Re: removing elements while looping with an index, and return values from Set add

[Paul Johnson]

Ken Gosier wrote:
> --- Paul Johnson <address@hidden> wrote:
>
> > Well, maybe I can.  Am I right in guessing that your
> > pseudo code like so:
> >         if (  condition[i]  )
> > is in the actual program something more elaborate, like
> >
> >          if (condition[i]== [thisElem getValue])
>
>
> It is pretty much like this, except that the conditions are
> not mutually exclusive. That is, in oldList, you have 3
> types of elements:
> - Those that satisfy condition[0]
> - Those that satisfy condition[1]
> - Those that satisfy neither.
>
> Basically, the problem is that the elements that satisfy
> condition[0] will get removed. Then when I'm looping to
> find those that satisfy condition[1], the index falls off
> the end of the List.
An index cannot "fall off" the end if you iterate like so:

 for (thisElem=[index next]; [index getLoc]==Member;
        thisElem=[index next])

As long as the thing is a Member, it gives it to  you. What you might 
not realize is that a nil object can be in a collection.

Now, if your index finds a nil element in your collection, that means 
you have a nil element in the collection. It does not mean you stepped 
off the end. So we need to find out how the nil got into your collection.

I was guessing before that happened because you removed an element from 
an Array, but if your oldList is just a List object, then it must mean 
at some time that you did the equivalent of:

  [oldList addLast: nil]

or you inserted an undefined thing into a collection.

I'd test by writing a diagnostic loop on oldList to find out what is in it.

pj


--
Paul E. Johnson                       email: address@hidden
Dept. of Political Science            http://lark.cc.ku.edu/~pauljohn
University of Kansas                  Office: (785) 864-9086
Lawrence, Kansas 66045                FAX: (785) 864-5700


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