Exercise 2.39 of SICP

2009-08-24

sicp

Exercise 2.39: Complete the following definitions of reverse (exercise 2.18) in terms of fold-right and fold-left from exercise 2.38:

(define (reverse sequence)
  (fold-right (lambda (x y) <??>) nil sequence))
(define (reverse sequence)
  (fold-left (lambda (x y) <??>) nil sequence))

(define (fold-right fn init-value items)
  (if (null? items)
    init-value
    (fn (car items)
        (fold-right fn init-value (cdr items)))))

(define (fold-left op initial sequence)
  (define (iter result rest)
    (if (null? rest)
        result
        (iter (op result (car rest))
              (cdr rest))))
  (iter initial sequence))

(define (reverse-right sequence)
  (fold-right (lambda (x y) (append y (list x))) '() sequence))

(define (reverse-left sequence)
  (fold-left (lambda (x y) (cons y x)) '() sequence))

> (reverse-right (list 1 2 3))

(3 2 1)

> (reverse-left (list 1 2 3))

(3 2 1)

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