The <xsl:variable> element can be used in three ways. The simplest form of the element creates a new variable whose value is an empty string (""). Here's how it looks:

<xsl:variable name="x"/>

This element creates a new variable named x, whose value is an empty string. (Please hold your applause until the end of the section.)

You can also create a variable by adding a select attribute to the <xsl:variable> element:

<xsl:variable name="favouriteColour" select="'blue'"/>

In this case, we've set the value of the variable to be the string "blue". Notice that we put single quotes around the value. These quotes ensure that the literal value blue is used as the value of the variable. If we had left out the single quotes, this would mean the value of the variable is that of all the <blue> elements in the current context, which definitely isn't what we want here.

WARNING: Some XSLT processors don't require you to put single quotes around a literal value if the literal value begins with a number. This is because the XML specification states that XML element names can't begin with a number. If I say the value should be 35, Xalan, XT, and Saxon all assume that I mean 35 as a literal value, not as an element name. Although this works with many XSLT processors, you're safer to put the single quotes around the numeric values anyway. A further aside: the value here is the string "35", although it can be converted to a number easily.

The third way to use the <xsl:variable> element is to put content inside it. Here's a brief example:

<xsl:variable name="y">
  <xsl:choose>
    <xsl:when test="$x &gt; 7">
      <xsl:text>13</xsl:text>
    </xsl:when>
    <xsl:otherwise>
      <xsl:text>15</xsl:text>
    </xsl:otherwise>
  </xsl:choose>
</xsl:variable>

In this more complicated example, the content of the variable y depends on the test attribute of the <xsl:when> element. This is the equivalent of this procedural programming construct:

int y;
if (x > 7)
  y = 13;
else
  y = 15;