Parse an XML file and return an R character vector

Question

I've parsed an XML document with R, e.g:

library(XML)
f = system.file("exampleData", "mtcars.xml", package="XML")
doc = xmlParse(f)

Using XPath expressions, I can select specific nodes in the document:

> getNodeSet(doc, "//record[@id='Mazda RX4']/text()")
[[1]]
   21.0   6 160.0 110 3.90 2.620 16.46  0  1    4    4 

    attr(,"class")
    [1] "XMLNodeSet"

But I can't figure out how to turn the result into an R character vector:

> as.character(getNodeSet(doc, "//record[@id='Mazda RX4']/text()"))
[1] "<pointer: 0x000000000e6a7fe0>"

How do I get text from an internal pointer to a C object?

Answer 1

Use xmlValue. Here's an extension of your example to help you see what the classes are:

v <- getNodeSet(doc, "//record[@id='Mazda RX4']/text()")
str(v)
#List of 1
#$ :Classes 'XMLInternalTextNode', 'XMLInternalNode', 'XMLAbstractNode' <externalptr> 
#- attr(*, "class")= chr "XMLNodeSet"
v2 <- sapply(v, xmlValue)  #this is the code chunk of interest to you
v2
#[1] "   21.0   6 160.0 110 3.90 2.620 16.46  0  1    4    4"
str(v2)
#chr "   21.0   6 160.0 110 3.90 2.620 16.46  0  1    4    4"

Answer 2

The following will also work: Instead of getNodeSet() and sapply(v,xmlValue), you can use xpathApply and add xmlValue as an argument

doc = xmlParse(f)
xpathApply(doc,"//record[@id='Mazda RX4']/text()")

[[1]]
   21.0   6 160.0 110 3.90 2.620 16.46  0  1    4    4 

attr(,"class")
[1] "XMLNodeSet"

xpathApply(doc,"//record[@id='Mazda RX4']/text()",xmlValue)

[[1]]
[1] "   21.0   6 160.0 110 3.90 2.620 16.46  0  1    4    4"

This is a character object in a list. You can transform it into a vector of numeric objects by unlisting, splitting the string with regex of one or more spaces, unlisting again and as.numeric()

 as.numeric(unlist(strsplit(unlist(v)," +")))
 [1]     NA  21.00   6.00 160.00 110.00   3.90   2.62  16.46   0.00   1.00   4.00   4.00