Re: space leak from `values'

[David Kastrup <dak@gnu.org>]

storm@cua.dk (Kim F. Storm) writes:

> Miles Bader <miles@lsi.nec.co.jp> writes:
> 
> > storm@cua.dk (Kim F. Storm) writes:
> > > + DEFUN ("setnthcdr", Fsetnthcdr, Ssetnthcdr, 2, 3, 0,
> > > +        doc: /* Set cdr of Nth element of LIST to VALUE (nil if omitted), returns the result.
> > 
> > What's wrong with (setcdr (nthcdr (1- N) LIST) VALUE) ?
> 
> Read the third line of the doc string:
> 
> > If list has less than N elements, do not modify list.
> 
> Your suggestion fails if N is less than the length of the list:
> 
> (let ((list '(1 2 3 4))
>       (n 10)     
>       (value nil))
>   (setcdr (nthcdr (1- n) list) value))
> 
> With (my version of) setnthcdr you replace
>     (if (> (length kill-ring) kill-ring-max)
> 	(setcdr (nthcdr (1- kill-ring-max) kill-ring) nil)))
> by
>     (setnthcdr kill-ring-max kill-ring)

It must also be noted that length requires a lot of prerequisites:
that the list is not circular, ends with a cons, and so on.  For
example, (length '(3 4 5 . 6)) throws an exception.

One could write this as

(setcdr (or (nthcdr (1- n) list) (cons nil nil)) value)

or as

(let ((tail (nthcdr (1- n) list)))
  (and (consp tail)
       (setcdr tail value)))

or something to avoid the length call, however.

-- 
David Kastrup, Kriemhildstr. 15, 44793 Bochum