## Re: [xsl] How to Do Random "Shuffle"?

 Subject: Re: [xsl] How to Do Random "Shuffle"? From: "Michael Kay mike@xxxxxxxxxxxx" Date: Sat, 13 Sep 2014 15:18:36 -0000
```What you describe is essentially the Fisher-Yates algorithm,

https://en.wikipedia.org/wiki/FisherYates_shuffle

which strikes me as so obvious I find it surprising it has a name. (Who knows,
it might even have a patent...)

The challenge is to do it efficiently, which depends on the time complexity of
the remove() function. If remove() is O(n) (i.e. if it involves copying all
the items other than the one that's removed), then the shuffle is O(n^2).

By contrast, sorting on a random sort key might well be O(n log n).

I've recently proposed a family of random number functions for XPath 3.1, and
I included permute() as a primitive on the theory that a native implementation
might be considerably more efficient than a user-written implementation.

Michael Kay
Saxonica
mike@xxxxxxxxxxxx
+44 (0) 118 946 5893

On 13 Sep 2014, at 15:29, Eliot Kimber ekimber@xxxxxxxxxxxx
<xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx> wrote:

> Using XSLT 2 I need to implement rendering of "match table" questions
> where you have two sets of items, the match item and the thing it matches
> to. I want to present this as a literal table, where the first column is
> the match-from items in source order and the second column is the match-to
> items, in random order.
>
> I think this is best characterized as a "shuffle" problem, where you want
> to reorder a list randomly but all items in the list must be accounted
> for.
>
> I can think of a recursive algorithm: given a list, generate a random
> integer between 1 and the list length, select that item and add it to the
> result list, then call this function on the original list minus the node
> you just selected.
>
> Is there an easier or more efficient way to do it?
>
> Thanks,
>
> Eliot
> 
> Eliot Kimber, Owner
> Contrext, LLC
> http://contrext.com

```