CS-2510: Lecture Notes #1

Course: Fund. 2
Me: Matthew Ekstrand-Abueg

What did we learn in Fund 1... (students)
  - Design Recipes
     * Structural recursion
     * Abstraction
     * Generative Recursion... Accumulators

  - Recursion (Structural? Generative?)

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Refresher:
  Sum: [Listof Number] -> Number
  1) Data... lists
      ;; A [Listof Number] is one of
      ;;  - empty
      ;;  - (cons Number [Listof Number])

  2) Contract/Purpose
      ;; sum : [Listof Number] -> Number
      ;; Sum the numbers in the list

  3) Examples
  4) Template
 
     (define (sum lon)
        (cond [(empty? lon) ...]
              [else ... (first lon)
                    ... (rest lon)
                    ... (sum (rest lon))]))

     ;; ** What does the recursion represent?
     ;;      (in English)

  5) Code!
     Case for Empty...
     Case for Cons...

  6) Run Tests ("all X tests passed")


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Administrivia
     Web: http://www.ccs.neu.edu/course/cs2510/
    Book: htdp.org, PDF, Course Packs in the Book Store (if desired)
  Office: 472 WVH
   Exams: In Lecture: Thursday, October 6th
                      Thursday, November 3rd
             6pm-9pm: Thursday, December 1st
  
       *** CCIS Accounts!!! ***
  Lab accounts, HW Handin, grades

Partners: 1 switch mid-semester... choose tomorrow in Lab
     Lab: Start tomorrow
    
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Discussion:
  - Step through an example:
      * stack buildup, too much to remember (R to L)
  - How else can we do it?

  - Examples
     ;; reverser : [Listof Posn] -> [Listof Posn]
     ;; Reverse a list

     ;; farthest : [Listof Posn] -> Posn
     ;; Find farthest from (0,0) assuming non-empty


  - If necessary, motivate with total distance

     ;; dist : Posn Posn -> Number
     (define (dist p1 p2)
        (sqrt (sqr (- (posn-x p1) (posn-x p2)))
              (sqr (- (posn-y p1) (posn-y p2)))))

     ;; total-dist : [Listof Posn] -> Number
     ;; Sum the total route length from start to finish
     (define (total-dist lop) ...)


  * Need to keep the previous Posn
  * Need helper to enforce non-empty list
     ;; total-dist-help : Posn [Listof Posn] -> Number
     (define (total-dist-help p lop) ...)

Better (sometimes):
  * Fold[r/l]
  * Map, Filter, BuildList