material

6.4 Example: Filtering

Given:

;; NumberList  →  NumberList
;; extract all items from l that are larger than pivot
(define (larger pivot l)
  (cond
    [(empty? l) empty]
    [else (cond
	    [(> (first l) pivot) (cons (first l) (larger pivot (rest l)))]
	    [else (larger pivot (rest l))])]))

;; test: 
(equal? (larger 3 (list 3 8 1 2 9 7)) (list 8 9 7))

Better:

;; NumberList  →  NumberList
;; extract all items from l that are larger than pivot
(define (larger pivot l)
  (local ((define (compare-item-to-pivot item)
	    (> item pivot)))
    (filter compare-item-to-pivot l)))

6.5 filtering with lambda, the Ultimate

Best:

;; NumberList  →  NumberList
;; extract all items from l that are larger than pivot
(define (larger pivot l)
  (filter (lambda (item) (> item pivot)) l))