;; The first three lines of this file were inserted by DrRacket. They record metadata
;; about the language level of this file in a form that our tools can easily process.
#reader(lib "htdp-intermediate-lambda-reader.ss" "lang")((modname |map and filter|) (read-case-sensitive #t) (teachpacks ()) (htdp-settings #(#t constructor repeating-decimal #f #t none #f ())))
;; Helene Martin, Garfield high school
;; 10/6/2010
;; Map/filter and anonymous functions

;; rewrite extract-seniors using filter
(define (extract-seniors2 a-los)
  (filter senior? a-los))

(check-expect (extract-seniors2 STUDENTS) (cons ADAM (cons QUYNH empty)))

;; extract-positives : list-of-number -> list-of-number
;; consumes list of num and produces list of all positive nums in list
;;  (uses built-in operator positive?)
(define (extract-positives alon)
  (cond [(empty? alon) empty]
        [(cons? alon) (cond [(positive? (first alon)) 
                             (cons (first alon) (extract-positives (rest alon)))]
                            [else (extract-positives (rest alon))])]))

(define LIST3 (list 6 5 -4 8 -3))
(check-expect (extract-positives LIST3) (list 6 5 8))

;; extract-positives2: list-of-number -> list-of-number
(define (extract-positives2 alon)
  (filter positive? alon))

(check-expect (extract-positives2 LIST3) (list 6 5 8))

;; short-strings : number list-of-string -> list-of-string
;; consumes num and list of string and produces list of all strings shorter than given num
(define (short-strings limit alos)
  (cond [(empty? alos) empty]
        [(cons? alos) (cond [(< (string-length (first alos)) limit)
                             (cons (first alos) (short-strings limit (rest alos)))]
                            [else (short-strings limit (rest alos))])]))

(define LIST4 (list "Garfield" "apple" "a" "the"))
(check-expect (short-strings 4 LIST4) (list "a" "the"))

(define (short-strings2 limit alos)
  (filter (lambda (a-string) (< (string-length a-string) limit)) alos))
(check-expect (short-strings2 4 LIST4) (list "a" "the"))