SICP_2.21-2.23

#lang racket

;;;;;;;;;;;;;;;;;;;2.21
(define (square-list items)
  (if (null? items)
      null
      (cons (square (car items))
            (square-list (cdr items)))))

(define (square x)
  (* x x))
;;
(define (square-list2 items)
  (map square items))

;;;;test
(square-list (list 1 2 3 4))
(square-list2 (list 1 2 3 4))

;;;;;;;;;;;;;;;;;;;;2.22
(define (square-list3 items)
  (define (iter things answer)
    (if (null? things)
        answer
        (iter (cdr things)
              (append answer
                      (list (square (car things)))))))
  (iter items '()))

;;test
(square-list3 (list 1 2 3 4))

;;;;;;;;;;;;;;;;;;;;2.23
(define (for-each proc items)
  (cond ((not (null? items))
         (proc (car items))
         (for-each proc (cdr items)))))

;;test
(for-each (lambda (x) (newline) (display x)) (list 57 321 88))