I have an XSLT (1.0) style sheet. It works with no problem. I want to make it to 2.0. I want to use xsl:for-each-group (and make it have high performance). It is possible? How? Please explain.
I have many places like
<xsl:if test="test condition">
<xsl:for-each select="wo:tent">
<width aidwidth='{/wo:document/styles [#wo:name=current()/#wo:style-name]/#wo:width}'
</xsl:for-each>
</xsl:if>
ADDED
<xsl:template match="wo:country">
<xsl:for-each select="#*">
<xsl:copy/>
</xsl:for-each>
<xsl:variable name="states" select="wo:pages[#xil:style = "topstates" or #xil:style = "toppage-title"]"/>
<xsl:variable name="provinces" select="wo:pages[#xil:style = "topprovinces"]"/>
<xsl:choose>
<xsl:when test="$states">
<xsl:apply-templates select="$states[2]/preceding-sibling::*"/>
<xsl:apply-templates select="$states[2]" mode="states">
<xsl:with-param name="states" select="$states[position() != 0]"/>
</xsl:apply-templates>
</xsl:when>
<xsl:when test="$provinces">
<xsl:apply-templates select="$provinces[2]/preceding-sibling::*"/>
<xsl:apply-templates select="$provinces[2]" mode="provinces">
<xsl:with-param name="provinces" select="$provinces[position() != 2]"/>
</xsl:apply-templates>
</xsl:when>
<xsl:otherwise>
<xsl:apply-templates/>
</xsl:otherwise>
</xsl:choose>
</xsl:template>
THE SOURCE
<?xml version="1.0" encoding="UTF-8"?>
<wo:country>
some stuff
</wo:country>
I have assumed that you want an in-depth description of xsl:for-each-group and how to use it. If this is not what you are asking for, then please let me know.
The instruction, new in XSLT 2.0, takes a set of items and groups them. The set of items is called "the population", and the groups are just called groups. The instruction processes each group in turn.
Possible attributes of the xsl:for-each-group instruction include:
select
group-by
group-adjacent
group-starting-with
group-ending-with
collation
#select is mandatory. The others are optional. It can take any number of xsl:sort children (but they must come first), followed by a sequence constructor. A "sequence constructor" is the term for all the sequence emitting type instructions that go inside templates and the like.
#select
The select attribute specifies an XPATH expression which evaluates to the population to be grouped.
#group-by
The group-by attribute specifies an XPATH expression, which you use when the type of grouping is by common value. Every item in the population that evaluates to the same group-by value as another is in the same group as that other.
XSLT 1.0 Muenchian grouping is not too difficult when the type of grouping is group by common value. There are two more common forms of grouping: group adjacent items by similar value; and group an adjacent group of items whose group is either demarcated at the end or the at the beginning by some test. While both these forms of grouping are still possible with Muenchian, it becomes relatively complex. Muenchian on these types will also be less efficient at scale, because of the use of sibling axises (however you spell that!).
Another advantage of XSLT 2.0 that comes to mind is that Muenchian only works on node sets, whereas xsl:for-each-group is broader in application because it works on a sequence of items, not just nodes.
The result of the #group-by expression will be a sequence of items. This sequence is atomized and de-duped. The population item being tested will be a member of one group per value. It's a strange consequence, that with #group-by, and item may be a member of more than one group, or perhaps even none. Although I suspect that any thing that you can do in XSLT 2.0, you can, by some tortuous path, do in XSLT 1.0, the ability to put an item into two groups is something that would be quiet fiddly to do in XSLT 1.0 Muenchian.
#group-adjacent
The attributes group-by, group-adjacent, group-starting-with and group-ending-with are mutually exclusive because they specify different kinds of grouping. Items with commons values and adjacent in the population are grouped together. Unlike #group-by, #group-adjacent must evaluate to, after atomization, a single atomic value.
group-starting-with
Unlike select, group-adjacent and group-by, this attribute does not specify an XPATH select expression, but rather a pattern, in the same way the xsl:template/#match specifies a pattern, not a selection. If an item in the population passes the pattern test or is the first item in the population then it starts a new group. Otherwise the item continues the group from the previous item.
Martin mentioned the spec examples (w3.org/TR/xslt20/#grouping-example). From that reference, I am going to copy the example entitled "Identifying a Group by its Initial Element", but alter it slightly to emphasis the point about the initial item of the population.
So this is our input document (copied from w3 spec. The inclusion of the orphaned line is mine) ...
