How do I get only those elements (<B val="3"/> and <B val ="1"/> in this case) for which the value of val is contained in the req attribute in condition element.

<?xml version="1.0"?>
<A>
<condition req="3|1"/>
<B val="1"></B>
<B val="2"></B>
<B val="3"></B>
...
...
</A>

Thanks!

/A/B[contains(../condition/@req, @val)]

Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference

But,
for <condition req="13"/> ,if
<B val="1"></B>
<B val="2"></B>
<B val="3"></B>
..
..
<B val="13"></B>,
how do I prevent <B val="1"></B> and <B val="3"></B> from appearing in my output.

I thought you might follow up with that. If the list is always separated with vertical bar and there are no spaces anywhere you could try

 /A/B[contains(
        concat('|', ../condition/@req, '|'),
        concat('|', @val, '|')]

In XSLT 2.0 you can do

/A/B[@val = tokenize(../condition/@req, '|')]



Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference

The concat fn did work.
How do I determine the output sequence (depending on the sequence of numbers appearing in req attribute )such that for the following xml
<A>
<condition req="3|1|2"/>
<B val="1"></B>
<B val="2"></B>
<B val="3"></B>
...
...
</A>
the output order should be
<B val="3"></B> ,<B val="1"></B> and <B val="2"></B>

Thanks in advance!

For that I think you really do need tokenize:

In XSLT 2.0:

<xsl:copy-of select="for $t in tokenize(condition/@req, '|')
                     return B[@val=$t]"/>

You can find a version of tokenize that works with XSLT 1.0 at www.exslt.org, but of course you'll have to expand the above code using xsl:for-each and variables.

Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference

I am using xslt 1.0
Could you help me to figure out a way of doing it on 1.0

Thanks in advance!

I find doing these things in 1.0 no fun at all, so I'll leave this to someone else.

Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference