XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (264 page)

This is true because nothing can change between the variables being defined and being used. The source document can't change, and the values of the variables
$this
and
$matches
can't change. The context (for example the current position in the source document) can change, but in this example (a) it doesn't, and (b) the expressions don't depend on the context anyway.

I call these
convenience variables
because you could get by without them if you had to (though there might be a performance hit). They can be used either as global variables or as local variables. Creating global convenience variables that refer to sets of nodes in the source document is often a useful programming technique; for example:

These act rather like views in an SQL database.

Variables to Capture Context-Sensitive Values

These variables are most often useful in conjunction with

, which changes the context item. Consider the following example.

Example: Using a Variable for Context-Sensitive Values

This example shows how a variable can be used to hold on to information that depends on the context, for use when the context has changed.

Source

The source file is
opera.xml
. It contains a list of operas and details of their composers.

   

      

      Mozart

      1791

   

   

      

      Mozart

      1787

   

      

      Verdi

      1843

   

   

      

      Verdi

      1850

   

   

      

      Puccini

      1897

   

   

      Wolfgang Amadeus Mozart

      1756

      1791

   

   

      Guiseppe Verdi

      1813

      1901

   

   

      Giacomo Puccini

      1858

      1924

   

Stylesheet

The stylesheet is the file
opera.xsl
. This is a complete stylesheet: It uses the simplified stylesheet syntax described on page 125, in Chapter 3.

The stylesheet contains two nested

loops. In the outer loop, it sets a variable
c
to the context node (the current composer). In the expression controlling the inner loop, this variable is used. It would not be correct to use
.
in place of
$c
, because the

element is no longer the context node. In this example it would be possible to use the
current()
function here (this function is described on page 734, in Chapter 13), but there are other cases where a variable is necessary.

   xmlns:xsl=“http://www.w3.org/1999/XSL/Transform”

   xsl:version=“2.0”>

   Programme

   

       

            select=“concat(fullname, ‘ (‘, born, ‘-’, died, ‘)’)”/>

       

       

           

       

   

Output

See
Figure 6-18
.

Figure 6-18

One case where context variables are very useful is when handling multiple source documents.

In any stylesheet that handles multiple source documents, it is useful to include a global variable that refers to the document node of the principal source document, thus:

This means it is always possible to refer to the source document by using this variable. Without this, when the context node is in a secondary document, there is no way of accessing data from the principal document.

For example, the expression
//item
refers to all

elements in the same document as the context node. If you actually want all

elements in the principal source document, then (provided you have included the global variable declaration above) you can use the expression
$root//item
.