Variables, and how to use them

[Derek Fountain]

I'm having trouble understanding the use of XSLT variables. I'm trying to add up the values from a set of elements. The code currently looks like this:

<xsl:variable name="orderTotal">
<xsl:value-of select="0" />
</xsl:variable>

<xsl:for-each select="item">
<xsl:variable name="orderTotal">
<xsl:value-of select="$orderTotal + qty*price" />
</xsl:variable>
</xsl:for-each>

<xsl:value-of select="$orderTotal" />

My 'item's each have a quantity and price, and I can print those values from inside the for-each, so I know they're there to be used.

Under Linux, $orderTotal is always given as zero. Under Windows the XSLT processor gives an error inside the loop saying 'orderTotal' cannot be redefined.

How do I update the value of 'orderTotal' inside my loop?

 

[Dimitre Novatchev]

"Derek Fountain" wrote:

I'm having trouble understanding the use of XSLT variables. I'm trying to add up the values from a set of elements. The code currently looks like this:

<xsl:variable name="orderTotal">
<xsl:value-of select="0" />
</xsl:variable>

<xsl:for-each select="item">
<xsl:variable name="orderTotal">
<xsl:value-of select="$orderTotal + qty*price" /> </xsl:variable> ...

from

inside the for-each, so I know they're there to be used.

Under Linux, $orderTotal is always given as zero. Under Windows the XSLT processor gives an error inside the loop saying 'orderTotal' cannot be redefined.

How do I update the value of 'orderTotal' inside my loop?

This is a FFFAQ ...

XSLT is a functional (side-effects free) language and it is not possible to change the value of an xsl:variable once initially defined.

You must use not xsl:for-each, but a named template that calls itself recursively.

=====
Cheers,

Dimitre Novatchev.
http://fxsl.sourceforge.net/ -- the home of FXSL

 

[Derek Fountain]

This is a FFFAQ ...

My apologies. } Is there a FAQ for this NG?

XSLT is a functional (side-effects free) language and it is not possible to change the value of an xsl:variable once initially defined.

You must use not xsl:for-each, but a named template that calls itself recursively.

This is a concept I'm not familiar with. Is there anything out there I can read to understand it?

 

[Andy Fish]

do a search for 'functional languages' or 'functional programming'

(and if you're used to normal 'imperative' programming, prepare to have your head completely messed up)

"Derek Fountain" wrote:

> This is a FFFAQ ...

My apologies. } Is there a FAQ for this NG?

> XSLT is a functional (side-effects free) language and it is not possible
> to change the value of an xsl:variable once initially defined.
>
> You must use not xsl:for-each, but a named template that calls itself
> recursively.

This is a concept I'm not familiar with. Is there anything out there I can read to understand it?

 

[Magnus Henriksson]

My apologies. } Is there a FAQ for this NG?

This is a concept I'm not familiar with. Is there anything out there I can read to understand it?

There are a few examples at
http://www.dpawson.co.uk/xsl/sect2/recursion.html.

Below is an example that works with any source document. This example uses a named template to implement a kind of 'for' loop. As you can see from the result, the $from parameter value is incremented until the "$from &lt; $to" test returns 'false' and the template is not called again.

-- begin stylesheet --

<?xml version="1.0" encoding="utf-8"?>
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">

<xslutput method="text"/>

<xsl:template match="/">
<xsl:call-template name="for">
<!-- use the default value for $from -->
<xsl:with-param name="to" select="10"/>
<!-- use the default value for $step-->
</xsl:call-template>
</xsl:template>

<xsl:template name="for">

<xslaram name="from" select="0"/>
<xslaram name="to" select="0"/>
<xslaram name="step" select="1"/>

<xsl:if test="$from &lt; $to">

<!-- Do something. Here, just output parameter values --> <xsl:text>

$from = </xsl:text>
<xsl:value-of select="$from"/>
<xsl:text>
$to = </xsl:text>
<xsl:value-of select="$to"/>
<xsl:text>
$to = </xsl:text>
<xsl:value-of select="$to"/>

<!-- Call itself with new parameter values -->
<xsl:call-template name="for">
<xsl:with-param name="from" select="$from + $step"/>
<xsl:with-param name="to" select="$to"/>
<xsl:with-param name="step" select="$step"/>
</xsl:call-template>

</xsl:if>

</xsl:template>

</xsl:stylesheet>

-- end stylesheet --

-- begin result --

$from = 0
$to = 10
$to = 10

$from = 1
$to = 10
$to = 10

$from = 2
$to = 10
$to = 10

$from = 3
$to = 10
$to = 10

$from = 4
$to = 10
$to = 10

$from = 5
$to = 10
$to = 10

$from = 6
$to = 10
$to = 10

$from = 7
$to = 10
$to = 10

$from = 8
$to = 10
$to = 10

$from = 9
$to = 10
$to = 10

-- end result --

// Magnus

 

[Vincent Lefevre]

In article <3f44d4fd$0$23612$>,
Derek Fountain wrote:

My apologies. } Is there a FAQ for this NG?

The XML FAQ is here: http://www.ucc.ie:8080/cocoon/xmlfaq and the XSL FAQ is here: http://www.dpawson.co.uk/xsl/xslfaq.html

(but I think that you may have found them with Google and perhaps you could find a FAQ on functional languages...).

