Re: How could I modify list element?

[Andreas Politz <politza@fh-trier.de>]

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