluating to a value. The procedure void takes no parameters, and produces no value.
b. Why does the definition of table-insert! need to do something different in the case where (table-entries table) is null?
Hint: try evaluating
(define lst null)
(append! lst 3)
lst
Our auction service will need procedures for adding bidders, posting items for sale and bidding on items.
Question 5: Describe the running time of your insert-bid! procedure. Your answer should use Θ notation, clearly explain what all variables you use mean, and include an explanation of why your answer is correct.
We have provided a table-display procedure in database.scm for printing out a table. (You don't need to understand the details of how table-display works.)
If you define add-bidder!, post-item! and insert-bid! correctly, you should obtain the following interactions:
> (table-display bidders)
name email ------------------------- -------------------------
> (add-bidder! "Tim Koogle" "tk@yahoo.com")
> (add-bidder! "Katie Couric" "katie@cbs.com")
> (add-bidder! "Dave Matthews" "dave@dmb.com")
> (table-display bidders)
name email ------------------------- ------------------------- Tim Koogle tk@yahoo.com Katie Couric katie@cbs.com Dave Matthews dave@dmb.comDevelop similar tests for your post-item! and insert-bid! procedures, and make sure they also work correctly.
(define (make-string-selector match) (lambda (fval) (string=? fval match)))
(define (get-bids item)
(table-entries (table-select bids 'item-name (make-string-selector item))))
The get-bids procedure evaluates to a list of bid entries
that are the entries in the bids table whose
item-name field matches the parameter item. If
you're not sure how this works, try evaluating some expressions using
make-string-selector by themselves.
If your table-select is working correctly, you should see interactions similar to these:
(insert-bid! "Tim Koogle" "SEAS" 10000000)
(insert-bid! "Dave Matthews" "CLAS" 2000000)
(insert-bid! "Tiki Barber" "COMM" 79000000)
(insert-bid! "Katie Couric" "CLAS" 37000000)
(table-select bids 'item-name (lambda (pitem) (string=? pitem "CLAS")))
((bidder-name item-name amount) ("Dave Matthews" "CLAS" 2000000) ("Katie Couric" "CLAS" 37000000))
(table-display (table-select bids 'item-name (lambda (pitem) (string=? pitem "CLAS"))))
bidder-name item-name amount ------------------------- ------------------------- ------------------------- Dave Matthews CLAS 2000000 Katie Couric CLAS 37000000
(table-entries (table-select bids 'item-name (lambda (pitem) (string=? pitem "SEAS"))))
(("Tim Koogle" "SEAS" 10000000))
(table-entries (table-select bids 'item-name (lambda (pitem) (string=? pitem "Rotunda"))))
()
(table-entries (table-select bids 'amount (lambda (pamount) (> pamount 10000000))))
(("Tiki Barber" "COMM" 79000000) ("Katie Couric" "CLAS" 37000000)
(table-entries
(table-select
(table-select bids 'amount (lambda (pamount) (<= pamount 10000000)))
'bidder-name
(lambda (pbidder) (string=? pbidder "Katie Couric"))))
()
Your get-highest-bid procedure should work like this:
> (setup-tables)
> (get-highest-bid "SEAS")
("Tim Koogle" "SEAS" 10000000)
> (get-highest-bid "Rotunda")
()
To report an error, use the (error "message") function. For example: (error bidder "is not an authorized bidder!"). For the purposes of grading, the exact prose of your error message doesn't matter — only whether or not you indicate an error. So be as creative as you like with your error messages.
Hint: place-bid may be a relatively long procedure: 25 to 30 lines is reasonable. Don't worry if yours is longer or shorter, as long as you understand what is going on and you are getting the right answers.
You should get interactions similar to these:
(setup-tables)
(place-bid "Tim Koogle" "SEAS" 20000000)
#t
(place-bid "Katie Couric" "SEAS" 18000000)
Bid amount does not exceed previous highest bid: {"Tim Koogle" "SEAS" 20000000}
(place-bid "Katie Couric" "SEAS" 22000000)
#t
(place-bid "Dave Matthews" "The Rotunda" 1000000)
The Rotunda "is not for sale!"
(place-bid "Westley Weimer" "SEAS" 10000000000)
Westley Weimer "is not a legitimate bidder!"
Here's an example:
(setup-tables)
(place-bid "Tim Koogle" "SEAS" 18000000)
#t
(place-bid "Katie Couric" "CLAS" 90000000)
#t
(table-display (end-auction))
bidder-name item-name amount ---------------- ------------- --------- Tiki Barber COMM 79000000 Tim Koogle SEAS 18000000 Katie Couric CLAS 90000000
Try some sample auctions to demonstrate your program. See if you can make enough money to save the University, but be careful not to sell off too many important assets — we will revisit the University in Charlottansville in Problem Set 6.
(define book-comments "You, book, are ugly. But in the morning, I shall be sober. A weakness of this book is that knowledge attained by rote memorization is tantamount to a defeat, for it is momentary. A strength of this book is that if you know this textbook and know yourself, you will not be imperiled in a hundred battles. Finally, I note a mistake on page 1937, where you suggest that it is peace in our time -- clearly a typo.")
More seriously, we covet comments on what you thought was well-explained in the book and what was confusing, as well as what you might have done differently. If you notice any errors in the text, please bring them to our attention. Your comments will reach the textbook author directly, and help me to make the course better.
If you are working with a partner, make a longer comment that addresses all of your concerns.
In any event, you will receive full credit for any comment longer than 20 characters.
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Your comments should be more substantive and helpful than that example. In any event, you will receive full credit for any comment longer than 20 characters. Students who submit exceptionally helpful book-comments and worst-section answers will receive extra credit.
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