I am reading one of the articles that Manfred has
suggested
(Michael Kay: What kind of language is
XSLT?) so that I can learn more about using node
sets, apply
templates and recursion but I do not fully
understand some of
the XPATH expressions.
Please note, that article was written in 2001 and
relates to XSLT 1.0.
These are the expressions used to compute the
teams
standings:
1 - <xsl:variable name="teams"
select="//team[not(.=preceding::team)]"/>
I know that this expression is getting a node set
of all the
unique team elements.
Yes. In XSLT 2.0 you would use <xsl:for-each-group>
here.
I am confused about the "." (dot) in this
expression and the
comparisson with the preceding::team.
For the first team, does the "." (dot) match the
value of the
node - 'Brazil' - and preceding::team match the
value as well?
The expressions "." and "preceding::team" both
evaluate to node-sets. ("."
is a node-set with one member). When you compare two
node-sets using "=" the
result is true if there is any node in the first set
whose string value is
equal to the string value of a node in the second
node-set. The string value
of an element such as <team
score="1">Scotland</team> is "Scotland".
2 - Here we are inside a loop
<xsl:template match="results">
<xsl:for-each select="$teams">
<xsl:variable name="this" select="."/>
Again does "." match the value of each team or the
whole team element?
There's no "matching" going on here. "." is an
expression whose value is a
(singleton) node-set, in this case one of the nodes
in the node-set $teams:
presumably a <team> element.
count($matches[team=$this])
When you use this predicate [team=$this] are we
comparing the
value of the element or the team elements ?
"team" is short for "child::team". So you are
selecting all the nodes in
$matches that have a child whose name is "team" and
whose string-value is
equal to [the string value of] $this.
4 - <xsl:variable name="won"
select="count($matches[team[.=$this]/@score >
team[.!=$this]/@score])"/>
I know that this expression is computing the
victories of a
particular team. However, I do not understand the
logic of
the expression. We have an extra predicate:
[team[.=$this]/@score
Why cant we just use [team=$this]/@score ?
Because the @score is an attribute of the team, not
of the match. The
predicate [team=$this] is fine for selecting matches
in which this team
participated, but we want to select one of the teams
in the match, not just
the match.
5 - <xsl:variable name="for"
select="sum($matches/team[.=current()]/@score)"/>
What does current() mean in here? Is it the same
as the $this
variable?
Yes, in this context. I don't know why I wrote some
of the expressions using
$this and some using current() - either would work.
In this case we are not using a predicate for the
$matches
variable. why?
We don't need to. To find out how many goals
Scotland scored, we need to
select all the <team> elements equal to Scotland,
across all the matches,
and then sum their @score attributes. Matches in
which Scotland didn't play
will contribute nothing to the result.
To put it another way, asking "how many goals did
Scotland score in all
matches where they played?" gives the same answer as
"how many goals did
Scotland score in all matches?"
Michael Kay