Re: REXML bug - descendant-or-self selector

On Sep 3, 2005, at 11:51 PM, Gavin Kistner wrote:
In XPath, "//" is an abbreviation for the "descendant-or-self" selector. My understanding (and Microsoft's XPath implementation) of this selector means that the following code should select the bar element as well as the descendants.

[Sliver:~/Desktop] gkistner$ cat tmp.rb
xml = <<ENDXML
<foo id="foo">
        <bar id="bar">
                <jim id="jim" />
                <jam id="jam" />
                <bork><whee id="whee" /></bork>
        </bar>
</foo>
ENDXML

require 'rexml/document'
include REXML
bar = XPath.first( Document.new( xml ), '//bar' )
bar.each_element( './/*[@id]' ){ |element_with_id|
        puts element_with_id
}

To set the record straight:
REXML is correct. My claim above (and what I believe is Microsoft's implementation) is incorrect.

The XPath expression:
    .//*[@id]
is XPath short-hand for:
    ./descendant-or-self::node()/child::*[attribute::id]

The implicit child:: axis is what causes the standard short-hand usage of // to act truly like a descendant selector. If you wanted the behavior that I describe above, then the legal XPath expression (which REXML supports) would be:
.//self::*[@id]

Way to go Sean! Thanks for setting me straight. (I filed a bug report, and he kindly explained why I was wrong. I researched the matter with other web-based XPath tools[1] and confirmed my incorrectness.)

[1] Such as the oh-so-useful http://www.zvon.org:9001/saxon/cgi-bin/ XLab/XML/extras.html?stylesheetFile=XSLT%2FxpathAxes.xslt&value=%2FAAA %2FBBB%2F%2F*%5B%40id%5D


.

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