XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (447 page)

BOOK: XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition
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Here are some other examples of simple mapping expressions at work:

Expression
Description
sum(order/(@price*@quantity))
Returns the result of calculating
@price
times
@quantity
for each child

and summing the results.
string-join(ancestor::*/name(), ‘/’)
Returns a string such as
book/chapter/section/para
giving the names of the ancestors of the context node, separated by
/
characters.
avg(product/number(substring-after(@price, ‘$’)))
Returns the result of taking the
@price
attributes of all the child

elements, stripping off the leading
$
sign, converting to a number, and then averaging.

In most cases the system will be able to distinguish a path expression from a simple mapping expression by the way it is written. In some cases, however—for example, if the expression calls a user-written function whose return type is not declared—the two cases can't be distinguished until runtime. The initial processing in both cases is the same: for each item in the sequence returned by the first expression, evaluate the second expression and concatenate the results. The rule is then:

  • If all the items in the result are nodes, we have a path expression. Duplicate nodes are eliminated, and those that remain are sorted into document order.
  • If all the items in the result are atomic values, we have a simple mapping expression, and we are done.
  • If there's a mixture of nodes and atomic values, an error is reported.

A simple mapping expression always maps from nodes to atomic values. You can't have atomic values on the left-hand side of the
/
operator. For example, you can't write
avg(*/name()/string-length())
to get the average length of the names of the child elements. Only the last step in the path can return atomic values, so you have to write this as
avg(*/string-length(name()))
. Why the restriction? Possibly because some people didn't like the consequence that the value of
1/10
would be 10.

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