My XSLT Toolbox – copy and copy-of

Using XSLT to copy elements is extremely common when you’re transforming a source document of a certain type (XML, HTML, etc.) to the same type. Often, you need an exact copy of an element verbatim, but other times you need to selectively choose certain elements to copy and others to discard. XSLT makes this process quite elegant using it’s xsl:copy-of and xsl:copy elements. The following is a setp-by-step tutorial on how these elements are used.

When you need an exact copy of an element and it’s children, you use the xsl:copy-of element, which makes an exact copy of the selected element and it’s children. Given the following XML data, which represents a (trivial) inventory of a store, let’s say you want an exact copy of any items with the name “XSLT”.

<pre lang="xml">
<?xml version="1.0" encoding="UTF-8"??><inventory><item id="1"><name>The Little Schemer</name><type>book</type><author>Friedman</author><author>Felleisen</author><list-price>29.95</list-price><sell-price>26.99</sell-price><cost>17.92</cost></item><item id="2"><name>XSLT</name><type>book</type><author>Tidwell</author><list-price>49.95</list-price><sell-price>34.99</sell-price><cost>22.92</cost></item><item id="3"><name>Romeo and Juliet</name><type>compact disc</type><conductor>Rostropovich</conductor><list-price>18.98</list-price><sell-price>13.99</sell-price><cost>9.92</cost></item></inventory>

You simply apply the following XSLT stylesheet to your source document:

<pre lang="xml">
<?xml version="1.0" encoding="UTF-8"??><stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform"><template match="/"><copy-of select="inventory/item[name = 'XSLT']"></copy-of></template></stylesheet>

Which gives you exactly what you were looking for, the “item” with the name “XSLT”.

<pre lang="xml">
<?xml version="1.0" encoding="utf-8"??><item id="2"><name>XSLT</name><type>book</type><author>Tidwell</author><list-price>49.95</list-price><sell-price>34.99</sell-price><cost>22.92</cost></item>

That was easy, so now let’s say you want to do a little more with your inventory document. Your boss wants a copy of it to look at the numbers and do some accounting. She doesn’t care about the authors or conductors, so she’d like that information left out. Also, she would like an additional piece of information for each item, the amount of profit off each item sold, the difference between the sell-price and the cost.

Because we are adding a piece of information and getting rid of elements that don’t affect the accounting, we can’t use a xsl:copy-of, because that would output an exact copy of the item element, it’s attribute nodes, and it’s child nodes. This exact copy is called a deep copy, because it not only copies the element, but all of it’s children as well. The solution is to use xsl:copy which performs a shallow copy, which means it only copies the current node, and ignores all children or attribute nodes.

Since xsl:copy only copies one element at a time, you need to explicitly specify that you want to continue copying attribute nodes and child nodes. xsl:apply-templates gives us the leverage to write a template that accomplishes that. The following template starts by matching attribute and children nodes, then copies the node, and recursively applies itself to any attribute or child nodes found in the source tree.