Extract sublists

Extract sublists

[Nordlöw]

Is there a function for extracting sublists of lists?

If not here is my suggestion for inclusion in Emacs.

(defun sublist (list from to)
  "Return a sublist of LIST, from FROM to TO.
Counting starts at 0. Like `substring' but for lists."
  (let (rtn (c from))
    (setq list (nthcdr from list))
    (while (and list (< c to))
      (push (pop list) rtn)
      (setq c (1+ c)))
    (nreverse rtn)))
;; Use: (sublist '(a b) 0 0)
;; Use: (sublist '(a b) 0 1)
;; Use: (sublist '(a b) 1 2)
;; Use: (sublist '(a b) 0 2)

Thanks,
Nordlöw

Re: Extract sublists

[Nordlöw]

On Nov 17, 2:22 pm, Nordlöw <per.nord...@gmail.com> wrote:
> Is there a function for extracting sublists of lists?
>
> If not here is my suggestion for inclusion in Emacs.
>
> (defun sublist (list from to)
>   "Return a sublist of LIST, from FROM to TO.
> Counting starts at 0. Like `substring' but for lists."
>   (let (rtn (c from))
>     (setq list (nthcdr from list))
>     (while (and list (< c to))
>       (push (pop list) rtn)
>       (setq c (1+ c)))
>     (nreverse rtn)))
> ;; Use: (sublist '(a b) 0 0)
> ;; Use: (sublist '(a b) 0 1)
> ;; Use: (sublist '(a b) 1 2)
> ;; Use: (sublist '(a b) 0 2)
>
> Thanks,
> Nordlöw

Modified from org-sublist() but with 0-begin-indexing analogously with
substring.

/Nordlöw

Re: Extract sublists

[LanX]

searching for (pop) led me to (butlast)

----------------ielm
ELISP> (defun splice (LIST OFFSET LENGTH)
	 (butlast
	  (nthcdr OFFSET LIST)
	  (- (length LIST) LENGTH OFFSET)
	  )
	 )
splice
ELISP> (setq list '(a b c d e f g h i j))
(a b c d e f g h i j)

ELISP> (splice list 3 4)
(d e f g)
------------------

Re: Extract sublists

[LanX]

---------------- M-x ielm
ELISP> (defun sublist (LIST FROM TO)
	 (butlast
	  (nthcdr FROM LIST)
	  (- (length LIST) TO)
	  )
	 )
sublist
ELISP> (sublist list 3 7)
(d e f g)
-----------------

Re: Extract sublists

[Ted Zlatanov]

On Tue, 17 Nov 2009 05:22:51 -0800 (PST) Nordlöw <per.nordlow@gmail.com> wrote: 

N> Is there a function for extracting sublists of lists?
N> If not here is my suggestion for inclusion in Emacs.

N> (defun sublist (list from to)
N>   "Return a sublist of LIST, from FROM to TO.
N> Counting starts at 0. Like `substring' but for lists."
N>   (let (rtn (c from))
N>     (setq list (nthcdr from list))
N>     (while (and list (< c to))
N>       (push (pop list) rtn)
N>       (setq c (1+ c)))
N>     (nreverse rtn)))
N> ;; Use: (sublist '(a b) 0 0)
N> ;; Use: (sublist '(a b) 0 1)
N> ;; Use: (sublist '(a b) 1 2)
N> ;; Use: (sublist '(a b) 0 2)

It would be really nice if either FROM or TO could be a function or a
number, so you can say "from 2 to the first place where the element
passed to this function doesn't return t."  This is not a filter because
you look for the first place the function fails.  Unfortunately it's
efficient only if you walk through the list yourself so nthcdr won't be
so useful.

Ted

Re: Extract sublists

[Nordlöw]

On Nov 17, 4:37 pm, Ted Zlatanov <t...@lifelogs.com> wrote:
> On Tue, 17 Nov 2009 05:22:51 -0800 (PST) Nordlöw <per.nord...@gmail.com> wrote:
>
> N> Is there a function for extracting sublists of lists?
> N> If not here is my suggestion for inclusion in Emacs.
>
> N> (defun sublist (list from to)
> N>   "Return a sublist of LIST, from FROM to TO.
> N> Counting starts at 0. Like `substring' but for lists."
> N>   (let (rtn (c from))
> N>     (setq list (nthcdr from list))
> N>     (while (and list (< c to))
> N>       (push (pop list) rtn)
> N>       (setq c (1+ c)))
> N>     (nreverse rtn)))
> N> ;; Use: (sublist '(a b) 0 0)
> N> ;; Use: (sublist '(a b) 0 1)
> N> ;; Use: (sublist '(a b) 1 2)
> N> ;; Use: (sublist '(a b) 0 2)
>
> It would be really nice if either FROM or TO could be a function or a
> number, so you can say "from 2 to the first place where the element
> passed to this function doesn't return t."  This is not a filter because
> you look for the first place the function fails.  Unfortunately it's
> efficient only if you walk through the list yourself so nthcdr won't be
> so useful.
>
> Ted

This gives superior performance in my benchmarks, thanks to very
little garbage collection.
For the case when to is nil it just returns (nthcdr from list) which I
hope is alright...

(defun sublist (list from &optional to)
  "Return a sublist of LIST, from FROM to TO.
If END is omitted, it defaults to the length of the sequence.
Counting starts at 0. Like `subseq' and `substring' but solely for
lists."
  (let ((start (nthcdr from list)))     ;start reference
    (if to (butlast start
                    (- (+ from (length start)) ;if extract list at the
end this makes it much faster
                       to))
      start)))

/Nordlöw

Re: Extract sublists

[Pascal J. Bourguignon]

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

> Is there a function for extracting sublists of lists?
>
> If not here is my suggestion for inclusion in Emacs.
>
> (defun sublist (list from to)
>   "Return a sublist of LIST, from FROM to TO.
> Counting starts at 0. Like `substring' but for lists."
>   (let (rtn (c from))
>     (setq list (nthcdr from list))
>     (while (and list (< c to))
>       (push (pop list) rtn)
>       (setq c (1+ c)))
>     (nreverse rtn)))

(require 'cl)
(list
 (subseq '(a b) 0 0)
 (subseq '(a b) 0 1)
 (subseq '(a b) 1 2)
 (subseq '(a b) 0 2))
--> (nil (a) (b) (a b))


-- 
__Pascal Bourguignon__