Sorting on Calculated Variables

The sorting performed by the <xsl:sort> elements above, in prodsort.xsl, would be suitable for an application like a catalog, in which a given product needs to be located easily.

Suppose, though, that the corporation's chief financial officer asks to see a list of all products, arranged in order by retail price. The first problem this presents is that our data contains a mixture prices in various currency denominations: US dollars (curr="USD"), Euros (curr="EU"), and British pounds Sterling (curr="GBP"). This might seem like an easy problem to fix. You could define a variable, such as usd_equiv, which would hold the retail price of the product, converted to its US dollar equivalent. Then, you might think, you could sort on the usd_equiv variable, instead of sorting directly on the value of the <price> element.

The problem is that you cannot sort directly on a variable. The sort key used as the value of the <xsl:sort> element's select attribute must be an XPath expression of some kind.

You can, however, sort on a calculated value, using the following general steps:

1. Clone the source tree into a variable, which will then hold a result tree fragment. For more information, see Introduction to Result Tree Fragments.
2. Create a new element to hold the calculated result, and append this new element to the cloned source tree created in step 1.
3. Select and sort the result tree fragment created by steps 1 and 2. Do not select and sort the original source tree. Note that this will require the result tree fragment to be converted to a node-set, using the msxsl:node-set() function. For more information, see Using msxsl:node-set() to Process Result-Tree Fragments.

The remainder of this topic works through each of these steps for the prodsort example. The complete solution to the problem stated above, the prodsort_curr.xsl style sheet, is available at Sample XSLT File for Sorting.

The solution to this problem requires the following steps.

• Step 1: Cloning the Source Tree
• Step 2: Creating the New Element
• Step 3: Sorting the Source Tree Clone