XPath query: Query not working as expected

Question

I was learning XPath using the following xml document: http://www.w3schools.com/xpath/xpath_examples.asp

Now, when I execute the query:

bookstore/book/author[contains(.,'G')]

I get the result: Giada De Laurentiis, James McGovern as expected. Now, since contains() returns a boolean value, I expected the following query to return all authors:

bookstore/book/author[true]

however, it returns an empty set. Can somebody explain?

roman · Accepted Answer · 2012-11-02 15:10:00Z

4

you need bookstore/book/author

UPDATE: to pass true into XPATH you have to use bookstore/book/author[true()]
author[true] just means that you want to get all author element which has true subelement.

You can check it yourself, try expressions

bookstore/book[author1] vs bookstore/book[author]

First one returns you nothing, because there's no book element with author1 subelements. Second one returns you all elements. But if you remove author subnodes from some of books nodes, you'll get only those having author subnode.

So if you take xml like this

<bookstore>
    <book category="COOKING">
        <title lang="en">Everyday Italian</title>
        <author>Giada De Laurentiis</author>
        <year>2005</year>
        <price>30.00</price>
    </book>
    <book category="CHILDREN">
        <title lang="en">Harry Potter</title>
    </book>
</bookstore>

then bookstore/book[author] returns

<book category="COOKING">
  <title lang="en">Everyday Italian</title>
  <author>Giada De Laurentiis</author>
  <year>2005</year>
  <price>30.00</price>
</book>

and bookstore/book[title] returns

<book category="COOKING">
  <title lang="en">Everyday Italian</title>
  <author>Giada De Laurentiis</author>
  <year>2005</year>
  <price>30.00</price>
</book>
<book category="CHILDREN">
  <title lang="en">Harry Potter</title>
</book>

edited Nov 2, 2012 at 15:10

answered Nov 2, 2012 at 7:18

roman

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2 Comments

TimeToCodeTheRoad Over a year ago

Roman Perker: yes, I know. But why does bookstore/book/author[true] not work, since if is similar to the situation when the contains() always returns true

BeniBela Over a year ago

Although it seems that in some implementations (e.g. Zorba) true is actually the same as true()

Dimitre Novatchev · Accepted Answer · 2012-11-02 12:45:59Z

I expected the following query to return all authors:
bookstore/book/author[true]
however, it returns an empty set. Can somebody explain?

The above expression selects all bookstore/book/author elements that have a child element named true. In the provided XML document no author element has a child named true -- therefore the XPath expression selects nothing.

In a comment the OP asks:

But why does bookstore/book/author[true] not work, since if is similar to the situation when the contains() always returns true

contains() never returns (the string) "true" -- it returns the boolean value true() -- this is different from the string "true".

Explanation:

You are confused by the fact that the serialization of the boolean value true() to sting is the string "true".

This fact doesnt mean that the string "true" and the boolean value true() are identical.

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