2.33.rkt

#lang racket
(provide (all-defined-out))
(define (filter predicate sequence)
  (cond [(null? sequence)
         '()]
        [(predicate (car sequence))
         (cons (car sequence)
               (filter predicate (cdr sequence)))]
        [else
         (filter predicate (cdr sequence))]))

(define (accumulate op initial sequence)
  (if (null? sequence)
      initial
      (op (car sequence)
          (accumulate op initial (cdr sequence)))))

;(define (map p sequence)
  ;(accumulate (lambda (x y) (cons (p x) y)) '() sequence))

;(define (append seq1 seq2)
  ;(accumulate cons seq2 seq1))

;(define (length sequence)
  ;(accumulate (lambda(x y) (+ 1 y)) 0 sequence))

;(length '(1 2 3 4 5))

;(append '(1 2 3) '(4 5 6))

;(map (lambda(x) (* x x)) '(1 2 3))