Re: [xsl] XPath proximity position in predicates

Subject: Re: [xsl] XPath proximity position in predicates
From: David Landwehr <david.landwehr@xxxxxxxxxxxx>
Date: Mon, 15 May 2006 13:04:42 +0200
Hi Ken,

Notice that your situation is different from what I have since you use the = operator. From a specification stand point this would never change as the result of the test is a boolean. The only situation where this "rounding" would apply would be in the special case where you have a predicate expression returning a number. I guess the specification would be in its right to say it default should use round (e.g. as it does for the arguments to the substring function) it would still allow the author to change the behavior by wrapping the calculation in a call to floor or ceiling. However I'm not trying to get a change into the specification, I simply found it interesting that it work in this way and thought way not ask the people who know why ;)

Best regards,

G. Ken Holman wrote:
At 2006-05-15 12:30 +0200, David Landwehr wrote:
It might be that a default of rounding the number expression isn't always the solution an author would want. The reason I took a guess for rounding is that if you write select="element[(((7 div 5) div 3) * 5) * 3]", you get from the constant expression select="element[6.99999999999]" which will never select an element. In that particular case rounding would be what I expected.

Yes, I understood that, and it seems intuitive but from a standards perspective the question would be would you want that to *always* happen, or should it be up to the user to decide what to do with inaccurate results as it might depend on the situation (as I tried to highlight in my example)?

Thanks for following up.

. . . . . . . . . Ken

Best regards,

G. Ken Holman wrote:
At 2006-05-15 11:19 +0200, David Landwehr wrote:
Reading XPath 1.0 it states that a predicate evaluating to a number will return true if equal to the proximity position of the current node. I was wondering if there is a reason the evaluated number isn't rounded by the XPath engine? Because XPath is based on IEEE 754 the result of an evaluation might be inaccurate on the last digit which will cause a predicate to return false where it should return true. I'm just asking this out of curiosity and accepts that an author has to call the round function if she uses arithmetic which can give inaccurate results.

As the typical use is merely ordinal position in a node set I've never had to worry about this in a predicate ... but I did have to think about position() and rounding in a standalone <xsl:if> when doing a two-column display in XSL-FO and I wanted to introduce a column break in a standalone block ... I might have wanted to use floor() instead of round() in the following:

 <xsl:if test="position()=round(last() div 2)">
   <block break-before="column"/>

... but I cannot readily extrapolate that into an XPath predicate.

I think you summarized correctly that if the author is doing things that might give inaccurate results they should do it explicitly ... as in my case they might want to make the decision between floor() and ceiling() but in a predicate rather than just round(), so I don't believe it makes sense to just implement round() by default.

I hope this helps.

. . . . . . . . . Ken

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