How to perform set operations in XPath 1.0

Question

I've seen on SO and other places that the following is supposed to work (this example is lifted directly from O'Reilly's XSLT Cookbook):

(: intersection :)
$set1[count(. | $set2) = count($set2)]

(: difference :)
$set1[count(. | $set2) != count($set2)]

and it looks like it should be OK, however this seems to fail when used with actual paths rather than variables. For example, given the following document

<a>
  <new>
    <val>1</val>
    <val>2</val>
  </new>
  <old>
    <val>2</val>
    <val>3</val>
  </old>
</a>

and the XPath expression /a/new/val[count(. | /a/old/val)=count(/a/old/val)]/text() I would expect to get the node-set { 2 } but instead am getting { 1 2 }. Any ideas what I'm doing wrong?

Answer 1

The formulas for node-set intersection use node-identity, not value identity.

Two nodes are identical if and only if count($n1|$n2) =1

However, you want to intersect based on value identity.

Solution:

Use:

/a/new/val[. = /a/old/val]

this selects any /a/new/val for which there exists at least one /a/old/val element such that the string values of these two elements is the same.