RE: Sorting a data structure array...

["Dane Cox" <DCox@xxxxxxxxxxxxx>]

Thanks Hans.  This works better, but it still isn't quite right.  It
looks as if once it hits the first element that is 'to be moved'...in
this case it is the second row in the ds array...then it starts doing
strange things again.

Here is the data set...they start out in the order in which they are
retrieved from a file...which is to say no particular order at all.  :-)

Original order...

Acct                    Status
12345                   0
885503                  2
8872384         0
18                      0
29                      0
30                      0
31                      0
542                     0
82098000                0
788800000008            0

Sorted order...
12345                   0
29                      0
8872384         0
18                      0
30                      0
31                      0
542                     0
82098000                0
788800000008            0
885503                  2

The fifth record '29' ends up getting moved up into the second record,
and all the others are left alone...except the original second record
that is now last...as it should be.

Any ideas?

Thanks again,
Dane

-----Original Message-----
From: rpg400-l-bounces@xxxxxxxxxxxx
[mailto:rpg400-l-bounces@xxxxxxxxxxxx] On Behalf Of Hans Boldt
Sent: Thursday, July 08, 2004 8:20 AM
To: rpg400-l@xxxxxxxxxxxx
Subject: Re: Sorting a data structure array...

Dane Cox wrote:
> I have a data structure array that I am sorting using qsort and I'm
> observing some 'unintended' sorting going on and thought I'd see if
> someone else has seen this.
>  
> My data structure array contains 9 fields, one of which is a status
> field (1.0) which is what I am using to sort by...I want to see the
> higher status' last.  When I load up the ds array only two rows
contain
> a status other than 0.  When qsort is called, the ds array comes back
> with the higher status' last, however, those that have a status of
zero
> are also re-ordered amongst themselves.  Does any body know why that
is?
> I would think that if there is no difference in the value of status,
> then these would not be 'touched'.
> ... 

I believe what you are seeing is a consequence of the fact that qsort() 
(normally) is not a "stable" sort. That is, elements with identical keys

do not necessarily return in the same order as when they started. But 
you can change the comparison function to turn the sort into a "stable" 
sort:

     if element1.status < element2.status;
        return -1;
     elseif element1.status > element2.status;
        return 1;
     elseif %addr(element1) < %addr(element2);
        return -1;
     endif;
     return 1;

That is, for elements with identical keys, ensure that items already 
sorted lower remain lower.

Cheers! Hans

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