page 48 self
axis Contains the context node itself. The
self
axis can be abbreviated with a single period
.
. The expressions
self::
and
.
are equivalent.
attribute
axis Contains the attributes of the context node. If the context node is not an element node,
this axis is empty. The
attribute
axis can be abbreviated with the at-sign . The
expressions
attribute::type
and
type
are equivalent.
ancestor
axis Contains the parent of the context node, the parents parent, etc. The
ancestor
axis always contains the root node unless the context node is the root node.
ancestor-or-self
axis Contains the context node, its parent, its parents parent, and so on. This axis always
includes the root node.
descendant
axis Contains all children of the context node, all children of all the children of the context
node, and so on. The children are all of the comment, element, processing instruction, and text nodes beneath the context node. In other words, the
descendant
axis does not include attribute or namespace nodes. As we discussed earlier, although an attribute
node has an element node as a parent, an attribute node is not considered a child of that element.
descendant-or-self
axis Contains the context node and all the children of the context node, all the children of
all the children of the context node, all the children of the children of all the children of the context node, and so on. As always, the children of the context node include all
comment, element, processing instruction, and text nodes; attribute and namespace nodes are not included.
preceding-sibling
axis Contains all preceding siblings of the context node; in other words, all nodes that have
the same parent as the context node and appear before the context node in the XML document. If the context node is an attribute node or a namespace node, the
preceding-sibling
axis is empty.
following-sibling
axis Contains all the following siblings of the context node; in other words, all nodes that
have the same parent as the context node and appear after the context node in the XML document. If the context node is an attribute node or a namespace node, the
following-sibling
axis is empty.
preceding
axis Contains all nodes that appear before the context node in the document, except
ancestors, attribute nodes, and namespace nodes.
following
axis Contains all nodes that appear after the context node in the document, except
descendants, attribute nodes, and namespace nodes.
page 49 namespace
axis Contains the namespace nodes of the context node. If the context node is not an
element node, this axis is empty.
3.2.7 Predicates
Theres one more aspect of XPath expressions that we havent discussed: predicates. Predicates are filters that restrict the nodes selected by an XPath expression. Each predicate is
evaluated and converted to a Boolean value either
true
or
false
. If the predicate is
true
for a given node, that node will be selected; otherwise, the node is not selected. Predicates
always appear inside square brackets
[]
. Heres an example:
xsl:apply-templates select=line[3]
This expression selects the third
line
element in the current context. If there are two or fewer
line
elements in the current context, this XPath expression returns an empty node- set. Several things can be part of a predicate; well go through them here.
3.2.7.1 Numbers in predicates
A number inside square brackets selects nodes that have a particular position. For example, the XPath expression
line[7]
selects the seventh
line
element in the context node. XPath also provides the boolean
and
and
or
operators as well as the union operator
|
to combine predicates. The expression
line[position=3 and style]
matches all
line
elements that occur third and have a
style
attribute, while
line[position=3 or style]
matches all
line
elements that either occur third or have a
style
attribute. With the union operator, the expression
line[3|7]
matches all third and seventh
line
elements in the current context, as does the more verbose
line[3] | line[7]
.
3.2.7.2 Functions in predicates
In addition to numbers, we can use XPath and XSLT functions inside predicates. Here are some examples:
line[last]
Selects the last
line
element in the current context.
line[position mod 2 = 0]
Selects all even-numbered
line
elements. The
mod
operator returns the remainder after a division; the
position
of any even-numbered element divided by 2 has a remainder of 0.
sonnet[type=Shakespearean]
Selects all
sonnet
elements that have a
type
attribute with the value
Shakespearean
.
ancestor::table[border=1]
Selects all
table
ancestors of the current context that have a
border
attribute with the value
1
.
countbodytable[border=1]
Returns the number of
table
elements with a
border
attribute equal to
1
that are children of
body
elements that are children of the root node. Notice that in this case were using an XPath predicate expression as an argument to a function.
page 50
3.3 Attribute Value Templates
Although theyre technically defined in the XSLT specification in section 7.6.2, to be exact, well discuss attribute value templates here. An attribute value template is an XPath
expression that is evaluated, and the result of that evaluation replaces the attribute value template. For example, we could create an HTML
table
element like this:
table border={size}
In this example, the XPath expression
size
is evaluated, and its value, whatever that happens to be, is inserted into the output tree as the value of the
border
attribute. Attribute value templates can be used in any literal result elements in your stylesheet for HTML
elements and other things that arent part of the XSLT namespace, for example. You can also use attribute value templates in the following XSLT attributes:
•
The
name
and
namespace
attributes of the
xsl:attribute element
•
The
name
and
namespace
attributes of the
xsl:element element
•
The
format
,
lang
,
letter-value
,
grouping-separator
, and
grouping-size
attributes of the
xsl:number element
•
The
name
attribute of the
xsl:processing-instruction element
•
The
lang
,
data-type
,
order
, and
case-order
attributes of the
xsl:sort element
3.4 XPath Datatypes
An XPath expression returns one of four datatypes:
node-set
Represents a set of nodes. The set can be empty, or it can contain any number of nodes.
boolean
Represents the value
true
or
false
. Be aware that the
true
or
false
strings have no special meaning or value in XPath; see
Section 4.2.1.2 in
Chapter 4 for a more
detailed discussion of boolean values.
number
Represents a floating-point number. All numbers in XPath and XSLT are implemented as floating-point numbers; the
integer
or
int
datatype does not exist in XPath and XSLT. Specifically, all numbers are implemented as IEEE 754 floating-
point numbers, the same standard used by the Java
float
and
double
primitive types. In addition to ordinary numbers, there are five special values for numbers: positive
and negative infinity, positive and negative zero, and
NaN
, the special symbol for anything that is not a number.
string
Represents zero or more characters, as defined in the XML specification. These datatypes are usually simple, and with the exception of node-sets, converting between
types is usually straightforward. We wont discuss these datatypes in any more detail here; instead, well discuss datatypes and conversions as we need them to do specific tasks.
