sicp-ex-2.31

Pretty nice.

  
 (define (tree-map proc tree) 
   (define nil '()) 
   (map (lambda (subtree) 
          (cond ((null? subtree) nil) 
                ((not (pair? subtree)) (proc subtree)) 
                (else (tree-map proc subtree)))) 
        tree)) 
  
 ;; Usage: 
 (define my-tree (list (list 1 2) 3 (list 4 5))) 
 (tree-map square my-tree) 
 (tree-map (lambda (x) (+ x 1)) my-tree) 
  

my way:

  
 (define (tree-map proc tree) 
   (cond ((null? tree) nil) 
         ((pair? tree)  
          (cons  
           (tree-map proc (car tree))  
           (tree-map proc (cdr tree)))) 
         (else (proc tree)))) 

my way. For me this is more succinct than using cond and there's no need to check for nil explicitly (or define it):

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

own map implementation without cons

  
 (define (tree-map proc tree) 
   (if (null? tree) 
       '() 
       (if (pair? tree) 
           (cons (tree-map proc (car tree)) 
                 (tree-map proc (cdr tree))) 
           (proc tree)))) 
  

using map

  
 (define (tree-map f tree) 
   (map  (lambda (x)  
                 (if (pair? x) 
                     (tree-map f x) 
                     (f x) 
                 ) 
         )  
         tree 
   ) 
 ) 
  

As I mentioned in the previous exercise, I think the idea of mapping over a tree is better expressed in the following way (although admittedly it's slightly more complex in the case where the procedure being mapped is not a unary procedure):

 (define (tree-map f tree) 
   (if (not (pair? tree)) 
       (f tree) 
       (map (lambda (subtree) (tree-map f subtree)) tree)))