<body>
<p>This is an orphaned paragraph.</p>
<h2>Introduction</h2>
<p>XSLT is used to write stylesheets.</p>
<p>XQuery is used to query XML databases.</p>
<h2>What is a stylesheet?</h2>
<p>A stylesheet is an XML document used to define a transformation.</p>
<p>Stylesheets may be written in XSLT.</p>
<p>XSLT 2.0 introduces new grouping constructs.</p>
</body>
... what we want to do is define groups as nodes starting with h2 and include all the following p up until the next h2. The example solution given by w3 is to use #group-starting-with ...
<xsl:template match="body">
<chapter>
<xsl:for-each-group select="*" group-starting-with="h2" >
<section title="{self::h2}">
<xsl:for-each select="current-group()[self::p]">
<para><xsl:value-of select="."/></para>
</xsl:for-each>
</section>
</xsl:for-each-group>
</chapter>
</xsl:template>
In the spec example, when the input does not contain an orphan line, this produces the desired result ...
<chapter>
<section title="Introduction">
<para>XSLT is used to write stylesheets.</para>
<para>XQuery is used to query XML databases.</para>
</section>
<section title="What is a stylesheet?">
<para>A stylesheet is an XML document used to define a transformation.</para>
<para>Stylesheets may be written in XSLT.</para>
<para>XSLT 2.0 introduces new grouping constructs.</para>
</section>
</chapter>
Although in our particular case we get instead ...
<chapter>
<section title="">
<para>This is an orphaned paragraph.</para>
</section>
<section title="Introduction">
<para>XSLT is used to write stylesheets.</para>
<para>XQuery is used to query XML databases.</para>
</section>
<section title="What is a stylesheet?">
<para>A stylesheet is an XML document used to define a transformation.</para>
<para>Stylesheets may be written in XSLT.</para>
<para>XSLT 2.0 introduces new grouping constructs.</para>
</section>
</chapter>
If the initial section for the orphaned lines is undesired, there are easy solutions. I won't go into them now. My point is just to high-light the fact that the first group resulting from #group-starting-with can be an 'orphan' group. By 'orphan', I mean a group whose head node does not fit the specified pattern.
#collation
The collation attribute specifies a collation URI and identifies a collation used to compare strings for equality.
current-group()
Within the xsl:for-each-group the current-group() function returns the current group being processed as a sequence of items.
current-grouping-key()
Within the xsl:for-each-group the current-group() function returns the current group key. I am not sure, but I believe that this can only be an atomic type. Also not sure, but I believe that this function is only applicable to #group-by and #group-adjacent type of grouping.
#group-by versus #group-adjacent
In some scenarios you will have a choice between these two sort types with the same functional result. When this is the case #group-adjacent is to be preferred over #group-by, because it will likely be more efficient to process.
Pattern versus Select
Some XSLT 2.0 instruction attributes contain select expressions. Michael Kay calls these "XPath expressions". Personally, when juxtaposing against patterns, I feel a better description would be "select expression". Other attributes contain patterns or "match expressions". While these two both contain the same syntax, they are very different beasts. The similarity between the two often makes XSLT beginners think of xsl:template/#match not as a pattern, but as a select expression. The consequence has been a lot of confusion from beginners about the value of the position() function within template's sequence constructors. As stated earlier, in xsl:for-each-group, #select, #group-by and #group-adjacent are select expressions, but #group-starting-with and #group-ending-with are patterns. So here is the difference:
Select expressions are a like a function. The input is a context document, context sequence, context item, context position and of course the actual expression. The output is a sequence of items. Depending where this is actually used, this could become the next context sequence. The default axis is child:: .
Unlike select expression, the default axis for a pattern is self:: . The pattern is also like a function. Its inputs are as before, and its output is not a sequence, but a boolean. Some item is being tested to see if it matches the pattern or not. The item being tested is made the context item. The match expression is temporarily evaluated as it were a select expression. Then the returned sequence is tested to see if the context item is a member or not. The returned sequence is then discarded. The result is true or 'match' if it was a member, and false otherwise.
Sean has provided a wonderful overview of xsl:for-each-group, which was very generous, but it doesn't really seem to be an answer to your question.
