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

[Thorsten Jolitz <tjolitz@gmail.com>]

"Drew Adams" <drew.adams@oracle.com> writes:

>> 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:
>> 
>> Debugger entered--Lisp error: (wrong-type-argument listp "c")
>>   length(("a" . "c"))
>
> It's not clear to me what you're asking.

I probably wasn't entirely clear to me either ..

> Are you looking for a "length" function that works for both true lists
> (but why do you mention 3 or more elements?) and a cons whose last cdr
> is a non-nil atom?
>
> If so then it is up to you to define what you want such a "length" to
> be/mean. Perhaps what you want is something like this?
>
> (defun thorsten-len (xs)
>   (cond ((null xs)       0)
>         ((null (cdr xs)) 1)
>         ((atom (cdr xs)) 2)
>         (t (1+ (thorsten-len (cdr xs))))))
>

But with this function you answered the question I really wanted to ask:
how to distinguish between a true list and a cons cell - by the use of
'atom' on the last cdr. Maybe pretty basic, but I haven't done it
before. 

> (thorsten-len '(1 2 3 4 . 5)) => 5
>
> (setq foo  '((a . 1) (b 2) (c (3 3 3)) (d 4 4 4 . 4)))
> (mapcar #'thorsten-len foo) => (2 2 2 5)
>
> (setq bar  ())
> (dolist (ff  foo) (push ff bar))
> (reverse bar) => ((a . 1) (b 2) (c  (3 3 3)) (d 4 4 4 . 4))

Thanks, nice solution - very useful. 

-- 
cheers,
Thorsten