Incrementing through s LIST

I have spent the last day working some things out with lists and blocks and have made a lot of progress.

The problem I can't seem to work out is how to increment through a LIST of text strings. I sort of thought I had it working using Nth command, but that doesn't seem to be working as I get an error at the end of my program as I think the program hasn't realised its finished.

What my program does is update Block Attributes for a Title Block. That all works. It updates only a block for each paperspace selected.

What happens is the command CTAB cracks it when the list runs out. I am using a while loop that doesn't seem to work as anticipated.

Here is a snippet of the offending code:

     [color=red]([/color][color=royalblue]while[/color][color=red] ([/color][color=royalblue]>[/color] ListNumber [color=seagreen]0[/color][color=red])[/color]

  [color=red] ([/color][color=royalblue]setq[/color] nthretlist [color=red]([/color][color=royalblue]nth[/color] no retlist2[color=red]))[/color]

   [color=red]([/color][color=royalblue]setavr[/color] [color=magenta]"CTAB"[/color] nthretlist[color=red])[/color] [color=gray];Error Happens Here After Running Through Loop[/color]

   [color=red]([/color][color=royalblue]setq[/color] no [color=red]([color=royalblue]+[/color][/color] no [color=seagreen]1[/color][color=red]))[/color]
   [color=red]([/color][color=royalblue]setq [/color]ListNumber [color=red]([/color]- ListNumber 1[color=red]))[/color]

THis is the error:

; error: AutoCAD variable setting rejected: "CTAB" nil

Never mind. After hours of mucking about, I added the following IF loop so the code now looks like this:

EDIT: Ok, now the Error is appearing again.....any thoughts?

     (while (> ListNumber 0)

   (setq nthretlist (nth no retlist2))

[color=green]    (if (= nthretlist nil)
     (princ "Done");(exit)
     )[/color]

   (setvar "CTAB" nthretlist)

   (setq no (+ no 1))
   (setq ListNumber (- ListNumber 1))

Andrew,

Post the list(s) you are using.

Seems to me a foreach loop would be better than a while loop

for your case.

The exit with an if is a bad idea.

ymg

Like ymg3 I'd use something like

(foreach layout (layoutlist))

or

(foreach layout retlist2)

to replace all the code you posted.

Agree with Tombu if playing with layouts use the for layoutlist, be carefull with lists (length list) returns the number of items but the first is 0 as it appears you are aware so last is -1 length

(setq x (length list))
(repeat (length list)
(setq val (nth (setq x (- x 1)) list) ; this way hit 0
)