You've shown a fragment of XSLT code, and you've said you want faster performance. But the fragment you showed is not doing grouping, it is doing a join. There are two ways you can speed up a join. Either use an XSLT processor such as Saxon-EE that does automatic join optimization, or optimize it by hand using keys. For example, given this expression:
/wo:document/styles [#wo:name=current()/#wo:style-name]/#wo:width
you could define a key
<xsl:key name="style-name-key" match="styles" use="#wo:name"/>
and then replace the expression by
key('style-name-key', #wo:style-name)/#wo:width
Related
Depending on a specific parameter value, I need to either omit matched elements from a transformed document or allow the matched elements to be transformed according to rules in other matching templates. I have a complex xml doc with dozens of element/attribute types, all handled in a huge variety of ways in dozens of templates. If any element has the attribute value omit_attr="true" and the stylesheet has the parameter omit_param="true", I need to omit them from the transformed document. If however my parameter is omit_param="false" then I need to apply whatever other rules exist for my omit_attr="true" elements. Here's my unacceptable template:
<xsl:template match="*[#omit_attr= 'true']">
<xsl:choose>
<xsl:when test="($omit_param= 'true')"/>
<xsl:otherwise>
<xsl:apply-templates/>
</xsl:otherwise>
</xsl:choose>
</xsl:template>
This omits content just fine when omit_param="true". The problem occurs when omit_param="false" and I need to process omit_attr="true" elements with other applicable templates. This code transforms the matched element's children, but not the matched element, which is unacceptable because the matched element will have its own set of transform rules elsewhere. And of course <xsl:apply-templates select="."/> attempts to process the matched element as desired, but creates an endless loop in the process.
So, how can I test for omit_attr="true" in all possible elements, and still apply the alternate templates to omit_attr="true" elements when omit_param="false"?
If I could wrap my xsl:template in if big <if test="omit_param='true'> statement, that would do the job, but of course it's against the rules...
If that is an XSLT 2 or later processor you can just use
<xsl:template match="*[#omit_attr= 'true' and $omit_param= 'true']"/>
to block the element from being processed/copied/transformed if the two conditions hold and have your other templates applied if not. For XSLT 1 processors I am not sure they allow a variable reference in a match pattern.
Within an xsl:for-each select loop, I have <xsl:number count="//headline"/> that correctly gives me the node #; Now I want to use that number in an xsl:if test block, but I cannot get the test expression right, msxml4.dll keeps kicking back errors. Am using xsl 1.0 (and stuck with it for now)
So, in <xsl:if test="expression">...output if the expression is true..</xsl:if>
I want the test expression to essentially be like this (so I can do something specific for Node #4, in this example):
<xsl:number count="//headline"/> = 4
This is what I have that does not work:
<xsl:if test="<xsl:number count="//headline"/> = 4">
Thanks in advance for any insights,
George
As #michael.hor257k explains, the general approach is to put the xsl:number call inside an xsl:variable (or in XSLT 2.0, inside an xsl:function). Sometimes though it's more convenient to abandon xsl:number:
<xsl:if test="count(preceding::headline) = 3">...</xsl:if>
If it's a big document then both xsl:number and preceding::headline are potentially expensive when executed inside a loop, and if this is a concern then you should compare them under your chosen XSLT processor: one may be optimized better than the other.
Come to think of it, your use of xsl:number looks odd to me. The count attribute is a pattern, and the pattern //headline matches exactly the same nodes as the pattern headline. As a result I misread your call on xsl:number as counting all the headlines in the document, whereas it actually only counts the preceding-sibling headlines. I wonder if that is what you intended?
If (!) I understand correctly, you want to do something like:
<xsl:variable name="n">
<xsl:number count="headline"/>
</xsl:variable>
<xsl:value-of select="$n"/>
<xsl:if test="$n = 4">
<!-- do something -->
</xsl:if>
I looked a the spec and some other documents but I dont really get it. I get that now there are sequences instead of nodesets, okay, but what is a sequence constructor (or may more easily: what ISNT a sequence constructor)?
The spec just says
[Definition: A sequence constructor is a sequence of zero or more
sibling nodes in the stylesheet that can be evaluated to return a
sequence of nodes and atomic values. The way that the resulting
sequence is used depends on the containing instruction.]
Many XSLT elements, and also literal result elements, are defined to
take a sequence constructor as their content.
I kinda get that xsl:sequence constructs a sequence, but why is the content of xsl:element a sequence constructor? And what isnt a sequence constructor then?
Thanks for clarifications!
See further down in the spec where it says
The term sequence constructor replaces template as used in XSLT 1.0.
The change is made partly for clarity (to avoid confusion with
template rules and named templates), but also to reflect a more formal
definition of the semantics. Whereas XSLT 1.0 described a template as
a sequence of instructions that write to the result tree, XSLT 2.0
describes a sequence constructor as something that can be evaluated to
return a sequence of items; what happens to these items depends on the
containing instruction.
