Re: 'length' function for lists and cons cells?

[Pascal J. Bourguignon]

Thorsten Jolitz <tjolitz@gmail.com> writes:

> Hi List, 
>
> which function could I use when I map an alist e.g. with dolist, that
> contains both types of associations as shown below: cons cells, or lists
> with 3 or more elements?
>
> 'length' doesn't work on cons cells:
>
> ,------------------------
> | (length '("a" "b" "c"))
> | 3
> `------------------------
>
> ,--------------------------------------------------------------
> | (length '("a" . "c"))
> | 
> | Debugger entered--Lisp error: (wrong-type-argument listp "c")
> |   length(("a" . "c"))
> `--------------------------------------------------------------

I don't understand what you want to do.
You can use length and dolist on alists and it will work perfectly:

(length '((a . 1) (b . 2) (c . 3)))
--> 3

(dolist (entry '((a . 1) (b . 2) (c . 3)))
  (destructuring-bind (key . value) entry
     (insert (format "key = %S, value = %S\n" key value))))
key = a, value = 1
key = b, value = 2
key = c, value = 3

If you want to count the cons cells, then why stop at the dotted-lists?
There are also circular lists.  See how you can do it at:
https://gitorious.org/com-informatimago/com-informatimago/blobs/master/common-lisp/cesarum/list.lisp#line303

(note: this is Common Lisp, some translation to emacs lisp is needed
(mostly, remove the colons from the loop keywords)).

-- 
__Pascal Bourguignon__                     http://www.informatimago.com/
A bad day in () is better than a good day in {}.