computer science

XPath 3

XPath 3 is a computer science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand XPath 3 rather than just read about it. In short: XPath 3 is the latest version of the XML Path Language, a query language for selecting nodes in XML documents. It supersedes XPath 1.0 and XPath 2.0.

Key takeaways

XPath 3 belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect XPath 3 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of XPath 3 from memory before moving on to harder problems.

Reference excerpt

XPath 3 is the latest version of the XML Path Language, a query language for selecting nodes in XML documents. It supersedes XPath 1.0 and XPath 2.0. XPath 3.0 became a W3C Recommendation on 8 April 2014, while XPath 3.1 became a W3C Recommendation on 21 March 2017.

New features in XPath 3.0 Compared to XPath 2.0, XPath 3.0 adds the following new features:

Inline function expressions Anonymous functions can be created in an expression context. For example, the expression function($a as xs:double, $b as xs:double) as xs:double { $a * $b } creates a function that returns the product of its two arguments. The expression collection()/(let $a := . return function() { $a }) creates a sequence of functions, each one returning a different node from a collection. Dynamic function calls Function values may be called without being referenced by name. For example, $f[2]("Hi there") fetches the second item from sequence $f, and invokes it as a function, passing the string "Hi there" as argument. Union types Union types, as defined in XML Schema, may be used in type conversions and function type signatures. Namespace literals XML namespaces may be referenced using braced URI literals. For example, the qualified name math:pi may be expanded to Q{http://www.w3.org/2005/xpath-functions/math}pi, embedding the namespace URI inside the prefix. String concatenation operator The new || operator may be used for string concatenation: $a || $b is equivalent to fn:concat($a, $b). Mapping operator The new ! operator performs simple mapping: E1 ! E2 evaluates E2 for each item in the sequence E1, and concatenates the resulting items. This is comparable to the path operator /, but the ! operator does not perform duplicate elimination nor document ordering of the results.

New features in XPath 3.1 XPath 3.1 mainly adds support for array and map (associative array) data types. These types and their associated functionality are intended to ease working with JSON data. Another innovation is the arrow operator => for function chaining. For example, the XPath 2.0 expression

can now be written

References "XML Path Language (XPath) 3.0". World Wide Web Consortium. 8 April 2014. "XML Path Language (XPath) 3.1". World Wide Web Consortium. 21 March 2017.

Worked examples

Example 1 — a first encounter with XPath 3

Start with the simplest possible case. Write down what XPath 3 claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to XPath 3 before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about XPath 3 ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of XPath 3

In research: XPath 3 appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses XPath 3 in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: XPath 3 is common in secondary-school and first-year university syllabi. It links to neighbouring topics Computing stubs, XML data access, so understanding it makes those chapters shorter.
In everyday life: Look for XPath 3 outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study XPath 3 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what XPath 3 means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain XPath 3 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is XPath 3 in simple terms?

XPath 3 is the latest version of the XML Path Language, a query language for selecting nodes in XML documents. It supersedes XPath 1.0 and XPath 2.0.

Why does XPath 3 matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study XPath 3?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on XPath 3.