Setting the nth element of a list

Setting the nth element of a list

[Nordlöw]

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

RE: Setting the nth element of a list

[Drew Adams]

> (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)?

Use setcar with nthcdr.

Re: Setting the nth element of a list

[Lennart Borgman]

Use nthcdr

On Thu, Jun 4, 2009 at 11:08 PM, Nordlöw <per.nordlow@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
>

Re: Setting the nth element of a list

[Barry Margolin]

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In article 
<8a7c4d36-195d-4517-827e-7fbeca7ae03f@r34g2000vba.googlegroups.com>,
 Nordlöw <per.nordlow@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)

-- 
Barry Margolin, barmar@alum.mit.edu
Arlington, MA
*** PLEASE post questions in newsgroups, not directly to me ***
*** PLEASE don't copy me on replies, I'll read them in the group ***

Re: Setting the nth element of a list

[Nordlöw]

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)
>
> --
> Barry Margolin, bar...@alum.mit.edu
> Arlington, MA
> *** PLEASE post questions in newsgroups, not directly to me ***
> *** PLEASE don't copy me on replies, I'll read them in the group ***

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?

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

Re: Setting the nth element of a list

[Nordlöw]

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)
>
> > --
> > Barry Margolin, bar...@alum.mit.edu
> > Arlington, MA
> > *** PLEASE post questions in newsgroups, not directly to me ***
> > *** PLEASE don't copy me on replies, I'll read them in the group ***
>
> 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?
>
> 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.

/Nordlöw

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__

Re: Setting the nth element of a list

[Nordlöw]

On Jun 5, 3:25 pm, p...@informatimago.com (Pascal J. Bourguignon)
wrote:
> Nordlöw <per.nord...@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__

I really believe such a function increases the readability of the code
that uses it, don't you?
What if I define it as a macro (defmacro) or inline (defsubst)?
Doesn't that give the best of both worlds+

/Nordlöw

Re: Setting the nth element of a list

[Pascal J. Bourguignon]

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

> I really believe such a function increases the readability of the code
> that uses it, don't you?
> What if I define it as a macro (defmacro) or inline (defsubst)?
> Doesn't that give the best of both worlds+

Readability is somewhat in the eyes of the reader.

That said, I can just repeat wat I wrote: setf is a standard and well
known operator designed to assign a value to any kind of place, so it
will be understood readily by any lisp programmer.

Defining redundant and specific operators just give you more work --
if you change the type of a sequence between list and vector, you'll
have to edit all the place where you used setnthcar or setnthref
instead of (setf (elt ...) ...) -- and it will give more work to the
reader who will have to refer to your functions definitions to know
what they mean.



If you were talking about a new kind of data structure, I'd agree that
it would be ok to define a setter,  but then it would have to be
defined with the arguments in the 'correct' order:

   (defun set-my-own-data-structure (new-value data-structure index1 ...) ...)

so you could bind it to setf:

   (defsetf my-own-data-structure set-my-own-data-structure)

and right away forget about set-my-own-data-structure, but write:

   (setf (my-own-data-structure my-ds i...) new-value)


-- 
__Pascal Bourguignon__

Re: Setting the nth element of a list

[bigfaceworm]

On Jun 5, 6:57 am, Nordlöw <per.nord...@gmail.com> wrote:
> I really believe such a function increases the readability of the code
> that uses it, don't you?
> What if I define it as a macro (defmacro) or inline (defsubst)?
> Doesn't that give the best of both worlds+
>
> /Nordlöw

In general, no.

Writing your own functions to increase "readability" decreases the
portability of code using those functions, in increases the effort
required to read such code (for a new person), and ultimately it leads
to bunches of little, disparate, libraries all doing the same thing
but with completely different nomenclature.  Once you learn and
understand 'setf, its meaning is incredibly clear, and adding layers
over that just obfuscates the code.

Using widely available, standard libraries (such as 'cl) is what leads
to more readable code.  Such libraries are the result of many years of
experience by people who know the language intimately.


TJ

Re: Setting the nth element of a list

[Miles Bader]

bigfaceworm <bigfaceworm@gmail.com> writes:
> Using widely available, standard libraries (such as 'cl) is what leads
> to more readable code.  Such libraries are the result of many years of
> experience by people who know the language intimately.

You have to be careful with cl.el.  In general if there's an
"elisp-native" way to do something (like `aset'), it's probably a better
idea to use that instead.

While I like common-lisp a lot, cl.el's attempt to fit it into elisp
doesn't always succeed very well (though the authors went to huge
lengths trying).  You can't just assume it's all good -- parts of it are
very useful, but parts are just horrid (e.g., anything related to
multiple-values, or that results in runtime-evaluated keyword
arguments).

-Miles

-- 
`To alcohol!  The cause of, and solution to,
 all of life's problems' --Homer J. Simpson

Re: Setting the nth element of a list

[Barry Margolin]

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In article 
<b775c362-a37c-4d38-bce1-8de6922acbd3@b1g2000vbc.googlegroups.com>,
 Nordlöw <per.nordlow@gmail.com> wrote:

> 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))

There's already a built-in function aset that does exactly that, except 
it takes its arguments in a different order.  Why do you need your own 
function?

-- 
Barry Margolin, barmar@alum.mit.edu
Arlington, MA
*** PLEASE post questions in newsgroups, not directly to me ***
*** PLEASE don't copy me on replies, I'll read them in the group ***