# Re: exslt random:random-sequence() and Math:random()

 Subject: Re: exslt random:random-sequence() and Math:random() From: a kusa Date: Thu, 18 Mar 2010 08:05:18 -0500
```Yes I was referring to the random:random-sequence function. Thank you

I have not been successful in generating unique random sequences using
any of the exslt functions.

So my next idea was to use the generate-id() function which generates
unique ids, and hack it to get a random number. Any thoughts on this
approach?

On Thu, Mar 18, 2010 at 3:28 AM, Michael Kay <mike@xxxxxxxxxxxx> wrote:
> The other question:
>
>>Does the exslt function generate unique random numbers in a sequential
> order?
>
> They are both exslt functions, I assume you are referring to
> random:random-sequence?
>
> (a) It's defined to return a sequence of numbers, so yes: there is a
> sequential order.
>
> (b) Don't assume the numbers will be unique. If the sequence is truly
> random, then occasionally the same number will appear more than once.
>
> (I remember an exercise in which students were asked to write down a
> sequence of 100 digits, chosen randomly. Not only were some digits such as
> 3, 7, and 9 much more frequent than others such as 0 and 5, but adjacent
> pairs (e.g. a 2 followed by a 2) were almost non-existent, whereas in a
> truly random sequence they would occur about once in every ten digits.)
>
> Regards,
>
> Michael Kay
> http://www.saxonica.com/
>
>> -----Original Message-----
>> From: Michael Kay [mailto:mike@xxxxxxxxxxxx]
>> Sent: 18 March 2010 08:16
>> To: xsl-list@xxxxxxxxxxxxxxxxxxxxxx
>> Subject: RE:  exslt random:random-sequence() and Math:random()
>>
>>
>> The math:random() function is defined to return a single
>> random number. This can be problematic, because you
>> presumably want it to return a different random number each
>> time; this means it's not a pure function. But the optimizer
>> doesn't know it's not a pure function, so it might do things
>> like moving the function out of a loop and only calling it
>> once (which means you get the same random number every time).
>> The Saxon-EE optimizer is quite likely to move all the calls
>> on random() into a single global variable.
>>
>> The function random:random-sequence() is designed to get
>> around these problems. Firstly, you can ask it to return a
>> sequence of say 1000 random numbers in a single call.
>> Secondly, you can supply a "seed" - typically a number
>> returned from a previous call on the function, or perhaps
>> just the value of position(). The function thus depends on
>> the value of its arguments, which means it is much less
>> likely to be optimized away.
>>
>> Calling "impure" functions in XSLT is always problematic; the
>> more aggressive your optimizer, the more problematic it becomes.
>>
>> Regards,
>>
>> Michael Kay
>> http://www.saxonica.com/
>>
>> > -----Original Message-----
>> > From: a kusa [mailto:akusa8@xxxxxxxxx]
>> > Sent: 18 March 2010 00:15
>> > To: xsl-list
>> > Subject:  exslt random:random-sequence() and Math:random()
>> >
>> > Hi
>> >
>> > I have a doubt and hoping that the experts can clarify for
>> me. Aren't
>> > the exslt function random:random-sequence() and
>> > Math:random() once and the same? What is the difference if any?
>> >
>> > Does the exslt function generate unique random numbers in a
>> sequential
>> > order?
>> >

```

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