Subject: Re: [xsl] How to Do Random "Shuffle"? From: "Eliot Kimber ekimber@xxxxxxxxxxxx" <xsllistservice@xxxxxxxxxxxxxxxxxxxxxx> Date: Sat, 13 Sep 2014 15:37:08 0000 
Interestingthanks for finding the reference (I was focusing my search on XSLT stuff). In my case the number of items to be sorted will be small, probably never more than 10, so efficiency is not that much of a concern at the moment, but of course this technique could get applied to larger sets (e.g., shuffling a large set of test questions, not just the answer options within a question. My challenge now is how to generate numbers between the limits: my math skills are sufficiently bad that I'm not thinking of any immediate solution. My hack, which works, is to generate random integers between 1 and 10 and simply see if, on each iteration, I get a hit on the current list. If I do, I shuffle and recurse. If I don't, I change the seed and try again. That should be guaranteed to finish but it's obviously wasteful. So having a builtin permute() function would be quite helpful for this particular problem. For the general task of rendering tests from question markup, randomizing both the questions and the answer options is general requirement. I suppose one could also implement this as a custom sort and handle it at the Java level, but I'd rather avoid any kind of Java extension dependency. Cheers, E. bbbbb Eliot Kimber, Owner Contrext, LLC http://contrext.com On 9/13/14, 10:18 AM, "Michael Kay mike@xxxxxxxxxxxx" <xsllistservice@xxxxxxxxxxxxxxxxxxxxxx> wrote: >What you describe is essentially the FisherYates algorithm, > >https://en.wikipedia.org/wiki/FisherbYates_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 userwritten >implementation. > >Michael Kay >Saxonica >mike@xxxxxxxxxxxx >+44 (0) 118 946 5893 > > > > >On 13 Sep 2014, at 15:29, Eliot Kimber ekimber@xxxxxxxxxxxx ><xsllistservice@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 matchfrom items in source order and the second column is the >>matchto >> 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 >> bbbbb >> Eliot Kimber, Owner >> Contrext, LLC >> http://contrext.com
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