setcdr question

setcdr question

[drain]

(setq list '(Trevor Jason John Alex Max))

(setcdr (nthcdr (1- 4) list) 'Christina)

list

"list" is now set to:

(Trevor Jason John Alex . Christina)

Why is the dot there?



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Re: setcdr question

[Pierre Lorenzon]

From: drain <aeuster@gmail.com>
Subject: setcdr question
Date: Mon, 1 Jul 2013 15:46:10 -0700 (PDT)

> (setq list '(Trevor Jason John Alex Max))
> 
> (setcdr (nthcdr (1- 4) list) 'Christina)
> 
> list
> 
> "list" is now set to:
> 
> (Trevor Jason John Alex . Christina)
> 
> Why is the dot there?
  Because you should have issued :
  (setcdr (nthcdr (1- 4) list) '(Christina)) 

  See how lists are made from cons cell. The cdr should always
  be a list itself or nil at the end.

  Regards 

  Pierre



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Re: setcdr question

[Nicolas Richard]

drain <aeuster@gmail.com> writes:

> (setq list '(Trevor Jason John Alex Max))
> (setcdr (nthcdr (1- 4) list) 'Christina)

As your question was already answered, I allow me to digress slightly
in suggesting that you may like setf for such operations:

(let ((list '(Trevor Jason John Alex Max)))
  (setf (nth (1- 4) list) 'Christina)
  list)
=> (Trevor Jason John Christina Max)

-- 
Nico.

Re: setcdr question

[Teemu Likonen]

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Nicolas Richard [2013-07-02 11:06:58 +02:00] wrote:

> drain <aeuster@gmail.com> writes:
>> (setq list '(Trevor Jason John Alex Max))
>> (setcdr (nthcdr (1- 4) list) 'Christina)

> (let ((list '(Trevor Jason John Alex Max)))
>   (setf (nth (1- 4) list) 'Christina)
>   list)

In Common Lisp a literal data shouldn't be edited. I suggest adopting
the same practice in Emacs Lisp too. So, create your list with LIST

    (list 'Trevor 'Jason 'John 'Alex 'Max)

or copy a literal sequence before editing:

    (copy-sequence '(Trevor Jason John Alex Max))

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Re: setcdr question

[drain]

Appreciate the answers.

However, where is the dot coming from? What is that supposed to represent?



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Re: setcdr question

[drain]

Ignore. Figured out.

That was pure laziness; apologies.



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Re: setcdr question

[Pierre Lorenzon]

From: drain <aeuster@gmail.com>
Subject: Re: setcdr question
Date: Tue, 2 Jul 2013 15:17:11 -0700 (PDT)

> Appreciate the answers.
> 
> However, where is the dot coming from? What is that supposed to represent?
  A cons cell : (cons 'a 'b) => (a . b)

  Regards 

  Pierre



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Re: setcdr question

[Rustom Mody]

On Wednesday, July 3, 2013 3:56:54 AM UTC+5:30, drain wrote:
> Ignore. Figured out.
> That was pure laziness; apologies.

Asking this question is the initiatory rite of starting lisp :-)

Re: setcdr question

[drain]

Things really open up once car / cons / cdr / nth / nthcdr are properly
understood.

I had so much inefficient code before!

Emacs über alles.



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