Re: Setting the nth element of a list

[Pascal J. Bourguignon]

Nordlöw <per.nordlow@gmail.com> writes:

> On Jun 5, 3:06 pm, Nordlöw <per.nord...@gmail.com> wrote:
>> On Jun 5, 4:25 am, Barry Margolin <bar...@alum.mit.edu> wrote:
>>
>>
>>
>> > In article
>> > <8a7c4d36-195d-4517-827e-7fbeca7ae...@r34g2000vba.googlegroups.com>,
>>
>> >  Nordlöw <per.nord...@gmail.com> wrote:
>> > > This simple example changes the second element in the list x:
>>
>> > > (setq x '(a nil c))
>> > > (setcar (cdr x 'b)
>>
>> > > How do I generalize this to a function that sets (changes) the nth
>> > > element of a list (for side effects only)?
>>
>> > > Thanks in advance,
>> > > Nordlöw
>>
>> > (require 'cl-macs)
>> > (setf (nth n x) 'b)
>>
>>
>> Okey, here is my solution:
>>
>> (defun setnthcar (n list x)
>>   "Set N:th element of LIST to X for side effects only."
>>   (setcar (nthcdr n list) x))
>>
>> Comments on that?

It's awful, since there's is already a more standard and more general
mechanism to do that: (setf (nth n x) 'b)


>> How do I accomplish the same with arrays? Using aref I guess. Here is
>> my mockup that doesn't yet do what I want:
>>
>> (defun setnthref (n array x)
>>   "Set N:th element of ARRAY to X for side effects only."
>>   (set (aref array n) x))
>>
>> Thanks in advance,
>> Nordlöw
>
> Okey I found a solution:
>
> (defun setnthref (n array x)
>   "Set N:th element of ARRAY to X for side effects only."
>   (setf (aref array n) x))
> ;; (equal (let ((l '[a b c])) (setnthref 1 l 'B) l) '[a B c])
>
> Is this the most efficient way? I see that setf() is quite
> complicated.

Setf is a macro that is quite complicated, but it generates the best
code to properly update a place.

(require 'cl)
(macroexpand '(setf (aref (aref (nth (incf a) list-of-arrays) (incf i)) n) 42))
-->
(let* ((#1=--cl-var-- (aref (nth (incf a) list-of-arrays) (incf i)))
       (#2=--cl-var-- n))
  (aset #1# #2# 42))

There's nothing more efficient to set an array than aset.
Once you've found the array to set...

So there's no need to define a setnthref for an array, since (setf
(aref a n) v) does the job better.


There's also no need to define a setnthcar for a list, since (setf
(nth l n) v) does the job better.


And if you don't know whether you have a list or a vector, there's no
need to define anything, since (setf (elt sequence n) v) works better.

(let (x)
   (setf (elt (setf x (funcall (if (oddp (random 2))
                                   (function list)
                                   (function vector)) 
                               1 2 3))
               1) 
         0)
   x)
--> [1 0 3] or (1 0 3)


-- 
__Pascal Bourguignon__