XSLT, 2nd Edition

Name

[2.0] zero-or-one()

Raises an error unless its argument is a sequence containing zero or one items. For example, if a <name> element can have at most one <title> child, zero-or-one(title) would raise an error if a <name> element had the wrong number of <title> elements. Be aware that zero-or-one() terminates processing; for a more flexible approach, use the count() function to determine the cardinality of a sequence.

Syntax

item()? zero-or-one(item()*)

Inputs

A sequence of items. The sequence must either be empty or a singleton.

Outputs

This function returns the input sequence, assuming it has zero or one items. If the input sequence has more than one item, the zero-or-one() function raises an error.

Defined in

XQuery 1.0 and XPath 2.0 Functions and Operators section 15.2, “Functions That Test the Cardinality of Sequences.” More details about this function can be found in XQuery 1.0 and XPath 2.0 Formal Semantics section 7.2, “Standard Functions with Specific Static Typing Rules.”

Example

We’ll look at a trivial example that illustrates the zero-or-one() function. Invoking zero-or-one() with a sequence containing more than one item raises an error. Here’s the stylesheet:

<?xml version="1.0"?>
<!-- zero-or-one.xsl --> <xsl:stylesheet version="2.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:xs="http://www.w3.org/2001/XMLSchema"> <xsl:output method="text"/> <xsl:template match="/"> <xsl:variable name="emptySequence" as="item()*"> <xsl:sequence select="()"/> </xsl:variable> <xsl:variable ...