Re: Sorting an array on multiple keys using SORTA

On Thu, 15 Feb 2007 12:43:40 +0530
ewart.desouza@xxxxxxxxxxx wrote:

I would like to sort the following array on the first 7 fields i.e.
from WWBCD through WATNNB. Is it possible using SORTA ???  According
to the manual you can sort only one field at a time.

It'd be nice to do a radix sort. A radix sort is a kind of super sort
that makes multiple passes. You'd make seven passes. The first sort
would be on the least significant field. The second would be on the next
most significant field, and so on. The last sort would be on the most
significant field.

You can kind of see it work in a miniature example. The leftmost column
is the input. I sort them by the ones column, then by the tens column,
then by the hundreds column.

329    651    329    329
757 -> 757 -> 651 -> 651
651    329    757    757

The one trick is that the sort you use to do a radix sort must be
"stable", meaning that if the compared fields are equal, they should
come out in the same order that they went in.

I think I see in the SORTA documentation that it isn't stable. That's
sad, because it would have been really nice.

Here are some stable sorts you could implement, if you choose to do it
the radix way: http://en.wikipedia.org/wiki/Category:Stable_sorts

I guess you could also use qsort to do it. It would just have kind of
a nasty comparison function. The comparison function would start by
comparing the most significant fields. If they were unequal, you could
return your decision. If they were equal, you'd have to check the next
most significant field, and so on.