CSC151 2010S, Class 26: Recursion with Helper Procedures

Overview:
* Notes on basic recursion.
* Helper recursion: Computing intermediate results "as you go"
* A term: Tail recursion.
* Lab

Admin:
* I hate Tuesdays!  (Too many students are missing today.)
* Reading for Wednesday: Unit Testing.
* Exam 2 distributed.  
  * Due next Wednesday.  
  * For this week, send me questions via email.
  * If you need in-person help, talk to Dr. Davis.
* Are there any final questions on Assignment 6?
* No CS Extras or CS Table this week.

Exam 6 questions
* Problem 1: Can we assume that the list of color names only include color names, and not things like "borgledyborgle"?  Yes, assuming you document that requirement.
* Problem 2: Can we have turtle-triangle?  Yes.
* Problem 3: Can we have some sample images?  Eventually.

Notes on Homework 6
* Your transformations of the Weinman image may be different.  
  (Weinman wrote his in a different language; we're not sure why he got different results.)
* For redder-greener-bluer, what do we do if two compnents are the same?
  * Whichever comes first
  * Return the original color
  * It's up to you

Notes on Basic Recursion:

Consider the filter-out-dark procedure:

(define filter-out-dark
  (lambda (lst)
    (if (null? lst)
        null
        (if (rgb-dark? (car lst))
            (filter-out-dark (cdr lst))
            (cons (car lst) (filter-out-dark (cdr lost)))))))

We may find it a bit easier to understand it in these terms

(define filter-out-dark
  (lambda (lst)
    (if (null? lst)
        null
        (let ((filtered-rest
               (filter-out-dark (cdr lst))))
          (if (rgb-dark? (car lst))
              filtered-rest
              (cons (car lst) filtered-rest))))))

Today's reading: Yet another technique for
recursion: We carry along an extra parameter
in which we build the result as we go

Questions!
* None

Lab!  Reflection

; A better way to define difference
(define difference
  (lambda (numbers)
    (if (null? numbers)
        0
        (- (car numbers) (sum (cdr numbers))))))

; A better way to define sum-red
(define sum-red
  (lambda (colors)
    (sum (map rgb-red colors))))

or, if your brain is wired right

(define sum-red (o sum (l-s map rgb-red)))