CSC151.01 2006S, Class 11: Recursion

Admin:
* Reminder: You all got a free extension on HW5 until Friday.  
  Questions on HW5?
* Please reread the reading on recursion for tomorrow's class.
* Upcoming EC: 
  * Today's CS Extra 
  * Today's community meal at 4:30 or so in Davis School
  * Thursday's Convo 
  * Saturday's Symphonic Band and Percussion Ensemble
    Saturday at 2:00 p.m. in Sebring-Lewis
  * Saturday's Con Brio and G-Tones Concert (4 p.m. Herrick)
  * Saturday's Football Game.
* Check Web for list of attendance EC
* We'll try the HW4 demo again.
* Partners for this week:
  * An and Howard
  * Brown and Rider
  * Berg and Cartagena
  * Brooks and Cloninger
  * Bonnin Cadogan and Miller
  * Harrington and Mertes
  * LaRue and Zamora (and Rich, today)
  * McArdle and Pan
  * McDonald and McFarlin
  * Potthoff and Thompson 
  * Rich and Wood

Overview:
* Questions.
* Lab.
* Reflection.

/HW4 Issues/

* Sometimes you're going to have to write stuff that other people will work with.
* Almost no one did numbers right
  * Wanted: (define numbers (list 1 2 3))
  * Got: (define numbers (list "1" "2" "3"))
  * (string-append "There were " 
                   (number->string (list-ref numbers 2))
                   " "
                   (list-ref nouns 1)
                   "s.")
  * Lesson: Converting types
* Lesson: Your stuff will get posted on the web.  (And, hey, if I'm okay with people seeing my mispellings, then you should be fine with that, too.)
  * If you object, please let me know.

/HW5 Questions/

* Can I use conditionals?  (E.g., for indefinite articles.)
  * Yes
* How do I determine whether the first letter of a word is a vowel?
  * string-ref
  * compare characters with eqv?
  * combine comprisons with and and or.
* How do I get started on #5?  "Chop off after the second decimal point."
  * Use the tools at your disposal.
    * truncate (round, floor, ceiling)
  * How do you get rid of less?
    * Hint: Shift the decimal point.
  * Talk to other people!

/Repetition through Recursion/

* Key idea in recursion: Procedures can call themselves.

(define RECURSIVE-PROCEDURE
  (lambda (PARAMS)
     ... (RECURSIVE-PROCEDURE ...) ...))

* A little odd, but some benefits
  (define RECURSIVE-PROCEDURE
    (lambda (PARAMS)
      (if ITS-SIMPLE
          GIVE-THE-SIMPLE-ANSWER
          USE-THE-PROCEDURE-TO-ANSWER-A-SIMPLER-PROBLEM)))
* Problem: Sum a list
  * What lists are easy to sum?
    * The one element list.  The sum is that element.
  * Lists that have more than one element are harder to sum.
    * How do I use the sum procedure to write the sum procedure?
    * Not using the same list.
    * Use a simpler list (say the cdr)
(define sum
  (lambda (values)
    (if (null? (cdr values))
        (car values)
        (+ (car values)  (sum (cdr values))))))

(sum (list 1 2 3))
=> ... (sum (list 2 3))
=> ... (sum (list 3))

/Reflection/

* Which do you prefer, sum or new-sum?
  * sum
  * new-sum
* Which do you prefer, new-sum or newer-sum?
  *