How do you identify duplicate elements in an XPath 2.0 sequence ?

Question

I have an XPath expression which provides me a sequence of values like the one below:

1 2 2 3 4 5 5 6 7

It is easy to convert this to a set of unique values "1 2 3 4 5 6 7" using the distinct-values function. However, what I want to extract is the list of duplicate values = "2 5". I can't think of an easy way to do this. Can anyone help?

Answer 1

Calculate the difference between your original set and the set of distinct values. This is the set of numbers that occur more than once. Note that numbers in this result set are not necessarily distinct if they occur more than twice in the original sequence so convert again to a set of distinct values if this is required.

Answer 2

Yes, but the problem is how do I calculate the difference between two sequences ? You can compare sequences using the union / intersect / except keywords, but none of these will provide the 'difference' between the 2 sets of values.

Answer 3

What about:

distinct-values(
  for $item in $seq
  return if (count($seq[. eq $item]) > 1)
         then $item
         else ())

This iterates through the items in the sequence, and returns the item if the number of items in the sequence that are equal to that item is greater than one. You then have to use distinct-values() to remove the duplicates from that list.

Answer 4

What about xsl? Is it applicable to your request?

 <xsl:for-each select="/r/a">
  <xsl:variable name="cur" select="." />
  <xsl:if test="count(./preceding-sibling::a[. = $cur]) > 0 and count(./following-sibling::a[. = $cur]) = 0">
   <xsl:value-of select="." />
  </xsl:if>
 </xsl:for-each>

Answer 5

Thanks to all who answered, JeniT gave just the kind of solution I was looking for - thanks !

Answer 6

Given the following xml:

<a>
    <b>1</b>
    <b>2</b>
    <b>2</b>
    <b>3</b>
    <b>4</b>
    <b>5</b>
    <b>5</b>
    <b>5</b>
    <b>6</b>
    <b>7</b>
</a>

The following XPath will give you a list of repeating values (in this case 2, 5, 5)

/a/b[.=following-sibling::b]

However if you wanted a distinct list of repeating values (in this case 2, 5) then the following XPath should do the business for you:

/a/b[.=following-sibling::b][not(.=preceding-sibling::b)]

Answer 7

Use this simple XPath 2.0 expression:

      $vSeq[index-of($vSeq,.)[2]]

where $vSeq is the sequence of values in which we want to find the duplicates.

For explanation of how this "works", see:

      http://dnovatchev.spaces.live.com/Blog/cns!44B0A32C2CCF7488!904.entry

Cheers,

Dimitre Novatchev