XPath and XPointer

Chapter 4. XPath Functions and Numeric Operators

The XPath 1.0 Recommendation specifies a number of functions and numeric operations that can be used to refine the results returned by an XPath expression.

Before getting into the details of these features’ uses, let’s take a look at a fundamental question: what are functions in the first place? (If you’re already familiar with the use of functions in programming languages, such as Java, C++, and Visual Basic, feel free to skip this section.)

Introduction to Functions

When I was a kid, I loved watching my father work on cars. He’d been a mechanic all his life, and the automotive toolkit he’d acquired over the course of the years was exotic (to my eyes, anyhow).

One of the smaller items in Dad’s toolkit was something he called a “spark-plug gapper.” It was something like a Swiss-Army knife, with a half-dozen or so stiff steel prongs that you could swivel out from the tool’s main body. Each L-shaped prong was of a slightly different thickness; depending on the model of car you were working on and the specific spark plug’s specifications, you’d tap the end of the spark plug on the pavement and, using the gapper, ensure that the distance across which the spark was to jump was just right. There was also a small, stiff plane of sheet metal attached to the gapper, which you could use to spread the gap if you’d already closed it up too much. The objective was the get the gap just right, to ensure that the spark plug fired in just exactly the ...