Re: How could I modify list element?

[Andreas Politz]

richardeng wrote:
> Alan Mackenzie wrote:
> > On Tue, 9 Dec 2008, richardeng wrote:
> >
> > > Hi all,
> > >    setcar/setcdr is not convenient.
> > >    In a long list, ex.
> > >    (setq a '(a b c d e f g))
> > >    I want to change 'e to 'E.
> > >    I need a function: (set-list-elt list old-elt new-elt)
> > >
> > >    How? translate list to vector, modify, then turn it back???
> >
> > Try this (not tested):
> >
> > (defun change-nth (liszt, n, nieuw)
> >   (while (> n 0)
> >     (setq liszt (cdr liszt)
> >           n (1- n)))
> >   (if liszt
> >      (setcar liszt nieuw)))
> (setq aaa '(1 2 3 4 5 b a "ccc"))
> (change-nth aaa 3 'BB)
> aaa  --> (1 2 3 BB 5 b a "ccc")    , it works, thank you!

I guess I misunderstood. You can do it with nthcdr,
which is a built-in function.

(let ((l '(0 1 2 3 4 5 6)))
  (setcar (nthcdr 3 l) -3)
  l)

-ap