[R] XML and str
Duncan Temple Lang
duncan at wald.ucdavis.edu
Sat Feb 10 16:18:30 CET 2007
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",
> 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"
>  "list"
> > is.list(obj) # TRUE
>  TRUE
> > length(obj) # 3
>  3
> > obj[] # ---> the error you see above.
> Error in obj$children[[...]] : subscript out of bounds
> Enter a frame number, or 0 to exit
> 1: obj[]
> 2: `[[.XMLDocumentContent`(obj, 3)
> Selection: 0
> > obj$children # works, should be identical to obj[]
> <!--A comment-->
> <foo x="1">
> <element attrib1="my value"/>
> This shows that the XML package implements the "[[" method
> wrongly IMHO and also inconsistently with the "$" method.
>>From a strict OOP view, the XML author could argue that
> this is not a bug in XML but rather str() which assumes that
> x[[length(x)]] works for objects of mode "list" even when they
> are not of *class* "list", but I hope he would still rather
> consider changing [[.XMLDocumentContent ...
More likely, the appropriate fix is to have
length() return the relevant value.
I even recall considering this at the time of writing
the package initially. But that was back in 1999/2000
and S4 and R/S-Plus compatibility were not what they
are now. It could be changed. Not certain when I will
get a chance.
> Ashley> examining components eg
> >> str(xeg$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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