[NTG-context] Request for xml.filter explanation!

Hans Hagen j.hagen at xs4all.nl
Thu Apr 11 17:33:18 CEST 2019


On 4/10/2019 9:27 PM, Geert Verhaag wrote:
> Hi,
> 
> I've the following xml file:
> 
> *<?xml version='1.0' standalone='yes?>
> 
> <content>
>   <object mid="a">
>    <text>a paragraph of text</text>
>    <value>40.2</value>
>   </object>
>   <object mid="b">
>    <text>another paragraph of text</text>
>    <value>10.7</value>
>   </object>
>   <object mid="c">
>    <text>still another paragraph!</text>
>    <value>20.5</value>
>   </object>
> </content>*
> 
> And want to calculate the total value of all the objects:
> 
> like so:
> 
> *\startxmlsetups xml:example:base
>    \xmlsetsetup{#1}{content|object}{xml:example:*}
> \stopxmlsetups
> 
> \xmlregisterdocumentsetup{example}{xml:example:base}
> 
> \startxmlsetups xml:example:content
> totalvalue: \xmlfunction{#1}{totalvalue}
> \xmlflush{#1} \endgraf
> \stopxmlsetups
> 
> \startxmlsetups xml:example:object
> attribute : \xmlatt{#1}{mid} \par
> \stopxmlsetups
> 
> % LUA
> 
> \startluacode
> function xml.functions.totalvalue(root)
> local score = 0
> for e in xml.collected(root,"/object") do
> tex.write('Okay!')
> --score = score + xml.filter(e,"xml:///value/number()") or 0
> end
> tex.write(score)
> end
> \stopluacode
> 
> \starttext
> 
> \xmlprocessfile{example}{content.xml}{}
> 
> \stoptext*
> 
> *
> *
> 
> When I run this example I get:
> 
> *totalvalue: Okay!Okay!Okay!0 attribute : a
> attribute : b
> attribute : c*
> 
> That looks allright!
> 
> But when I incorporate the line:
> 
> *score = score + xml.filter(e,"xml:///value/number()") or 0*
> 
> the following Lua error is thrown:
> **
> 
> *! LuaTeX error [string "\directlua "]:5: attempt to perform arithmetic 
> on a nil value*
> 
> Has anybody any idea what I'm doing wrong here? The manual 
> *xml-mkiv.pdf* doesn't give any clue about this!
\startluacode
     function xml.functions.totalvalue(root)
         local score = 0
         for e in xml.collected(root,"/object/value") do
             score = score + tonumber(xml.text(e)) or 0
         end
         context(score)
     end
\stopluacode


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