Re: [xml-dev] Beware of XPath expressions that produce false positives

[Dimitre Novatchev <dnovatchev@gmail.com>]

 Hi Roger,

> Example of a Condition:
>
> The <B> element is empty.
>
> Will the following XPath correctly indicate when the condition is present?

A few notes:

1. You haven't defined what should  "empty" mean exactly. Therefore,
*any* XPath expression attempting to express this undefined concept of
"empty" is potentially wrong.

2. The provided example of expressions with "false positive" results
only reflect the misunderstanding in 1) above.

3. In case "empty" is strictly defined, then it is not correct to say
that an XPath expression "returns false positives" -- the correct
terminology is: the XPath expression is *wrong*. And the reason for
having wrong expressions in any language -- not only in XPath, that
surprise us, is lack of adequate testing.

Cheers,
Dimitre

On Sat, Sep 24, 2016 at 5:27 AM, Costello, Roger L. <costello@mitre.org> wrote:
> Hi Folks,
>
> A few days ago David Carlisle posted a message with an excellent analysis of
> several XPath expressions. His message got me to thinking: It’s really easy
> to write XPath expressions that yield false positives. Stated differently,
> it’s really hard to write correct XPath expressions.
>
> [Definition] False positive: a test result which incorrectly indicates that
> a particular condition or attribute is present.
>
> Example of a Condition:
>
> The <B> element is empty.
>
> Will the following XPath correctly indicate when the condition is present?
>
> normalize-space(B) eq ''
>
> Evaluating the XPath expression on the following XML correctly indicates
> that the condition is present.
>
> <Row>
>     <A>foo</A>
>     <B/>
>     <C>bar</C>
> </Row>
>
>
>
> Evaluating the XPath expression on the following XML correctly indicates
> that the condition is not present.
>
> <Row>
>     <A>foo</A>
>     <B>99</B>
>     <C>bar</C>
> </Row>
>
>
>
> Evaluating the XPath expression on the following XML incorrectly indicates
> that the condition is present.
>
>
>
> <Row>
>     <A>foo</A>
>     <B><bad/></B>
>     <C>bar</C>
> </Row>
>
>
>
> Evaluating the XPath expression produces a false positive.
>
>
>
> Evaluating the XPath expression on the following XML incorrectly indicates
> that the condition is present.
>
>
>
> <Row>
>     <A>foo</A>
>     <B><!-- Hello, world --></B>
>     <C>bar</C>
> </Row>
>
>
>
> Evaluating the XPath expression produces a false positive.
>
>
>
> /Roger



-- 
Cheers,
Dimitre Novatchev
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