numbering backgrounds (was: background owners)

[miscdas at boxfrog.com]

Wilhelm Sanke writes: 

> While dealing with  the special features of  backgrounds we have
> discussed so far under "background owners", I noticed another specialty
> that seems to be undocumented, namely how to refer to backgrounds by
> number: 
> 
> As stated in the documentation, the backgroundnames and backgroundIDs
> functions return the "toplevel" backgrounds - the primary ones without
> considering nested groups. 
> 
> "put the number of backgrounds of this stack" likewise returns the
> number of these toplevel backgrounds. 
> 
> However, if you want to refer to a background by number (which, of
> course, you need not do) 
> 
> - do something with background x - 
> 
> you may end up with a background quite different from what you expected,
> because - this is the point - 
> 
> the *internal* numbering of backgrounds does additionally include all
> nested backgrounds. 
> 
> "Background 4" may refer to the "toplevel"background  number 4 - if
> there are no nested groups, but in case there are nested groups,
> "background 4" could refer to the second-nested group of  toplevel
> background 2. 
> 
> Apparently there is no function to get the total number of backgrounds,
> maybe something like "the effective number of backgrounds" should be
> introduced. 
> 
> It has to be scripted, as in the following example: 
> 
>   "put 0 into counter
>   repeat
>     add 1 to counter
>     if there is a background counter of stack SName then
>       # do something with background counter
>     else
>       exit repeat
>     end if
>   end repeat" 
> 
> The final value of "counter" now contains the total number of
> backgrounds and if you have produced a list of background names or IDs
> with the help of  this script, you can now accurately refer to a
> background also by number. 
> 
> Regards, 
> 
> Wilhelm Sanke
=============== 

But, beware if you add groups, ungroup/regroup, because the numbering will 
almost always change. This is another case where it is usually better to use 
ID values or names than numbers. 

miscdas