What's the difference between //node and /descendant::node in xpath?

Question

I use a lot of XPath when locating elements in web pages using Selenium, and have moved away from using node1//node2 towards using node1/descendant::node2 more recently. What's the difference between the two methods? Is one more efficient than the other?

Example XML snippet to demonstrate:

<div id="books">
  <table>
    <tr><td class="title">Lord of the Rings</td><td class="author">JRR Tolkein</td></tr>
    <tr><td class="title">The Hitch-Hikers Guide to the Galaxy</td><td class="author">Douglas Adams</td></tr>
  </table>
</div>

So it'd be:

id('books')//td[@class='title']

or:

id('books')/descendant::td[@class='title']

Solution 1

see http://www.w3.org/TR/xpath#path-abbrev

// is just an abbreviation for the descendant:: axis

Edit

To quote:

//para is short for /descendant-or-self::node()/child::para

That is, it refers to all para which are a child of the context node or any node descended from the context node. As far as I can tell that translates into any descendant para of the context node.

Solution 2

There's a difference in the context group. //para[1] is short for /descendant-or-self::node()/child::para[1], which returns every para that is the first child of its parent. /descendant::para[1] returns only the first para in the entire subtree.

Solution 3

In your case

 id('books')//td[@class='title']

and:

 id('books')/descendant::td[@class='title']

return the same result.

But in fact, like it was already stated before, id('books')//td[@class='title'] means id('books')/descendant-or-self::node()/td[@class='title'] which is different from id('books')/descendant::td[@class='title'] in concept.

See the following note:

NOTE: The location path //para[1] does not mean the same as the location path /descendant::para[1]. The latter selects the first descendant para element; the former selects all descendant para elements that are the first para children of their parents.

this note was taken from http://www.w3.org/TR/xpath#path-abbrev

Solution 4

Other than terseness, I'm not aware of any difference.

Solution 5

I realize this answer is more of a functional correctness, but in case it helps anyone, I stumbled across a key difference today.

When selecting elements based on some property of a descendant (like attribute). For example:

...
<foo id="ID1">
    <bar>
        <baz at2="c" />
    </bar>
</foo>
<foo2 id="ID2">
    <bar>
        <baz at2="d" />
    </bar>
</foo2>
...

To select the id attribute of all elements who have a descendant with an attribute at2=c:

This xpath will select foo and foo2:

//*[//*[@at2="c"]]/@id

while this xpath will only select foo:

//*[descendant::*[@at2="c"]]/@id

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