Remove items contained on an even position in the list.

How to remove items contained on an even position in the list.

Example:

'(1 2 3 4 5 6 7 8 9)

Result:

'(2 4 6 

(defun odd (lst)
 (if lst (cons (car lst) (odd (cddr lst))))
)

(defun even (lst)
 (if lst (cons (cadr lst) (even (cddr lst))))
)

FTFY:

(defun even (lst)
 (if (cdr lst) (cons (cadr lst) (even (cddr lst))))
)

Iterative:

(defun remove-odd ( l )
   ((lambda ( i ) (vl-remove-if '(lambda ( x ) (= 1 (rem (setq i (1+ i)) 2))) l)) -1)
)
(defun remove-even ( l )
   ((lambda ( i ) (vl-remove-if '(lambda ( x ) (= 0 (rem (setq i (1+ i)) 2))) l)) -1)
)

This works with both Integers & Real numbers including duplicate elements :-

(defun even (l)
 (vl-remove-if-not
   '(lambda (x) (wcmatch (vl-princ-to-string (/ x 2.)) "*.0"))
   l
 )
)
(defun odd (l)
 (vl-remove-if
   '(lambda (x) (wcmatch (vl-princ-to-string (/ x 2.)) "*.0"))
   l
 )
)

Examples :

_$ (EVEN '(1.0 2.0 3.0 4.0 5.0 6.0 5.0 4.0 3.0 2.0 1.0))
(2.0 4.0 6.0 4.0 2.0)
_$ (ODD '(1.0 2.0 3.0 4.0 5.0 6.0 5.0 4.0 3.0 2.0 1.0))
(1.0 3.0 5.0 5.0 3.0 1.0)
_$ (ODD '(1 2 3 4 5 6 5 4 3 2 1))
(1 3 5 5 3 1)
_$ (EVEN '(1 2 3 4 5 6 5 4 3 2 1))
(2 4 6 4 2)

If you actually want to remove the atoms from the list:

[b][color=BLACK]([/color][/b]defun c:removep [b][color=FUCHSIA]([/color][/b]/ lst a i rl[b][color=FUCHSIA])[/color][/b]
 [b][color=FUCHSIA]([/color][/b]setq lst '[b][color=NAVY]([/color][/b]1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16[b][color=NAVY])[/color][/b][b][color=FUCHSIA])[/color][/b]

 [b][color=FUCHSIA]([/color][/b]initget 7 [color=#2f4f4f]"Even Odd"[/color][b][color=FUCHSIA])[/color][/b]
 [b][color=FUCHSIA]([/color][/b]setq a [b][color=NAVY]([/color][/b]getint [color=#2f4f4f]"\nRemove Filter Type [b][color=MAROON]([/color][/b] Even/Odd [b][color=MAROON])[/color][/b] Or Atom Placement:   "[/color][b][color=NAVY])[/color][/b][b][color=FUCHSIA])[/color][/b]

 [b][color=FUCHSIA]([/color][/b]setq i 0[b][color=FUCHSIA])[/color][/b]
 [b][color=FUCHSIA]([/color][/b]foreach v lst
   [b][color=NAVY]([/color][/b]cond [b][color=MAROON]([/color][/b][b][color=GREEN]([/color][/b]eq a [color=#2f4f4f]"Odd"[/color][b][color=GREEN])[/color][/b]
          [b][color=GREEN]([/color][/b]and [b][color=BLUE]([/color][/b]zerop [b][color=RED]([/color][/b]rem [b][color=PURPLE]([/color][/b]1+ i[b][color=PURPLE])[/color][/b] 2[b][color=RED])[/color][/b][b][color=BLUE])[/color][/b]
               [b][color=BLUE]([/color][/b]setq rl [b][color=RED]([/color][/b]cons v rl[b][color=RED])[/color][/b][b][color=BLUE])[/color][/b]
               [b][color=GREEN])[/color][/b][b][color=MAROON])[/color][/b]
         [b][color=MAROON]([/color][/b][b][color=GREEN]([/color][/b]eq a [color=#2f4f4f]"Even"[/color][b][color=GREEN])[/color][/b]
          [b][color=GREEN]([/color][/b]and [b][color=BLUE]([/color][/b]zerop [b][color=RED]([/color][/b]rem i 2[b][color=RED])[/color][/b][b][color=BLUE])[/color][/b]
               [b][color=BLUE]([/color][/b]setq rl [b][color=RED]([/color][/b]cons v rl[b][color=RED])[/color][/b][b][color=BLUE])[/color][/b]
               [b][color=GREEN])[/color][/b][b][color=MAROON])[/color][/b]
         [b][color=MAROON]([/color][/b][b][color=GREEN]([/color][/b]numberp a[b][color=GREEN])[/color][/b]
          [b][color=GREEN]([/color][/b]and [b][color=BLUE]([/color][/b]not [b][color=RED]([/color][/b]zerop [b][color=PURPLE]([/color][/b]rem [b][color=TEAL]([/color][/b]1+ i[b][color=TEAL])[/color][/b] a[b][color=PURPLE])[/color][/b][b][color=RED])[/color][/b][b][color=BLUE])[/color][/b]
               [b][color=BLUE]([/color][/b]setq rl [b][color=RED]([/color][/b]cons v rl[b][color=RED])[/color][/b][b][color=BLUE])[/color][/b][b][color=GREEN])[/color][/b][b][color=MAROON])[/color][/b][b][color=NAVY])[/color][/b]
   [b][color=NAVY]([/color][/b]setq i [b][color=MAROON]([/color][/b]1+ i[b][color=MAROON])[/color][/b][b][color=NAVY])[/color][/b][b][color=FUCHSIA])[/color][/b]

 [b][color=FUCHSIA]([/color][/b]setq rl [b][color=NAVY]([/color][/b]reverse rl[b][color=NAVY])[/color][/b][b][color=FUCHSIA])[/color][/b]
 [b][color=FUCHSIA]([/color][/b]prin1 rl[b][color=FUCHSIA])[/color][/b]
 [b][color=FUCHSIA]([/color][/b]prin1[b][color=FUCHSIA])[/color][/b][b][color=BLACK])[/color][/b]

This could also work with integer values. Say remove every 3rd atom

-David

PS Extracting would do just the opposite.

(defun even (l)
(vl-remove-if ''((x)(zerop (logand (vl-position x l)1))) l))

(defun odd (l)
(vl-remove-if-not ''((x)(zerop (logand (vl-position x l)1))) l))

(defun ListRemoveAlt (lst includeFirstP)
 (vl-remove-if
   '(lambda (x) (setq includeFirstP (not includeFirstP)))
   lst
 )
)

A bit late to party.

(defun even (lst / l)
 (mapcar '(lambda (n) (and (zerop (rem n 2)) (setq l (append l (list n))))) lst)
l
)

(defun odd (lst / l)
 (mapcar '(lambda (n) (and (not (zerop (rem n 2))) (setq l (append l (list n))))) lst)
l
)

Do we have TEN ways !!! Oh yeah (nth (+ x 1) lst)

Some may read this post mistaking it for a even and odd routines for a list but.

(setq lst (list 1 2 3 3 4 5 6 6 7 8 8 8 ))
(1 3 4 6 7