Lesson Plan - 6.001 SP04 - recitation 6

lists and hops

let
 desugar

writing list procs

(define (make-units C L H)
  (list C L H))
(define get-units-C car)
(define get-units-L cadr)
(define get-units-H caddr)

(define (make-class number units)
  (list number units))
(define get-class-number car)
(define get-class-units cadr)
(define (get-class-total-units class)
  (+ (get-units-C (get-class-units class))
     (get-units-L (get-class-units class))
     (get-units-H (get-class-units class))))
(define (same-class? c1 c2)
  (= (get-class-number c1) (get-class-number c2)))

note - ask not to use hops

(define (total-scheduled-units sched)
  (if (null? sched)
      0
      (+ (get-class-total-units (car sched))
         (total-scheduled-units (cdr sched)))))

(define (drop-class sched classnum)
  (cond ((null? sched) nil)
        ((= (get-class-number (car sched)) classnum) (cdr sched))
        (else
         (cons (car sched) (drop-class sched classnum)))))

(define (credit-limit sched max-credits)
  (if (> (total-scheduled-units sched) max-credits)
      (credit-limit (cdr sched) max-credits)
      sched))


HOPs - using procedures like normal data

(make-student 3495873459 (lambda (sched) (not (null? sched))))

(make-student 3495873459 (lambda (sched) (< (total-scheduled-units sched) 54)))

(define (class-numbers sched)
  (map get-class-number sched))

(define (drop-class sched classnum)
  (filter (lambda (class) (not (= (get-class-number class) classnum)))
          sched))

(define (total-scheduled-units sched)
  (fold-right 0 + (map get-class-total-units sched)))

(define (credit-limit sched limit)
  (fold-right
   (empty-schedule)
   (lambda (class sched)
     (if (< (total-scheduled-units (add-class class sched)) limit)
         (add-class class sched)
         sched))
   sched))

slick solution: (O(n), not O(n^2))
(define (credit-limit sched limit)
  (car
   (fold-right
    (list (empty-schedule) 0)
    (lambda (class sched)
     (if (< (+ (get-class-total-units class) (cadr sched)) limit)
         (list (add-class class (car sched))
               (+ (get-class-total-units class) (cadr sched)))
         sched))
    sched)))