SICP/ex-3_12-xx.scm

(load "util.scm")

(display "\nex-3.12 - append!\n")

(define (append x y)
  (if (null? x)
      y
      (cons (car x) (append (cdr x) y))))

(define (append! x y)
  (set-cdr! (last-pair x) y)
  x)

(define (last-pair x)
  (if (null? (cdr x))
      x
      (last-pair (cdr x))))

(define x (list 'a 'b))
(define y (list 'c 'd))
(define z (append x y))

(assert z '(a b c d))
(assert (cdr x) '(b))
(define w (append! x y))
(assert w '(a b c d))
(assert (cdr x) '(b c d))

(display "\nex-3.13 - make cycle\n")

(define (make-cycle x)
  (set-cdr! (last-pair x) x)
  x)

(define z (make-cycle (list 'a 'b 'c)))

(display "[see comment]\n")
; infinite loop
; (last-pair z)

(display "\nex-3.14 - mystery\n")

; reverse
(define (mystery x)
  (define (loop x y)
    (if (null? x)
        y
        (let ((temp (cdr x)))
          (set-cdr! x y)
          (loop temp x))))
  (loop x '()))

(define v (list 'a 'b 'c 'd))
(define w (mystery v))

(display v) (newline)
(display w) (newline)

(display "\nex-3.15\n")

(define x (list 'a 'b))
(define z1 (cons x x))
(define z2 (cons (list 'a 'b) (list 'a 'b)))

(define (set-to-wow! x)
  (set-car! (car x) 'wow)
  x)

(assert z1 '((a b) a b))
(set-to-wow! z1)
(assert z1 '((wow b) wow b))
(set-to-wow! z2)
(assert z2 '((wow b) a b))


(display "\nex-3.16 - count pairs\n")

(define (count-pairs x)
  (if (not (pair? x))
      0
      (+ (count-pairs (car x))
         (count-pairs (cdr x))
         1)))

(define r3 '(1 2 3))
;(define r4 '())
;(define r7 '())
;(define r-never '())

(assert (count-pairs r3) 3)
;(assert (count-pairs r4) 4)
;(assert (count-pairs r7) 7)
;(assert (count-pairs r-never) '())

(display "\nex-3.17 - count pairs improved\n")


(display "\nex-3.18\n")