Re: [xsl] Is this the best way to compute a sigmoid function?

Subject: Re: [xsl] Is this the best way to compute a sigmoid function?
From: "Michael Kay mike@xxxxxxxxxxxx" <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx>
Date: Sat, 2 May 2020 18:09:19 -0000
> On 2 May 2020, at 18:55, Costello, Roger L. costello@xxxxxxxxx
<xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx> wrote:
> I want to compute the result of evaluating this sigmoid function:
>     1
> ----------------
> (1 + e**-x)
> That is, compute 1 divided by (1 + e raised to the -x power)

In recent versions of XPath, you can simply do

(1 div (1 + math:exp(-$x)))

> (However, it already appears to be mighty accurate -- look at all those
digits to the right of the decimal point)

I hope you don't seriously believe that precision implies accuracy - that
someone who claims the population of the world is 7,345,651,523 is more likely
to be right than someone who says it is 7 billion.

> Notice that for the variable $e-to-the-minus-x-power I specified it this
way: as="xs:decimal". Should I have specified it this way: as="xs:float"

For calculations like this it's best to use xs:double. You don't need any more
precision than this, and exponential/trigonometric functions are likely to be
implemented as double-precision floating point by whatever underlying library
is used.

Michael Kay

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