XSLT 2 - find first missing element in source list

Question

I have problem with XSLT and/or XPATH. Let's say I have XML Input:

<context>
    <pdpid-set>
      <list>
        <item>1</item>
        <item>2</item>
        <item>4</item>
        <item>6</item>
        <item>7</item>
        <item>8</item>
      </list>
    </pdpid-set>
</context>

Task is: find FIRST missing element in array pdpid-set/list. In example above answer is 3.

I tried to use <xsl:for-each to find missing element but there is no possibility to break such loop so my XSL produce more than one element in output:

<xsl:variable name="list" select="context/pdpid-set/list"/>
<xsl:variable name="length" select="count(context/pdpid-set/list/item)"/>

<xsl:for-each select="1 to ($length)">
  <xsl:variable name="position" select="position()"/>
    <xsl:if test="$list/item[$position] > $position">
      <missing-value>
        <xsl:value-of select="$position"/>
      </missing-value>
    </xsl:if>
</xsl:for-each>

in code above output will be:

<missing-value>3</missing-value><missing-value>4</missing-value><missing-value>5</missing-value>...

I don't want to have more than one missing-value. Any suggestion?

Answer 1

Task is: find FIRST missing element in array pdpid-set/list. In example above answer is 3

Here is a correct XPath 1.0 expression that when evaluates to the wanted result (3):

/*/*/*/item[not(. +1 = following-sibling::*[1])][1] + 1

The XPath expression in the currently selected answer, on the other side, selects this element:

<item>4</item>

And the complete correct XSLT 1.0 transformation is:

<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
 <xsl:output omit-xml-declaration="yes" indent="yes"/>

  <xsl:template match="/">
    <missing-value>
      <xsl:copy-of select="/*/*/*/item[not(. +1 = following-sibling::*[1])][1] + 1"/>
    </missing-value>
  </xsl:template>
</xsl:stylesheet>

When applied on the provided XML document, the wanted, correct result is produced:

<missing-value>3</missing-value>

Finally, if the task is to find all missing elements:

<xsl:stylesheet version="2.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
 xmlns:xs="http://www.w3.org/2001/XMLSchema" exclude-result-prefixes="xs">
 <xsl:output omit-xml-declaration="yes" indent="yes"/>
 <xsl:strip-space elements="*"/>

  <xsl:template match=
  "item[following-sibling::* and not(number(.) +1 = following-sibling::*[1]/number())]">
     <xsl:for-each select="xs:integer(.) + 1 to following-sibling::*[1]/xs:integer(.) -1">
       <missing-value><xsl:copy-of select="."/></missing-value>
     </xsl:for-each>
  </xsl:template>
  <xsl:template match="text()"/>
</xsl:stylesheet>

when this XSLT 2.0 transformation is applied on the following XML document (missing 3, 5, and 6):

<context>
    <pdpid-set>
      <list>
        <item>1</item>
        <item>2</item>
        <item>4</item>
        <item>7</item>
        <item>8</item>
      </list>
    </pdpid-set>
</context>

the wanted, correct result is produced:

<missing-value>3</missing-value>
<missing-value>5</missing-value>
<missing-value>6</missing-value>