Topic: how would you shift items in a list?

[Shay Gaghe]

hi

how would you shift items of a list?

_$ (setq lst '((0)(1)(2)(3)(4)(5)(6)(7)))

((0)(1)(2)(3)(4)(5)(6)(7)))
((1)(2)(3)(4)(5)(6)(7)(0)))
((2)(3)(4)(5)(6)(7)(0)(1)))
((3)(4)(5)(6)(7)(0)(1)(2)))

[irneb]

hi

how would you shift items of a list?

_$ (setq lst '((0)(1)(2)(3)(4)(5)(6)(7)))

Simplest I can think of:

Code - Auto/Visual Lisp: [Select]

1. (defun rotate-list (lst)
2.   (reverse (cons (car lst) (reverse (cdr lst)))))

Not necessarily the most efficient.

[ronjonp]

Here's another ( not so efficient ) for the list you provided.

Code - Auto/Visual Lisp: [Select]

1. (defun _shift (l n) (append (member (nth n l) l) (reverse (cdr (member (nth n l) (reverse l))))))
2. (_shift '((0) (1) (2) (3) (4) (5) (6) (7)) 3)
3. ;; ((3) (4) (5) (6) (7) (0) (1) (2))

[ymg]

From Reini's std-lib

Code: [Select]

(defun rot- (l)
  (append (cdr l) (list (car l))))


[Shay Gaghe]

what about  the most efficient code for the task?

[ronjonp]

what about  the most efficient code for the task?

Not mine    (unless you want to jump to something in the middle without having to iterate)

Benchmarking ...................Elapsed milliseconds / relative speed for 65536 iteration(s):


    (ROTATE-LIST L).....1281 / 1.16 <fastest>
    (ROT- L)............1312 / 1.13
    (_SHIFT L 1)........1484 / 1.00 <slowest>

[ymg]

shay,

It depends on the size of the list.

For short list up to about 50 items Irneb's is a bit more efficient.

On long one the advantage goes to rot-

Here benchmark for 10, 50, 500 and 5000 items list.

10 items
 (benchmark '((rotate-list pl) (rot- pl)))
Benchmarking .................Elapsed milliseconds / relative speed for 16384 iteration(s):

    (ROTATE-LIST PL).....1513 / 1.01 <fastest>
    (ROT- PL)............1529 / 1.00 <slowest>

50 items
(benchmark '((rotate-list pl) (rot- pl)))
Benchmarking .................Elapsed milliseconds / relative speed for 16384 iteration(s):

    (ROTATE-LIST PL).....1529 / 1.00 <fastest>
    (ROT- PL)............1529 / 1.00 <slowest>

500 items:
(benchmark '((rotate-list pl) (rot- pl)))
Benchmarking .................Elapsed milliseconds / relative speed for 16384 iteration(s):

    (ROT- PL)............1685 / 1.09 <fastest>
    (ROTATE-LIST PL).....1841 / 1.00 <slowest>

5000 items:
(benchmark '((rotate-list pl) (rot- pl)))
Benchmarking ..............Elapsed milliseconds / relative speed for 2048 iteration(s):

    (ROT- PL)............1233 / 1.99 <fastest>
    (ROTATE-LIST PL).....2449 / 1.00 <slowest>