RE: [xsl] Use pure XPath to test a sequence for being a valid Fibonacci sequence

["Michael Kay" <mike@xxxxxxxxxxxx>]

Not sure why you are using index-of (or some, for that matter). It looks to
me like

$fib[1] = 0 and $fib[2] = 1 and
  every $i in 2 to count($fib) satisfies
    $fib[$i] = $fib[$i - 1] + $fib[$i - 2]

Michael Kay
http://www.saxonica.com/ 

> -----Original Message-----
> From: Abel Braaksma [mailto:abel.online@xxxxxxxxx] 
> Sent: 31 January 2007 17:13
> To: xsl-list@xxxxxxxxxxxxxxxxxxxxxx
> Subject: [xsl] Use pure XPath to test a sequence for being a 
> valid Fibonacci sequence
> 
> Hi Xslt'ers,
> 
> I was wondering about the following: using some/every XPath 
> expressions, is it possible to test values in a sequence 
> against preceding values? 
> The requirement pops up when you want to test pairs of 
> values, where, say, the even indexed value must be greater 
> than the odd indexed value, etc. I often have this 
> requirement, and although it is resolvable with for-each 
> and/or nested for-in-return, I was looking for a simpler solution.
> 
> Since many people understand the requirements of Fibonacci 
> numbers, I think it is a good example use case and represents 
> my actual use case pretty well (without the blur):
> 
> 0, 1, 1, 2, 3, 5, 8, 13, 21...
> 
> Now, for each given Fibonacci number, it is correct if:
> 
> F(n) = F(n-1) + F(n-2)
> 
> For positions 1 and 2, if the value for F(n-1) and F(n-2) are zero.
> 
> This is what I could think of. It is not working, because 
> index-of() function return two indices for the the number '1' 
> (because it appears twice). It works if I omit these numbers, 
> but then I cannot apply the 'every'. Does someone know of a 
> way to use some/every instructions in a way that it satisfies 
> the Fibonacci series above with 'every'? Perhaps it is simply 
> not possible (or requires an awkward construction)?
> 
> some $i in $fib satisfies
>     $fib[index-of($fib, $i) - 1] + $fib[index-of($fib, $i) - 2] = $i
> 
> $fib was defined as:
> <xsl:variable name="fib" as="xs:integer*" select="(0, 1, 1, 
> 2, 3, 5, 8, 13, 21)" />
> 
> Cheers,
> -- Abel Braaksma
>    http://www.nuntia.nl