2.30.rkt

#lang sicp
;; Exercise 2.30:
;; Define a procedure square-tree analogous to the square-list procedure of Exercise 2.21. That is, square-tree should behave as follows:
;;
(square-tree
 (list 1
       (list 2 (list 3 4) 5)
       (list 6 7)))
;; (1 (4 (9 16) 25) (36 49))
;;
;; Define square-tree both directly (i.e., without using any higher-order procedures) and also by using map and recursion.

(define (square-tree tree)
  (map (lambda (sub-tree)
         (if (pair? sub-tree)
             (square-tree sub-tree)
             (* sub-tree sub-tree)))
       tree))

(define (map proc items)
  (if (null? items)
      nil
      (cons (proc (car items))
            (map proc (cdr items)))))

(define (square-tree tree)
  (cond ((null? tree) nil)
        ((not (pair? tree))
         (square tree))
        (else (cons (square-tree (car tree))
                    (square-tree (cdr tree))))))

(define (square n) (* n n))