Matching by Key

Keys give you an easy way to identify elements, and you can match specific keys with the pattern "key()" . Chapter 9 discusses how to use keys in detail, but Ill include a quick example here.

You use the <xsl:key> element to create a key. This element is a top-level element, so it appears outside templates and as a child element of <xsl:stylesheet> . In the following example, I use a key to match planets whose COLOR attribute is set to BLUE, which means Earth:

 <?xml version="1.0"?>  <?xml-stylesheet type="text/xml" href="planets.xsl"?>  <PLANETS>          .          .          .      <PLANET COLOR="BLUE">          <NAME>Earth</NAME>          <MASS UNITS="(Earth = 1)">1</MASS>          <DAY UNITS="days">1</DAY>          <RADIUS UNITS="miles">2107</RADIUS>          <DENSITY UNITS="(Earth = 1)">1</DENSITY>          <DISTANCE UNITS="million miles">128.4</DISTANCE><!--At perihelion-->      </PLANET>  </PLANETS>

Now I can create a key named COLOR that matches <PLANET> elements by checking their COLOR attribute:

 <?xml version="1.0"?>  <xsl:stylesheet version="1.0"  xmlns:xsl="http://www.w3.org/1999/XSL/Transform">      <xsl:key name="COLOR" match="PLANET" use="@COLOR"/>          .          .          .

Now I can use the pattern "key()" to match <PLANET> elements with the COLOR attribute set to BLUE as follows :

 <?xml version="1.0"?>  <xsl:stylesheet version="1.0"  xmlns:xsl="http://www.w3.org/1999/XSL/Transform">      <xsl:key name="COLOR" match="PLANET" use="@COLOR"/>      <xsl:template match="/PLANETS">          <HTML>              <HEAD>                  <TITLE>                      The Planets Table                  </TITLE>              </HEAD>              <BODY>                  <H1>                      The Planets Table                  </H1>                  <TABLE BORDER="2">                      <TR>                          <TD>Name</TD>                          <TD>Mass</TD>                          <TD>Radius</TD>                          <TD>Day</TD>                      </TR>                      <xsl:apply-templates select="key('COLOR', 'BLUE')"/>                  </TABLE>              </BODY>          </HTML>      </xsl:template>          .          .          .

And heres the resultas you can see, Earth was the only planet that matched the pattern I used:

 <HTML>      <HEAD>          <TITLE>              The Planets Table          </TITLE>      </HEAD>      <BODY>          <H1>              The Planets Table          </H1>          <TABLE BORDER="2">              <TR>                  <TD>Name</TD>                  <TD>Mass</TD>                  <TD>Radius</TD>                  <TD>Day</TD>              </TR>              <TR>                  <TD>Earth</TD>                  <TD>1 (Earth = 1)</TD>                  <TD>2107 miles</TD>                  <TD>1 days</TD>              </TR>          </TABLE>      </BODY>  </HTML>