Re: elisp: returning a local variable

[Stephen Berman]

On Sun, 10 Mar 2013 10:39:53 +0000 (UTC) Glen Stark <mail@glenstark.net> wrote:

> Hi everyone.
>
> I'm doing some stuff in elisp, largely in an effort to become competent 
> in it.  I would like to obtain a list of subdirectories in the current 
> working directory.  I couldn't find a method that does this, so I tried 
> writing my own.  I got this far:
>
>
> (defun get-all-subdirectories ()
>   "Returns a list of directories in the current working folder"
>   (interactive)
>   (let ((src-list (file-expand-wildcards "*"))
>         (retval ()))
>     (dolist (fname src-list)
>       (when (file-directory-p fname) (push fname retval))
>       )
>     )
>   )   
>
> Which does indeed assemble a list of subdirectories into retval, but I 
> can't figure out how to return it as the return value of the function 
> (yes, I could setq to global variable, but I prefer not to do this).
>
> Can anyone help me out here?  
>       1)  how do I accomplish this?  

The dolist macro returns the value of retval if you add it to the
dolist "argument" list:

(defun get-all-subdirectories ()
  "Return a list of directories in the current working folder."
  (interactive)
  (let ((src-list (file-expand-wildcards "*"))
        (retval))
    (dolist (fname src-list retval)
      (when (file-directory-p fname) (push fname retval)))))

>       2)  Is there an existing method that does this?

I don't know of any.  But note that file-expand-wildcards does not
include "." or "..", so if you want these, you should use
directory-files instead.  Also, by using push the list order in retval
is the reverse of what file-expand-wildcards or directory-files returns.
To keep the same order, return the reversed list (or use append instead
of push):

(defun get-all-subdirectories ()
  "Return a list of directories in the current working folder."
  (interactive)
  (let ((src-list (file-expand-wildcards "*"))
        (retval))
    (dolist (fname src-list (reverse retval))
      (when (file-directory-p fname) (push fname retval)))))

Alternatively, instead of building up a new list and reversing it you
could eliminate the unwanted elements from the list by mapping them to
nil and deleting them:

(defun get-all-subdirectories ()
  "Return a list of directories in the current working folder."
  (interactive)
  (delq nil (mapcar #'(lambda (f) (and (file-directory-p f) f))
                    (directory-files default-directory))))

Steve Berman