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

[Paul Johnson]

Ken Gosier wrote:
> Hello all again. Many thanks for all the replies. I am now using index
> remove and not getting the seg faults.
>
> However, I am now having a new problem. I am able to resolve it myself,
> but not happy with my solution. Specifically, the removing of elements I
> posted yesterday actually takes place in a 2-step loop:
>
>
> for (i=0; i<2; i++)
> {
>    id <List> newList = [List create: self];
>    id <Index> index = [oldList begin: self];
>    id thisElem;
>
>    for (thisElem=[index next]; [index getLoc]==Member;
>         thisElem=[index next])
>    {
>       if (  condition[i]  )
>       {
>          [newList addLast: [index remove]];
>       }
>    }
>    [index drop];
>
>    ... do stuff with newList ...
> }
>
>
> I first want to get those elements that satisfy condition[0], then those
> that satisfy condition[1]. I find that everything goes smashing, wonderful
> in the first pass through. But then the second time through, the index
> steps off the end of the List, and I get seg faults, b/c messages get sent
> to nil id's.

If oldList is a Swarm Array, this could happen, because removing things 
puts nil in their place.  Otherwise, I can't explain it.

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])

otherwise this loop does not make any sense to me at all.  Either all or 
none of the objects will get removed on the first pass, and then on the 
second pass the list will be empty and, I have a hazy recollection that 
everything goes to hell if you try to create an index from an empty 
Swarm Collection.

--
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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