xpath /descendant-or-self - Searching for a node in a particular tree

Question

I am reading about the short-cut, '//', which apparently is a shortcut for:

'/descendant-or-self'

it is clear what to expect, say, from a simple example of such an expression, eg,

//myNode

It will return a node list of all instances in the document, found from the root, of elements called 'myNode'.

However, what is the meaning of a more complicated expression, such as:

//aNode//myNode

?

Since // (being the shortcut for '/descendant-or-self') matches the root node twice, does this mean the first part of the expression '//aNode' is redundant, and only adds to the time it takes to complete the execution of the expression (after having still only found all expressions throughout the whole document, of 'myNode') ?

Are '//myNode' and '//aNode//myNode' going to result in exactly the same thing?

Finally, if I was searching through the document for an instance of node 'myNode' which was an indirect descendant of node 'interestingTree'. But I don't want the instance of node 'myNode' which is an indirect descendant of node 'nonInterestingTree', how should I do this ?

for example, searching in the document:

<root>
    <anode>
        <interestingTree>
            <unknownTree>
                <myNode/><!-- I want to find this one, not the other, where I don't know the path indicated by 'unknownTree' -->
            </unknownTree>
        </interestingTree>
        <nonInterestingTree>
            <unknownTree>
                <myNode/>
            </unknownTree>
        </nonInterestingTree>
    </anode>
    <anode>
        <someOtherNode/>
    </anode>
</root>

Thanks!

Answer 1

Are '//myNode' and '//aNode//myNode' going to result in exactly the same thing?

Yes, in this case, because all myNodes are also descendants of anode. In the general sense however, //aNode//myNode will obviously not match nodes which do not have an anode parent in their ancestor tree.

The xpath:

//aNode//myNode

will ignore any intermediate hierarchy between aNode and myNode, i.e. it will match /aNode/myNode, /anyNodes/anode/myNode, and /anyNodes/anode/xyzNode/myNode

Which answers your last question, you can find the nodes in the interesting subpath like so: (and again, ignoring any intermediate elements in the hierarchy)

//anode//interestingTree//myNode

ideally of course, you should be as explicit as possible with your pathing, as // can incur performance overhead due to the potentially large number of elements it needs to search.

Edit Possibly this helps?

I've adjusted your xml input for clarity to:

<root>
    <anode>
        <interestingTree>
            <unknownTree>
                <myNode>
                    MyNode In Interesting Tree
                </myNode>
            </unknownTree>
        </interestingTree>
        <nonInterestingTree>
            <unknownTree>
                <myNode>
                    MyNode In Non-Interesting Tree
                </myNode>
            </unknownTree>
        </nonInterestingTree>
    </anode>
    <anode>
        <someOtherNode/>
    </anode>
    <bnode>
        <myNode>
            MyNode in BNode
        </myNode>
    </bnode>
</root>

When parsed through the stylesheet:

<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
    <xsl:output method="xml" encoding="UTF-8" indent="yes"/>
    <xsl:template match="/">
        Matched by `//myNode`
        <xsl:apply-templates select="//myNode">
        </xsl:apply-templates>

        Matched by `//aNode//myNode`
        <xsl:apply-templates select="//anode//myNode">
        </xsl:apply-templates>

        Matched by `//aNode//interestingTree//myNode`
        <xsl:apply-templates select="//anode//interestingTree//myNode">
        </xsl:apply-templates>
    </xsl:template>

    <xsl:template match="myNode">
        <xsl:value-of select="text()"/>
    </xsl:template>
</xsl:stylesheet>

Returns the following:

Matched by `//myNode`
        MyNode In Interesting Tree
        MyNode In Non-Interesting Tree
    MyNode in BNode

Matched by `//aNode//myNode`
        MyNode In Interesting Tree
        MyNode In Non-Interesting Tree

Matched by `//aNode//interestingTree//myNode`
        MyNode In Interesting Tree