xsl: xml to csv conversion - search list for value, otherwise output empty field

I have an xml structure like:

   <foos>
      <foo>
        <foo_type>1</foo_type>
        <foo_date>2011_1_1</foo_date>
      </foo>
      <foo>
        <foo_type>1</foo_type>
开发者_C百科        <foo_date>2012_1_1</foo_date>
      </foo>
      <foo>
        <foo_type>2</foo_type>
        <foo_date>2011_1_1</foo_date>
      </foo>
    </foos>

What I need to do is produce one value from this, where foo_type==1 and the date is latest date. Previously I have used a sort descending for a list of dates. If there is no foo of foo_type==1 it should produce an empty field. (Fields are surrounded with quotes, comma is the delimiter.)

I was envisioning doing this by building a list of all foos of type 1. I'm not sure how to do that in xsl. something like this perhaps:

        <xsl:variable name="fooList">
            <xsl:for-each select="foos/foo">
                <xsl:if test="contains(foo_type, '1')">
                    <xsl:value-of select="concat($fooList, ', ', foo_date)"/>
                </xsl:if>
            </xsl:for-each>
        </xsl:variable>

then i would parse the comma delimited list and sort to determine the latest date:

       <xsl:variable name="latestFooDate">
            <xsl:sort select="split(fooList)" order="descending" />
       </xsl:variable>

and finally call the template to format the result:

        <xsl:call-template name="dateFormat">
            <xsl:with-param name="date" select="latestFooDate" />
        </xsl:call-template>

Keep in mind this is pseudo code because I dont know how to do this. Now if someone can help turn my pseudo into the real thing.

I don't think you need to create a comma-delimited list before getting the first date. This seems like a bit of extra effort. You could sort your foo elements in your xsl:for-each statement, and then just get the first one (i.e. the one with position() = 1)

Try this template for matching a foos element

   <xsl:template match="foos">
      <xsl:for-each select="foo[foo_type='1']"> 
         <xsl:sort select="foo_date" />
         <xsl:if test="position() = 1">
            <xsl:value-of select="foo_date" />
         </xsl:if>
      </xsl:for-each> 
   </xsl:template>

This loops over all foo elements with a foo_type of 1, but then only performs any processing on the first one.

When you use this template to match your given input XML, the output should then be

2011_1_1

You can then go ahead and perform extra date formatting on this as required.