Finding unique nodes with xslt

With JavaScript 1.6 / ECMAScript 5 you can use the native filter method of an Array in the following way to get an array with unique values:

function onlyUnique(value, index, self) {
  return self.indexOf(value) === index;
}

// usage example:
var a = ['a', 1, 'a', 2, '1'];
var unique = a.filter(onlyUnique);

console.log(unique); // ['a', 1, 2, '1']

The native method filter will loop through the array and leave only those entries that pass the given callback function onlyUnique.

onlyUnique checks, if the given value is the first occurring. If not, it must be a duplicate and will not be copied.

This solution works without any extra library like jQuery or prototype.js.

It works for arrays with mixed value types too.

For old Browsers (<ie9), that do not support the native methods filter and indexOf you can find work arounds in the MDN documentation for filter and indexOf.

If you want to keep the last occurrence of a value, simple replace indexOf by lastIndexOf.

With ES6 it could be shorten to this:

// usage example:
var myArray = ['a', 1, 'a', 2, '1'];
var unique = myArray.filter((v, i, a) => a.indexOf(v) === i);

console.log(unique); // unique is ['a', 1, 2, '1']

Thanks to Camilo Martin for hint in comment.

ES6 has a native object Set to store unique values. To get an array with unique values you could now do this:

var myArray = ['a', 1, 'a', 2, '1'];

let unique = [...new Set(myArray)];

console.log(unique); // unique is ['a', 1, 2, '1']

The constructor of Set takes an iterable object, like Array, and the spread operator ... transform the set back into an Array. Thanks to Lukas Liese for hint in comment.

multi array

var a = [[2,4],[2,3],[1,3],[1,3],[1,8]]

var unique = (value, index, self) =>{
  var findIndex = (element) => element[0] == value[0];
  return self.findIndex(findIndex) === index;
}

console.log(a.filter(unique))

Here is a stylesheet that incorporates Nicholas suggestion with the union, then taking the last() for the previous item and the first for the next item:

<xsl:stylesheet
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
  version="2.0">

  <xsl:output method="html" indent="yes"/>

  <xsl:template match="manual">
    <table>
      <thead>
        <tr>
          <th>Selected section</th>
          <th>Previous section</th>
          <th>Next section</th>
        </tr>
      </thead>
      <tbody>
        <xsl:apply-templates select="descendant::section"/>
      </tbody>
    </table>
  </xsl:template>

  <xsl:template match="section">
    <tr>
      <td>
        <xsl:value-of select="@id"/>
      </td>
      <xsl:apply-templates select="." mode="prev"/>
      <xsl:apply-templates select="." mode="next"/>
    </tr>
  </xsl:template>

  <xsl:template match="section" mode="prev">
    <td>
      <xsl:value-of select="(preceding::section | ancestor::section)[last()]/@id"/>
    </td>
  </xsl:template>

  <xsl:template match="section" mode="next">
    <td>
      <xsl:value-of select="(following::section | descendant::section)[1]/@id"/>
    </td>
  </xsl:template>

</xsl:stylesheet>

With your sample input Saxon 6.5.5 outputs

<table>
   <thead>
      <tr>
         <th>Selected section</th>
         <th>Previous section</th>
         <th>Next section</th>
      </tr>
   </thead>
   <tbody>
      <tr>
         <td>1</td>
         <td></td>
         <td>2</td>
      </tr>
      <tr>
         <td>2</td>
         <td>1</td>
         <td>3</td>
      </tr>
      <tr>
         <td>3</td>
         <td>1</td>
         <td>4</td>
      </tr>
      <tr>
         <td>4</td>
         <td>1</td>
         <td>5</td>
      </tr>
      <tr>
         <td>5</td>
         <td>1</td>
         <td>6</td>
      </tr>
      <tr>
         <td>6</td>
         <td>1</td>
         <td>7</td>
      </tr>
      <tr>
         <td>7</td>
         <td>1</td>
         <td>8</td>
      </tr>
      <tr>
         <td>8</td>
         <td>1</td>
         <td>9</td>
      </tr>
      <tr>
         <td>9</td>
         <td>1</td>
         <td>10</td>
      </tr>
      <tr>
         <td>10</td>
         <td>1</td>
         <td>11</td>
      </tr>
      <tr>
         <td>11</td>
         <td>1</td>
         <td>12</td>
      </tr>
      <tr>
         <td>12</td>
         <td>1</td>
         <td></td>
      </tr>
   </tbody>
</table>