CSC151.02, Class 23: Review of Exam 1

Admin:
* Fruit juice (plus blue stuff)
* Are there questions on the homework?
* I've flipped the schedule around once again.
* Two cool talks (both for extra credit)
  * Thursday, noon, Science 2413, Shaka McGlotten '97, "A Conversation on Gendered, Racial, and Sexual Dynamics of Online Spaces".
  * Friday, noon, Science 2413, Joshua Shaw Vickery '02, "Real-World Java Programming".

Overview:
* General issues
* Problem 1
* Problem 2
* Problem 3
* Problem 4
* Problem 5
* Problem 6

/Problem 1: Types/
* Quiz!  
  * Name
  * List all the types you know in Scheme.  (Don't bother with the variants of number)
  * Count the number of types you wrote
    * list, number, character, symbol, string, boolean, procedure
    * null-list, exact-number, inexact-number
(define type-of
  (lambda (val)
    (cond
      ((list? val) 'list)
      ((number? val) 'number)
      ((character? val) 'character)
      ...
      (else 'unknown)
      )))

/General Issues/
* I do not report averages, means, media, max, min, ...
* I do not usually permit redos
* I will discuss and argue grading
* I pay attention to stupid anal-retentive stylistic issues
  * Format of six P's
	;;; Procedure whatever. Purpose to do something, but
	;;; I'm not sure what. Parameters
  * Choice of where to break lines
	(if (some-test) (string-append
	  "a"
	  "b")
	  "Hello")
  * Returning #t and #f explicitly
	(if test #t #f)

/Problem 2: Extending Sentences/
* "Compare the last character in the string to ..."
(define extend
  (lambda (sentence)
    (cond
      ((equal? (string-ref str (- (string-length str) 1)) #\.)
       (string-append (substring str 0 (- (string-length str) 1))
                      ", I think."))
      (... #\?)
      (... #\!))))
* Moral one: Work with the type as is, don't convert to another type.
* Moral two: Preconditions are your friends.

/Problem 3: singleton?/
(define singleton?
  (lambda (lst)
    (and (not (null? lst)) (null? (cdr lst)))))

(define singleton?
  (lambda (lst)
    (if (null? lst) #f
        (if (and (not (null? lst)) (null? (cdr lst))) #t
            (if (and (not (null? lst))) (not (null? (cdr lst))) #f)))))

/Problem 4: Close Numbers/
* What does it mean to be close?
  * Within 1% of each other.
  * Not a great metric, but you're stuck with it.
* What are some potentially problematic inputs?  (Inputs that are correct, but hard to process.)
  * Negative numbers
  * Zero
  * Repeated numbers
  * Very large
  * Very small
  * Exact vs. inexact
* Some strategies used. 

/Problem 5: Choosing an Element/

/Problem 6/
(if test0
    (if test1
        #f
        (if test2
            (if test3 #t test4)
            (if test5 #f test7)))
    (if #f test8 test9))

(if test0
    (and (not test1)
         (if test2
             (or test3 test4)
             (and (not test5) test7))
    test9)

(if test5 #f test7) => (and (not test5) test7)
  Try some examples.  Suppose both hold.  Suppose test5 doesn't hold.


Note: Two different ways of thinking about the tests
* The "if" way: Do this test, then this test, then this test.
* The "defined" way: This holds when this combination holds

Feel free to leave when you want.  Feel free to come up and ask me questions.