;; The first three lines of this file were inserted by DrScheme. 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 functional_abstraction2) (read-case-sensitive #t) (teachpacks ((lib "testing.ss" "teachpack" "htdp"))) (htdp-settings #8(#t constructor repeating-decimal #f #t none #f ((lib "testing.ss" "teachpack" "htdp")))))
;;Contract: (listof X) (X -> boolean) -> (listof X)
;; The function takes a list of X and a criterion function
;; and creates a list of elements that satisfy the criterion
(define (filter2 criterion alist) 
   (cond
     [(empty? alist) empty]
     [(criterion (first alist)) (cons (first alist) (filter2 criterion (rest alist)))]
     [else (filter2 criterion (rest alist))]
   )
)

(filter2 even? (list 1 2 3 4 5 6 7 8 9)) 
;; expected value
(list 2 4 6 8)

;; posn -> boolean
;; returns true if both x and y coordinates of a 
;; position are between 0 and 300 (included)
(define (within-canvas a-posn)
  (and (>= (posn-x a-posn) 0) (<= (posn-x a-posn) 300) (>= (posn-y a-posn) 0) (<= (posn-y a-posn) 300)) 
)

(define positions (list (make-posn 200 2000) (make-posn 150 230) (make-posn -100 20)
                        (make-posn 400 50) (make-posn 100 300)))

(filter2 within-canvas positions)

;; given a list of mixed data, use filter2 to create a list of all
;; symbols in the original list

;; given a list of mixed data, use filter2 to create a list of all 
;; non-symbols in the original list