Re: RANK

[John Darrington]

On Wed, May 18, 2005 at 10:24:15AM -0700, Ben Pfaff wrote:

     However, I don't know whether that's really the best approach.
     First, do we actually need the additional pointer in all cases or
     just in casefiles?  Just in casefiles, I think, and in that case
     we can add it in a wrapper in the casefile instead of in the
     ccase itself.  

Not sure that I understand you correctly.
We'd still need to keep one pointer/casenum  per case.  This could either be
encapsulated in the ccase itself, or it could be maintained in a map in the 
casefile.    In any event, there'd need to be a concept of a 
casefile-which-duplicates-another-casefile, and a means to create one.


                    Second, how would we deal with on-disk casefiles?
     I don't think a pointer makes much sense in that case; perhaps a
     case number instead?

If there was also a function to get a case from a casefile by number,
perhaps you're right.  I don't know enough about the casefile internals.
     
     Third, a basic RANK can be implemented something like this:
            compute orig_order=$casenum.
            sort cases by var_to_rank.
            compute rank=$casenum.
            sort cases by orig_order.
            modify vars/drop=orig_order.

But this involves an extra sort, which is an expensive operation.

J'

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