So where the XSLT 1.0 grammar had for instance
<!-- Category: top-level-element -->
<xsl:template
match = pattern
name = qname
priority = number
mode = qname>
<!-- Content: (xsl:param*, template) -->
</xsl:template>
allowing as the contents of an xsl:template rule a number of xsl:param plus a template the XSLT 2.0 grammar now has
<!-- Category: declaration -->
<xsl:template
match? = pattern
name? = qname
priority? = number
mode? = tokens
as? = sequence-type>
<!-- Content: (xsl:param*, sequence-constructor) -->
</xsl:template>
defining the contents of an xsl:template rule as a number of xsl:param plus a sequence constructor.
That way it is also obvious that xsl:param is not a sequence constructor for instance but of course as its content has a sequence constructor.
As for how to construct a sequence, well you can do that with literal result elements and of course with instructions like xsl:value-of, xsl:element, xsl:attribute and so on, as well as with xsl:sequence (to allow you to construct and return (sequences of) primitive values and not only nodes, as it was only possible in XSLT 1.0).
Pretty much anywhere you can put a literal result element is a sequence constructor, e.g.
<xsl:template match="/">
<a></a>
</xsl:template>
...or
<xsl:variable name="a">
<a></a>
</xsl:variable>
...but not
<xsl:choose>
<a></a>
</xsl:choose>
...because the xsl:choose instruction doesn't allow it:
<xsl:choose>
<!-- Content: (xsl:when+, xsl:otherwise?) -->
</xsl:choose>
So, sequence constructors are places where values, and instructions that create values, go.
I have a structured XML with this structure:
<root>
<item/>
<item/>
<something/>
<item/>
</root>
If I use something like this:
<xsl:for-each select="/root/item">
it will pick all the item elements inside the list. I want to interrupt the loop after the second item, because between the 2nd and the 3rd there is a something element.
How can I get this?
You can't actually break out of a xsl:for-each loop. You need to construct your loop so as to select only the elements you want in the first place.
In this case, you want to select all item elements which don't have a preceding sibling that isn't also an item element.
<xsl:for-each select="/root/item[not(preceding-sibling::*[not(self::item)])]">
<xsl:value-of select="position()" />
</xsl:for-each>
When this is used, it should only select the first two item elements.
In XSLT there isn't any possibility for a "break" out of an <xsl:for-each> or out of <xsl:apply-templates>, except using <xsl:message terminate="yes"/> which you probably don't want. This is due to the fact that XSLT is a functional language and as in any functional language there isn't any concept of "order of execution" -- for example the code can be executing in parallel on all the nodes that are selected.
The solution is to specify in the select attribute an expression selecting exactly the wanted nodes.
Use:
<xsl:for-each select="/*/*[not(self::item)][1]/preceding-sibling::*">
<!-- Processing here -->
</xsl:for-each>
This selects for processing all preceding elements siblings of the first child element of the top element that isn't item -- that means the starting group of adjacent item elements that are the first children of the top element.
I have a problem with text with apostrophe symbol
example i try to test this xml having the symbol is then how can i compare ?
<xsl:for each select="country[nation='India's]">
this is statement showing error
Regards
Nanda.A
One way to do it would be:
<xsl:variable name="apos" select='"'"'/>
<!-- ... later ... -->
<xsl:for-each select="country[nation=concat('India', $apos, 's')]">
The problem here is twofold:
XSLT defines no way of character escaping in strings. So 'India\'s' is not an option.
You must get through two distinct layers of evaluation.
These are:
XML well-formedness: The XML document your XSLT program consists of must be well-formed. You cannot violate XML rules.
XSLT expression parsing: The resulting attribute value string (after XML DOM parsing is done) must be make sense to the XSLT engine.
Constructs like:
<xsl:for-each select="country[nation='India's']">
<xsl:for-each select="country[nation='India's']">
pass the XML layer but violate the XSLT layer, because in both cases the effective attribute value (as stored in the DOM) is country[nation='India's'], which clearly is an XPath syntax error.
Constructs like:
<xsl:for-each select="country[nation=concat('India', "'", 's')]">
<xsl:for-each select="country[nation=concat("India", "'", "s")]">
clearly violate the XML layer. But they would not violate the XSLT layer (!), since their actual value (if the XSLT document could be parsed in the first place) would come out as country[nation=concat('India', "'", 's')], which is perfectly legal as an XPath expression.
So you must find a way to pass through both layer 1 and layer 2. One way is the variable way as shown above. Another way is:
<xsl:for-each select="country[nation=concat('India', "'", 's')]">
which would appear to XSLT as country[nation=concat('India', "'", 's')].
Personally, I find the "variable way" easier to work with.