;; Solutions to double-all and extract-positives

;; double-all:  list-of-number -> list-of-number
;; doubles all the numbers in the list
(define (double-all alon)
  (cond [(empty? alon)  empty     ]
        [(cons? alon) (cons (* (first alon) 2)
                            (double-all (rest alon)))]))




;; test function
(check-expect (double-all empty) empty)
(check-expect (double-all (cons 3 (cons 9 (cons 14 empty)))) (cons 6 (cons 18 (cons 28 empty))))


;; extract-positives:  list-of-number -> list-of-number
;; consumes a list of numbers and produces a list of only the positive numbers (consider 0 positive)
(define (extract-positives alon)
  (cond [(empty? alon) empty      ]
        [(cons? alon) (cond [(>= (first alon) 0)
                             (cons (first alon) (extract-positives (rest alon)))]
                            [else (extract-positives (rest alon))])]))


;;tests

(check-expect (extract-positives empty) empty)
(check-expect (extract-positives (cons 3 (cons 9 (cons 14 empty)))) (cons 3 (cons 9 (cons 14 empty))))
(check-expect (extract-positives (cons -2 (cons -6 empty))) empty)
(check-expect (extract-positives (cons -2 (cons 6  (cons -1 empty)))) (cons 6 empty))
(check-expect (extract-positives (cons -2 (cons 0 (cons 6 empty)))) (cons 0 (cons 6 empty)))