sicp 2.30

Exercise 2.30.  Define a procedure square-tree analogous to the square-list procedure of exercise 2.21. That is, square-list 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.

 

 

直接定义：

 

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

(define (square x)
  (* x x))

 

 

使用map：

 

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

(define (square x)
  (* x x))