About `current-kill'

About `current-kill'

[Xue Fuqiao]

In the definition of the function `current-kill':

[...]
      (let ((ARGth-kill-element
	     (nthcdr (mod (- n (length kill-ring-yank-pointer))
			  (length kill-ring))
		     kill-ring)))
[...]

I understand what `ARGth-kill-element' does (from its name).  But how
does it work?  I cannot understand it.

-- 
Xue Fuqiao
http://www.gnu.org/software/emacs/

Re: About `current-kill'

[Óscar Fuentes]

Xue Fuqiao <xfq.free@gmail.com> writes:

> In the definition of the function `current-kill':
>
> [...]
>       (let ((ARGth-kill-element
> 	     (nthcdr (mod (- n (length kill-ring-yank-pointer))
> 			  (length kill-ring))
> 		     kill-ring)))
> [...]
>
> I understand what `ARGth-kill-element' does (from its name).  But how
> does it work?  I cannot understand it.

ARGth-kill-element *does* nothing. It *is* a variable that takes the
value of the expression that begins with (nthcdr ...

Re: About `current-kill'

[Stephen Berman]

On Mon, 18 Mar 2013 06:32:42 +0800 Xue Fuqiao <xfq.free@gmail.com> wrote:

> In the definition of the function `current-kill':
>
> [...]
>       (let ((ARGth-kill-element
> 	     (nthcdr (mod (- n (length kill-ring-yank-pointer))
> 			  (length kill-ring))
> 		     kill-ring)))
> [...]
>
> I understand what `ARGth-kill-element' does (from its name).  But how
> does it work?  I cannot understand it.

The kill-ring is a list of buffer substrings.  As a ring, you can cycle
through it, e.g. if its length is 5, taking its cdr 8 times returns the
same result as taking its cdr 3 times; that's what the mod function
guarantees.  So nthcdr skips over the first n elements of the kill-ring,
and the car of the resulting list becomes the current-kill.

Steve Berman

Re: About `current-kill'

[Xue Fuqiao]

On Mon, 18 Mar 2013 01:38:24 +0100
Stephen Berman <stephen.berman@gmx.net> wrote:

> > In the definition of the function `current-kill':
[...]
> >       (let ((ARGth-kill-element
> > 	     (nthcdr (mod (- n (length kill-ring-yank-pointer))
> > 			  (length kill-ring))
> > 		     kill-ring)))
[...]
> > I understand what `ARGth-kill-element' does (from its name).  But how
> > does it work?  I cannot understand it.

> The kill-ring is a list of buffer substrings.  As a ring, you can cycle
> through it

That's what I've forgotten, thanks.  It will be good if the {mark,kill}
ring uses ring.el, I think.

> e.g. if its length is 5, taking its cdr 8 times returns the
> same result as taking its cdr 3 times; that's what the mod function
> guarantees.  So nthcdr skips over the first n elements of the kill-ring,
> and the car of the resulting list becomes the current-kill.

That's true, and `(mod -3 5)' evaluates 2.  It suits this feature exactly.

-- 
Xue Fuqiao
http://www.gnu.org/software/emacs/