Copy object to middle of line or polyline

Hi everyone!

I have problem with my program. It run unstable.

I hope everyone helps me

(defun mid (p1 p2)
 (list
   (/ (+ (car p1) (car p2)) 2.0)
   (/ (+ (cadr p1) (cadr p2)) 2.0)
 )
)
;______________________________________
(defun c:test ( / ob1 ob2 d0 i j ) 
 (setq ob0 (ssget))
 (setq d0 (getpoint))
 (setq ob1 (ssget "_:L" '((0 . "LINE"))))
 ;(setq i (- (sslength ob1) 1))
(repeat (setq i (sslength ob1))
 (setq j i)
 (setq d1 (cdr (assoc 10 (entget (ssname ob1 (setq i (1- i)))))))
 (setq d2 (cdr (assoc 11 (entget (ssname ob1 (setq j (1- j)))))))
 (setq mi (mid d1 d2))
 (command ".copy" ob0 "" "m" d0 mi "")
)
(princ)
)

Please read this:

http://www.cadtutor.net/forum/showthread.php?9184-Code-posting-guidelines

Regarding your issue:

Your problem is most likely caused by the 'Running Osnap' setting (OSMODE). When using command calls this setting can have an impact on the outcome of Lisp programs.

See here for a possible solution ("_non"):

http://www.cadtutor.net/forum/showthread.php?102187-Getpoint-doesn-t-allow-osnap&p=693767&viewfull=1#post693767

You have said also you want plines so p1 p2 may not give correct mid if say your pline has more than 2 pts. I would use Vla-getpointatdist take length / 2.0 this will give mid of a line and pline.