[R] XML and str

[Duncan Temple Lang]

Martin Maechler wrote:
>>>>>> "Ashley" == Ashley Ford <ford at signal.QinetiQ.com>
>>>>>>     on Wed, 07 Feb 2007 17:18:56 +0000 writes:
> 
>     Ashley> If I read in an .xml file eg with 
> 
>     >> xeg <- xmlTreeParse(system.file("exampleData", "test.xml",
>                                        package="XML"))
> 
>     Ashley> It appears to be OK however examining it with str() gives an apparent
>     Ashley> error
> 
>     >> str(xeg, 2)
>     Ashley> List of 2
>     Ashley> $ doc:List of 3
>     Ashley> ..$ file    : list()
>     Ashley> .. ..- attr(*, "class")= chr [1:2] "XMLComment" "XMLNode"
>     Ashley> ..$ version :List of 4
>     Ashley> .. ..- attr(*, "class")= chr "XMLNode"
>     Ashley> ..$ children:Error in obj$children[[...]] : subscript out of bounds
> 
>     Ashley> I am unsure if this is a feature or a bug and if the latter whether it
>     Ashley> is in XML or str, it is not causing a problem but I would like to
>     Ashley> understand what is happening, any ideas ?
> 
> Yes -  thank you for providing a well-reproducible example.
> After setting  
>       options(error = recover)
> 
> I do
> 
>    > obj <- xeg$doc
>    > mode(obj)     # "list"
>    [1] "list"
>    > is.list(obj)  # TRUE
>    [1] TRUE
>    > length(obj)   # 3
>    [1] 3
>    > obj[[3]]      # ---> the error you see above.
>    Error in obj$children[[...]] : subscript out of bounds
> 
>    Enter a frame number, or 0 to exit   
> 
>    1: obj[[3]]
>    2: `[[.XMLDocumentContent`(obj, 3)
> 
>    Selection: 0
> 
>    > obj$children  # works, should be identical to obj[[3]]
>    $comment
>    <!--A comment-->
> 
>    $foo
>    <foo x="1">
>     <element attrib1="my value"/>
>    ......
> 
> This shows that the XML package implements the "[[" method
> wrongly IMHO and also inconsistently with the "$" method.
> 
>>From a strict OOP view, the XML author could argue that
> this is not a bug in XML but rather str() which assumes that
> x[[length(x)]] works for objects of mode "list" even when they
> are not of *class* "list", but I hope he would still rather
> consider changing [[.XMLDocumentContent ...
> 


More likely, the appropriate fix is to have
length() return the relevant value.
I even recall considering this at the time of writing
the package initially.  But that was back in 1999/2000
and S4 and R/S-Plus compatibility were not what they
are now.  It could be changed.  Not certain when I will
get a chance.

 D.


> Martin
> 
>     Ashley> examining components eg 
>     >> str(xeg$doc$children,2)
> 
>     Ashley> List of 2
>     Ashley> $ comment: list()
>     Ashley> ..- attr(*, "class")= chr [1:2] "XMLComment" "XMLNode"
>     Ashley> etc 
> 
>     Ashley> is OK.
> 
>     Ashley> XML Version 1.4-1, 
>     Ashley> same behaviour on Windows and Linux, R version 2.4.1 (2006-12-18)
> 
> 
> 
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