2.21¶

; The procedure square-list takes a list of numbers as argument and 
; returns a list of the squares of those numbers.
; (square-list (list 1 2 3 4)) 
; (1 4 9 16)
; Here are two different definitions of square-list. Complete both of 
; them by filling in the missing expressions:
; (define (square-list items) 
;   (if (null? items)
;       nil
;       (cons ⟨??⟩ ⟨??⟩)))
;
; (define (square-list items)
;    (map ⟨??⟩ ⟨??⟩)) 

(define (square-list items)
    (if (null? items)
        items
        (cons (square (car items))
              (square-list (cdr items))))
)
(square-list (list 1 2 3 5)) ; (1 4 9 25)

(define (square-list items)
    (map square items))
(square-list (list 2 4 5 6)) ; (4 16 25 36)