Re: number() floated?

On 3 January 2012 22:33, Vincent Lefevre <[email protected]> wrote:
> On 2012-01-03 23:00:16 +0100, Vincent Lefevre wrote:
>> On 2012-01-03 20:53:40 +0000, Laurence Rowe wrote:
>> > On 16 December 2011 17:06, Konrad Korzeniowski <[email protected]> wrote:
> [...]
>> > >     <xsl:value-of select="number('8.95')"/>
> [...]
>>
>> Except that this seems to be a bug here:
>
> After looking at the spec, I confirm this is a bug. The XPath spec
> says for number(): "a string that consists of optional whitespace
> followed by an optional minus sign followed by a Number followed
> by whitespace is converted to the IEEE 754 number that is nearest
> (according to the IEEE 754 round-to-nearest rule) to the mathematical
> value represented by the string"
>
> The nearest value X is the following one (represented exactly in
> radix 16, thanks to %a):
>
>> $ /usr/bin/printf "%a\n" 8.95
>> 0x8.f33333333333333p+0
>
> But actually (see below) the internal representation doesn't matter
> here.
>
> Concerning xsl:value-of, the XSLT spec says that for a number, the
> XPath string() fucntion is used, and the XPath spec says for the
> above value: "otherwise, the number is represented in decimal form
> as a Number including [...]; beyond the one required digit after
> the decimal point there must be as many, but only as many, more
> digits as are needed to uniquely distinguish the number from all
> other IEEE 754 numeric values." (FYI, that's the IEEE 754 rule for
> conversions.)
>
> As a consequence, the output value should be "8.95" (like the input)
> because 8.95 rounds to X and has less than 17 digits.

You're right, xpath.c just prints DBL_DIG digits (a constant from
float.h). However, doing this properly is difficult, see:
http://bugs.python.org/issue1580.

Laurence