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

2014-09-18 07:05:28
Wolfgang, the same type of weakness will appear in pretty much any
version of this algorithm. And I suspect that the above is actually
less susceptible to such bizarre cases than other algorithms because
only one sequence is calculated rather than many. In any event he
random:random-sequence function provides values to 64-bit precision,
so the likelihood that a problem arises is practically 0.

The probability that two random values in a sequence are equal is less
than n^2 / 2 * precision, so you would have to have an N of about a
1,000,000,000 before any significant probability of error (about a 3%
chance)

You could always do a check on count(distince-values($rand)) if you
were really worried about it, but unless your code is meant to
navigate nuclear weapons, I doubt such a step is necessary.


On Thu, Sep 18, 2014 at 1:48 PM, Wolfgang Laun 
wolfgang(_dot_)laun(_at_)gmail(_dot_)com
<xsl-list-service(_at_)lists(_dot_)mulberrytech(_dot_)com> wrote:
What if the unsorted sequence is (0.413,0.192,0.888,0.513,0.522,0.413)?

Admittedly, given existing implemenations of random generators for doubles
in [0,1.0), this is rather unlikely, but you may have a hard time proving
that it is impossible.

-W


On 18 September 2014 13:35, David Rudel fwqhgads(_at_)gmail(_dot_)com
<xsl-list-service(_at_)lists(_dot_)mulberrytech(_dot_)com> wrote:

That is not what random:random-sequence does. It creates a sequence of
N random numbers between 0 and 1.

But if you then find the index of each of these numbers in the sorted
version of this sequence, **then** you have created a random
permutation of the numbers from 1 to N, as the OP requested.

So, by way of example, let's say random:random-sequence(5) spits out
(0.413,0.192,0.888,0.513,0.522)

Then the sorted version is (0.192,0.413,0.513,0.522,0.888).

Taking each element (in sequence) from the original output of
random:random-sequence() and finding the index in the sorted sequence
yields (2,1,5,3,4), a random permutation of the numbers from 1 to 5.


On Thu, Sep 18, 2014 at 1:14 PM, Wolfgang Laun 
wolfgang(_dot_)laun(_at_)gmail(_dot_)com
<xsl-list-service(_at_)lists(_dot_)mulberrytech(_dot_)com> wrote:
random:random-sequence(N)

If this is supposed to produce a sequence of numbers in the range 1..N
while
expecting it to contain every number of that range exactly once: would
this
truly be a "random" sequence? I don't think so.

-W



On 18 September 2014 11:05, David Rudel fwqhgads(_at_)gmail(_dot_)com
<xsl-list-service(_at_)lists(_dot_)mulberrytech(_dot_)com> wrote:

When I have to do this (essentially create a permutation of the numbers
from 1 to N), I combine random:random-sequence with saxon:sort

I'm away right now so I'm not working on a machine with XSLT, so the
following syntax may be off, but I use:

<xsl:variable name="rand" select="random:random-sequence(N)"/>

<xsl:variable name="sorted.rand" select="saxon:sort($rand)"/>

<xsl:variable name="permutation"
select="$rand!index-of($sorted.rand,.)"/>

The select attribute of the last can also be written as "for $i in
$rand
return index-of($sorted.rand,$i)"  .


On Saturday, September 13, 2014, Eliot Kimber 
ekimber(_at_)contrext(_dot_)com
<xsl-list-service(_at_)lists(_dot_)mulberrytech(_dot_)com> 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




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