The XPath id() function
function, xml, xpath
Solution
It appears that based on the spec that you would only use the `id()` function if you have defined unique IDs in your DTD.
The id function selects elements by their unique ID.
...
An element node may have a unique identifier (ID). This is the value of the attribute that is declared in the DTD as type ID.
I also came across this SO question that discusses the ID type in a DTD and how to define that.
Problem
I can't find any good explanation of the `id()` function in XPath. What does it do? How do you use it? From the available descriptions, it sounds like it will give you `id` of a node. So I played with it like this, but got an error: ``` //bookstore/id(book) ``` Then got a little fancier and tried ``` //bookstore/book[name = id(book/@category)] ``` which didn't return an error, but got no hits. Can someone explain this function for me, and future Stack fans and Googlers? Here's the XML I'm playing with: ``` <?xml version="1.0" encoding="ISO-8859-1"?> <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> <author>J. K. Rowling</author> <price>29.99</price> </book> <book category="WEB"> <title lang="en">XQuery Kick Start</title> <author>James McGovern</author> <author>Per Bothner</author> <author>Kurt Cagle</author> <author>James Linn</author> <author>Vaidyanathan Nagarajan</author> <year>2003</year> <price>49.99</price> </book> <book category="WEB"> <title lang="en">Learning XML</title> <author>Erik T. Ray</author> <year>2003</year> <price>39.95</price> </book> <book category="OPENSOURCE"> <title lang="en">Open Source</title> <year>2003</year> <price>39.95</price> </book> <book category="WEB"> <title lang="en">WEB</title> <year>2012</year> <price>21.99</price> </book> </bookstore> ```