Produce context data for first and last occurrences of every value of an element

Question

Given the following xml:

<container>
    <val>2</val>
    <id>1</id>
</container>
<container>
    <val>2</val>
    <id>2</id>
</container>
<container>
    <val>2</val>
    <id>3</id>
</container>
<container>
    <val>4</val>
    <id>1</id>
</container>
<container>
    <val>4</val>
    <id>2</id>
</container>
<container>
    <val>4</val>
    <id>3</id>
</container>

I'd like to return something like

2 - 1
2 - 3
4 - 1
4 - 3

Using a nodeset I've been able to get the last occurrence via:

exsl:node-set($list)/container[not(val = following::val)]

but I can't figure out how to get the first one.

Answer 1

Basic XPath:

//container[position() = 1]  <- this is the first one
//container[position() = last()]  <- this is the last one

Here's a set of XPath functions in more detail.

Answer 2

I. XSLT 1.0

Basically the same solution as the one by Tomalak, but more understandable Also it is complete, so you only need to copy and paste the XML document and the transformation and then just press the "Transform" button of your favourite XSLT IDE:

<xsl:stylesheet version="1.0"
 xmlns:xsl="http://www.w3.org/1999/XSL/Transform"&gt;
 <xsl:output method="text"/>

    <xsl:key name="kContByVal" match="container"
     use="val"/>

    <xsl:template match="/*">
      <xsl:for-each select=
       "container[generate-id()
                 =
                  generate-id(key('kContByVal',val)[1])
                 ]
       ">

       <xsl:variable name="vthisvalGroup"
        select="key('kContByVal', val)"/>

       <xsl:value-of select=
        "concat($vthisvalGroup[1]/val,
              '-',
              $vthisvalGroup[1]/id,
              '&#xA;'
              )
      "/>
       <xsl:value-of select=
        "concat($vthisvalGroup[last()]/val,
              '-',
              $vthisvalGroup[last()]/id,
              '&#xA;'
              )
        "/>
      </xsl:for-each>
    </xsl:template>
</xsl:stylesheet>

when this transformation is applied on the originally-provided XML document (edited to be well-formed):

<t>
    <container>
     <val>2</val>
     <id>1</id>
    </container>
    <container>
     <val>2</val>
     <id>2</id>
    </container>
    <container>
     <val>2</val>
     <id>3</id>
    </container>
    <container>
     <val>4</val>
     <id>1</id>
    </container>
    <container>
     <val>4</val>
     <id>2</id>
    </container>
    <container>
     <val>4</val>
     <id>3</id>
    </container>
</t>

the wanted result is produced:

2-1
2-3
4-1
4-3

Do note:

1. We use the Muenchian method for grouping to find one container element for each set of such elements that have the same value for val.
2. From the whole node-list of container elements with the same val value, we output the required data for the first container element in the group and for the last container element in the group.

II. XSLT 2.0

This transformation:

<xsl:stylesheet version="2.0"
    xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
    xmlns:xs="http://www.w3.org/2001/XMLSchema"&gt;
    <xsl:output method="text"/>

    <xsl:template match="/*">
      <xsl:for-each-group select="container"
           group-by="val">
        <xsl:for-each select="current-group()[1], current-group()[last()]">
          <xsl:value-of select=
           "concat(val, '-', id, '&#xA;')"/>
        </xsl:for-each>
    </xsl:for-each-group>
    </xsl:template>
</xsl:stylesheet>

when applied on the same XML document as above, prodices the wanted result:

2-1
2-3
4-1
4-3

Do note:

1. The use of the <xsl:for-each-group> XSLT instruction.

2. The use of the current-group() function.