English Idiom in Unix: Directory Recursively

[Roland Hutchinson]

On Wed, 18 May 2011 07:19:08 +0200, Pascal J. Bourguignon wrote:

> Roland Hutchinson <my.spamtrap at verizon.net> writes:
> 
>> Sorry to have to contradict you,
> 
> Don't be sorry.
> 
> 
>> but it really is a textbook example of recursion.  Try this psuedo-code
>> on for size:
>>
>> FUNCTION DIR-DELETE (directory)
>>   FOR EACH entry IN directory
>>   IF entry IS-A-DIRECTORY THEN DIR-DELETE (entry).
>>
>> Well, now that's not just recursion; it's tail recursion.
> 
> It's not tail recursion.  If you had indented your code properly, you'd
> see why it's not:
> 
>     (defun dir-delete (directory)
>       (loop for entry in directory
>             do (if (is-a-directory entry)
>                    (dir-delete entry))))
> 

You are right, of course.  Thanks for the correction.

> (I put parentheses, so my editor knows what I mean and can do the
> indentation for me).

My editor would have done that, too--if I had bothered to be thinking 
clearly.

> That's why walking a directory is done with a recursive procedure,
> instead of an iterative one: it's much simplier.  To implement an
> iterative procedure, you would have to manage a stack yourself, instead
> of using the implicit stack of the recursive procedure.

Got it!  Thanks again.


-- 
Roland Hutchinson		

He calls himself "the Garden State's leading violist da gamba,"
... comparable to being ruler of an exceptionally small duchy.
--Newark (NJ) Star Ledger  ( http://tinyurl.com/RolandIsNJ )