Question

A variable $rows has a list of node ids that key into some source XML.

<row>
   <node id="d0113" />
   <node id="d0237" />
   <node id="d0321" />
</row>
<row>
   <node id="c0278" />
   <node id="d0137" />
   <node id="e0021" />
</row>

What is a good way to test, before processing each <row>, whether any of nodes keyed to actually exist in a node-set $set?

All I’ve come up with is

<xsl:for-each select="row">
  <xsl:variable name="test">  
    <xsl:for-each select="node">
      <xsl:value-of select="boolean($set//*[generate-id()=current()/@id]) * 1"/>
    </xsl:for-each>                  
  </xsl:variable>
  <xsl:if test="$test&gt;0">
    <!-- go ahead and process the row -->
  </xsl:if>
</xsl:for-each>

Answer 1

  <xsl:value-of select="boolean($set//*[generate-id()=current()/@id]) * 1"/>

This will in most cases return false() so this whole method of “indexing” is incorrect.

This is because the values of generate-id() for any node in an XML document aren’t guaranteed to be the same from transformation to transformation.

Even if in the provided XML document any node/@id attribute was generated to have the value of generate-id() on some particular node of the second document, nothing guarantees that in a consequent new transformation generate-id() on that same node will produce the same valu as the one that was used to generate the respective node/@id.

Recommendation:

For such indexing use a more stable function of a node — one example of such function is the XPath expression that selects exactly that node.

If the document hasn’ been modified, that XPath expression is going always to select that node.

Update:

In a comment the OP maintains that the node/@id attributes are both generated and used in the same transformation.

In this case, this single XPath expression produces a boolen that indicates whether or not $set contains at least one node whose generate-id() is one of the node/@id attributes of another document:

boolean($set[generate-id() = someExpression/row/node/@id])

In a test attribute of a conditional instruction, just the argument to boolean() above can be used.

Explanation:

The expression:

$set[generate-id() = someExpression/row/node/@id])]

Selects all nodes in $set whose value of generate-id() is equal to at least one someExpression/row/node/@id attribute’s value (someExpression here stands for the missing location steps about which we know nothing, since no XML document has been provided).

By definition, boolean($someNodeSet) is always true() if the node-set $someNodeSet contains at least one node, and is false if $someNodeSet is the empty node-set.