GNU Info

(emacs-lisp-intro.info)lengths-list-many-files

---

Next: append Prev: Several files Up: Several files

---

Enter node , (file) or (file)node

Determine the lengths of `defuns'
---------------------------------

   The design using a `while' loop is routine.  The argument passed the
function is a list of files.  As we saw earlier (Note: Loop Example),
you can write a `while' loop so that the body of the loop is evaluated
if such a list contains elements, but to exit the loop if the list is
empty.  For this design to work, the body of the loop must contain an
expression that shortens the list each time the body is evaluated, so
that eventually the list is empty.  The usual technique is to set the
value of the list to the value of the CDR of the list each time the
body is evaluated.

   The template looks like this:

     (while TEST-WHETHER-LIST-IS-EMPTY
       BODY...
       SET-LIST-TO-CDR-OF-LIST)

   Also, we remember that a `while' loop returns `nil' (the result of
evaluating the true-or-false-test), not the result of any evaluation
within its body.  (The evaluations within the body of the loop are done
for their side effects.)  However, the expression that sets the
lengths' list is part of the body--and that is the value that we want
returned by the function as a whole.  To do this, we enclose the
`while' loop within a `let' expression, and arrange that the last
element of the `let' expression contains the value of the lengths'
list.  (Note: Loop Example with an Incrementing Counter.
)

   These considerations lead us directly to the function itself:

     ;;; Use `while' loop.
     (defun lengths-list-many-files (list-of-files)
       "Return list of lengths of defuns in LIST-OF-FILES."
       (let (lengths-list)
     
     ;;; true-or-false-test
         (while list-of-files
           (setq lengths-list
                 (append
                  lengths-list
     
     ;;; Generate a lengths' list.
                  (lengths-list-file
                   (expand-file-name (car list-of-files)))))
     
     ;;; Make files' list shorter.
           (setq list-of-files (cdr list-of-files)))
     
     ;;; Return final value of lengths' list.
         lengths-list))

   `expand-file-name' is a built-in function that converts a file name
to the absolute, long, path name form of the directory in which the
function is called.

   Thus, if `expand-file-name' is called on `debug.el' when Emacs is
visiting the `/usr/local/share/emacs/21.0.100/lisp/emacs-lisp/'
directory,

     debug.el

becomes

     /usr/local/share/emacs/21.0.100/lisp/emacs-lisp/debug.el

   The only other new element of this function definition is the as yet
unstudied function `append', which merits a short section for itself.