How to specify an ordered list in XSD

Question

I have a list of numbers that need to be ordered. Currently they are in the XML as such:

0.2
0.4
&l         


        

Answer 1

Elements in XML are inherently ordered per their order in the document. This order is significant (unlike that of attributes) and will be preserved by parsers.

XML documents must have a single root element, so let's wrap your example elements in a single containing element:

<values>
  <value_1>0.2</value_1>
  <value_2>0.4</value_2>
  <value_3>0.6</value_3>
  <!-- ... -->
  <value_N>1.8</value_N>
</values>

An XSD can be written for this XML:

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:element name="values">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="value_1" type="xs:decimal"/>
        <xs:element name="value_2" type="xs:decimal"/>
        <xs:element name="value_3" type="xs:decimal"/>
        <!-- ... -->
        <xs:element name="value_N" type="xs:decimal"/>
      </xs:sequence>
    </xs:complexType>
  </xs:element>
</xs:schema>

But an improvement is immediately obvious: Eliminate the indexing number from the component name:

<values>
  <value>0.2</value>
  <value>0.4</value>
  <value>0.6</value>
  <!-- ... -->
  <value>1.8</value>
</values>

The XSD for this improved XML is similarly streamlined (and also can actually handle an indefinite number of value elements as well):

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <xs:element name="values">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="value" type="xs:decimal" maxOccurs="unbounded"/>
      </xs:sequence>
    </xs:complexType>
  </xs:element>
</xs:schema>

Now one might ask whether the XSD can enforce a sorted order on the elements such that

<values>
  <value>0.2</value>
  <value>0.4</value>
  <value>0.6</value>
</values>

would be valid, but

<values>
  <value>0.6</value>
  <value>0.2</value>
  <value>0.4</value>
</values>

would be invalid.

This is not possible to do in XSD 1.0, however XSD 1.1 can express such a constraint via assertions.

Asserting sorted ordering in XSD 1.1

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
  xmlns:vc="http://www.w3.org/2007/XMLSchema-versioning"
  vc:minVersion="1.1">
  <xs:element name="values">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="value" type="xs:decimal" maxOccurs="unbounded"/>
      </xs:sequence>
      <xs:assert test="every $v in value 
                       satisfies not(number($v) lt 
                                     number($v/preceding-sibling::value[1]))"/>
    </xs:complexType>
  </xs:element>
</xs:schema>

Credit: The idea for the assertion test is based on one from Michael Kay.