substring XSLT over multiple value lines for an atttribute - xslt

Question

I know the following xslt will work:
<xsl:attribute name="test">
<xsl:value-of select="substring(title, 1, 4000)"/>
</xsl:attribute>
But not sure what to do if there is something like the following and you want the substring over the whole attribute value not just the title or the substitle.
<xsl:attribute name="test">
<xsl:value-of select="title"/>
<xsl:if test="../../sub_title != ''">
<xsl:text> </xsl:text>
<xsl:value-of select="../sub_title"/>
</xsl:if>
</xsl:attribute>
Is it even possible to apply a substring function over multiple lines that define an attribute?

Answer 1

I think what you are saying is that you want to build up a long string, consisting of the values of a number of other elements, and then truncate the result.
What you could do, is use the concat function to build the attribute value, and then do a substring on that.
<xsl:attribute name="test">
<xsl:value-of select="substring(concat(title, ' ', ../sub_title), 1, 4000)" />
</xsl:attribute>
In this case, if sub_title was empty, you would end up with a space at the end of the test attribute, so you might want to add a normalize-space to this expression
<xsl:value-of select="normalize-space(substring(concat(title, ' ', ../sub_title), 1, 4000))" />
An alternate approach, if you did want to use a more complicated expression, is to do the string calculation in a variable first
<xsl:variable name="test">
<xsl:value-of select="title"/>
<xsl:if test="../../sub_title != ''">
<xsl:text> </xsl:text>
<xsl:value-of select="../sub_title"/>
</xsl:if>
</xsl:variable>
<xsl:attribute name="test">
<xsl:value-of select="substring($test, 1, 4000)" />
</xsl:attribute>
As an aside, you can simplify your code by using "Attribute Value Templates" here, instead of using the more verbose xsl:attribute command. Simply do this..
<myElement test="{substring($test, 1, 4000)}">
Here, the curly braces indicate an expression to be evaluated, rather than output literally.