Re: Sorting a data structure array...

[Hans Boldt <boldt@xxxxxxxxxx>]

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