Can I create a value for a missing tag in XPath?

Question

I have an application which extracts data from an XML file using XPath. If a node in that XML source file is missing I want to return the value "N/A" (much like the Oracle NVL function). The trick is that the application doesn't support XSLT; I'd like to do this using XPath and XPath alone.

Is that possible?

Answer 1

It can be done but only if the return value when the node does exist is the string value of the node, not the node itself. The XPath

substring(concat("N/A", /foo/baz), 4 * number(boolean(/foo/baz)))

will return the string value of the baz element if it exists, otherwise the string "N/A".

To generalize the approach:

substring(concat($null-value, $node),
          (string-length($null-value) + 1) * number(boolean($node)))

where $null-value is the null value string and $node the expression to select the node. Note that if $node evaluates to a node-set that contains more than one node, the string value of the first node is used.

Answer 2

Short answer: no. Such a function was considered and explicitly rejected for version 2 of the XPath spec (see the non-normative Illustrative User-written Functions section).