 

[Magnus Henriksson]

There are a few examples at
http://www.dpawson.co.uk/xsl/sect2/recursion.html.

Below is an example that works with any source document. This example uses a named template to implement a kind of 'for' loop. As you can see from the result, the $from parameter value is incremented until the "$from &lt; $to" test returns 'false' and the template is not called again.

Stupid copy-paste-error. Here is what I intended:

-- begin stylesheet --

<?xml version="1.0" encoding="utf-8"?>
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">

<xslutput method="text"/>

<xsl:template match="/">
<xsl:call-template name="for">
<!-- use the default value for $from -->
<xsl:with-param name="to" select="10"/>
<!-- use the default value for $step-->
</xsl:call-template>
</xsl:template>

<xsl:template name="for">

<xslaram name="from" select="0"/>
<xslaram name="to" select="0"/>
<xslaram name="step" select="1"/>

<xsl:if test="$from &lt; $to">

<!-- Do something. Here, just output parameter values --> <xsl:text>

$from = </xsl:text>
<xsl:value-of select="$from"/>
<xsl:text>
$to = </xsl:text>
<xsl:value-of select="$to"/>
<xsl:text>
$step = </xsl:text>
<xsl:value-of select="$step"/>

<!-- Call itself with new parameter values -->
<xsl:call-template name="for">
<xsl:with-param name="from" select="$from + $step"/>
<xsl:with-param name="to" select="$to"/>
<xsl:with-param name="step" select="$step"/>
</xsl:call-template>

</xsl:if>

</xsl:template>

</xsl:stylesheet>

-- end stylesheet --

-- begin result --

$from = 0
$to = 10
$step = 1

$from = 1
$to = 10
$step = 1

$from = 2
$to = 10
$step = 1

$from = 3
$to = 10
$step = 1

$from = 4
$to = 10
$step = 1

$from = 5
$to = 10
$step = 1

$from = 6
$to = 10
$step = 1

$from = 7
$to = 10
$step = 1

$from = 8
$to = 10
$step = 1

$from = 9
$to = 10
$step = 1

-- end result --

// Magnus

 

[Dimitre Novatchev]

My apologies. } Is there a FAQ for this NG?

This is a concept I'm not familiar with. Is there anything out there I can read to understand it?

See for example:

http://www-106.ibm.com/developerworks/xml/library/x-xslrecur/?loc=dwmain

=====
Cheers,

Dimitre Novatchev.
http://fxsl.sourceforge.net/ -- the home of FXSL

 

[Andy Dingley]

This is a concept I'm not familiar with.

Think of it this way (which is extremely partial), but not a bad way to approach it.

Variables in XSLT cannot be modified once set.

Variables in XSLT are typed (in some ways very strongly) to contain a set of nodes. XSLT sees these nodes as atomic, and manipulating the values _within_ these nodes is not something that it's appropriate for.

So assemble a set of the nodes you want, as an XSLT variable, then use something like a JavaScript extension to operate upon them.

There's also the "functional programming" approach. You still can't modify variables, but you can work with the contents of a stack.

Write a recursive template that traverses the set of nodes you want (get this working first). Then give it a parameter, and a local variable. Pass the sum of these two as the actual parameter to the next call. When you reach the end, return the value accumulated.

After doing this for a while, you'll probably grow a beard and start pining for Lisp !

You'll also find that code in this style is dificult to maintain by other team members. Code with extensions often survives better, although excessive use of them can turn ugly too.

If you don;t already have it, Michael Kay's XSLT book is a good one.

 

[Dimitre Novatchev]

So assemble a set of the nodes you want, as an XSLT variable, then use something like a JavaScript extension to operate upon them.

Using any extension function with side effects is dangerous, unpredictable/unreliable and should not be recommended.

=====
Cheers,

Dimitre Novatchev.
http://fxsl.sourceforge.net/ -- the home of FXSL

 

[Andy Dingley]

Using any extension function with side effects is dangerous, unpredictable/unreliable and should not be recommended.

Side effects are certainly a bad thing. I was thinking more of some global state, accessed only through a few extension functions (clear, accumulate, get total) and then returned by calling a function.

Certainly I would be most cautious about any extension function that had side-effects _that_affected_the_document_. But I don't think we need that here,

 

[Dimitre Novatchev]

Certainly I would be most cautious about any extension function that

had side-effects _that_affected_the_document_. But I don't think we need that here,

Not only "that affected the document"...

Any keeping and modifying of "program state" (e.g. using static Javascript variables for counters/accumulators) will have unpredictable results.

See for real practical examples these:

http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&threadm=uASG4eE2CHA.1560@TK2MSFTNGP09&r num=7&prev=/groups%3Fq%3Ddimitre%2Bxslt%2Bfunctions%2Bside%2Beffects%26hl%3D en%26lr%3D%26ie% 3DUTF-8%26selm%3DuASG4eE2CHA.1560%2540TK2MSFTNGP09%26rnum%3D7 (read the whole thread)

http://groups.google.com/groups?hl=de&frame=right&th=418985883ecc2e2c&seekm= uZk9iKYiBHA.2000%40tkmsftngp04#link1

=====
Cheers,

Dimitre Novatchev.
http://fxsl.sourceforge.net/ -- the home of FXSL