Attributes

JoshC · February 24th, 2010, 06:01 PM

Hi Everyone,

How do you only output an attribute if it exists? For example, if you have a table and a row element that can contain 0-3 attributes:

<table>
<row backgroundcolor="#ccc">
<data>row one</data>
</row>
<row backgroundcolor="00f" text-align="center">
<data>row one</data>
</row>
<row backgroundcolor="00f" text-align="center" text-weight="bold">
<data>row one</data>
</row>
</table>

I know how to output the attributes, something like the following block (outputting an html table row element):

<xsl:for-each select="row">
<tr bgcolor="{@background-color}" align="{@text-align}" style="font-weight:{@text-weight}">

The problem is that this will output empty attributes if the attribute doesn't exist in the source xml, and the processor I'm using will throw warnings if there are empty attributes. I've used <choose> blocks to test if an attribute exists and then outputting it if it does exist, but this can become really messy with nested <choose> blocks if there are many attributes.

So my question is, how would experienced xslt users approach this problem? How would you only output an attribute if it exists? Is there a simple check that one can do other than nesting a bunch of <choose> blocks?

Thanks,
Josh

mhkay · February 24th, 2010, 07:26 PM

Good question.

If the attribute name and value are the same in the input and output, I would use <xsl:copy-of select="@x"/>, which outputs nothing if @x is absent.

If the name or value are different, I would probably use <xsl:apply-templates select="@*"/>, with template rules like

Code:

<xsl:template match="@background-color">
  <xsl:attribute name="bgcolor" select="."/>
</xsl:template>

(the select attribute of xsl:attribute is 2.0 only: in 1.0, use a nested xsl:value-of).

JoshC · February 25th, 2010, 12:18 PM

In my project the attribute names and values are the same in the input and output, so I could use <xsl:copy-of select="@*"/>. I'm not clear how to use the xsl:copy-of element properly, however. How do I get the values that it grabs into my <tr> element? If I use the same for-each block as in my example, where do I put the copy-of, and how do I get the values into the <tr> element? I'm thinking something like the following, but I'm still unsure how to get the values into the <tr>:

<xsl:for-each select="row">
<xsl:copy-of select="@*"/>
<tr ???>

Thanks,
Josh

samjudson · February 25th, 2010, 12:22 PM

Code:

<xsl:for-each select="row">
  <tr>
    <xsl:copy-of select="@*"/>
  </tr>
</xsl:for-each>

JoshC · February 25th, 2010, 02:16 PM

Thanks for the reply. This works great, and simplifies my code significantly. I'm not sure I understand how this is working--I guess I never expected the xsl:copy-of statement to insert the attributes in the previous <tr> element. If anyone cares to explain the logic behind this I would love to hear it.

At any rate, thanks for the help. This was exactly what I was looking for.

Thanks,
Josh

Martin Honnen · February 25th, 2010, 03:05 PM

Quote:

Originally Posted by JoshC

Thanks for the reply. This works great, and simplifies my code significantly. I'm not sure I understand how this is working--I guess I never expected the xsl:copy-of statement to insert the attributes in the previous <tr> element. If anyone cares to explain the logic behind this I would love to hear it.

The xsl:copy-of is a child of that literal result 'tr' element and that way it adds nodes to that 'tr' result element, in the case of xsl:copy-of select="@*" it adds attribute nodes to the 'tr' element.

mhkay · February 25th, 2010, 08:14 PM

For the semantics of this, see the section "Constructing Complex Content" in the XSLT 2.0 spec. Within a literal result element like <tr>...</tr>, there's a sequence of instructions (called a "sequence constructor"). These instructions are evaluated to create (usually) a sequence of nodes, typically some attribute nodes and then some elements and/or text nodes. The attributes become attributes of the new (tr) element, the elements become its children. You can also of course create the attributes using logic such as

Code:

<tr>
  <xsl:if test="condition">
     <xsl:attribute name="valign" select="'top'"/>
  </xsl:if>
  <td>...</td>
</tr>

but in this case xsl:copy-of is easier.