如何从XML文件读取base64数值向量？

Question

I am trying to read the Ydata from an XML file. The Base64 string containing the Ydata is a 1245-elements numerical vector and is stored in data$ATR. The file has been encoded according to the gaml.org standards.

Tried too many things the whole day, but none is working... Will not post here all the countless things I tried and that does not work. I am out of ideas. How to convert it to R numeric vector?

library(XML)
x = XML::xmlTreeParse("http://utsav.podzone.net/T0011VAP1.0.xml")
xmltop = xmlRoot(x)
data = xmlSApply(xmltop, function(x) xmlSApply(x, xmlValue))
(data$ATR)

From an independent source, I know that the first 10 elemens of Ydata are :

-0.0903
-0.0751
-0.0605
-0.0471
-0.0353
-0.0249
-0.0159
-0.0082
-0.0017
 0.0035

Answer 1

I dont have access to that URL (company filtering), but this example may help:

The actual sample XML is

<root>
<value>123</value>
<value>234</value>
<value>345</value>
</root>

encoded as

PHJvb3Q+DQo8dmFsdWU+MTIzPC92YWx1ZT4NCjx2YWx1ZT4yMzQ8L3ZhbHVlPg0KPHZhbHVlPjM0NTwvdmFsdWU+DQo8L3Jvb3Q+

and the code to access the XML:

library("RCurl")
library("XML")
tmp <- "PHJvb3Q+DQo8dmFsdWU+MTIzPC92YWx1ZT4NCjx2YWx1ZT4yMzQ8L3ZhbHVlPg0KPHZhbHVlPjM0NTwvdmFsdWU+DQo8L3Jvb3Q+";
xml <- base64(tmp, encode=FALSE)

x = XML::xmlTreeParse(xml)
xmltop = xmlRoot(x)
data = xmlSApply(xmltop, function(x) xmlSApply(x, xmlValue))

Answer 2

That is one ugly file ;-) It looks to be output from these guys .

Using the numvalues from this XML tag in the data file:

<values byteorder="INTEL" format="FLOAT32" numvalues="1245">

if you do:

library(base64enc)

head(readBin(base64decode(data$ATR), "double", 1245, 4), 10)

I made the assumption that the 1245 for numvalues as the number of floating point #'s in the binary field—which turned out to be a good assumption.

it gives:

## [1] -0.090307593 -0.075070500 -0.060486197 -0.047122478 -0.035274029 -0.024934530 -0.015949965 -0.008214951
## [9] -0.001725793  0.003505349

That output tracks nicely with your 10 known elements.

The readBin call says to use double as the mode (data type) for the vector it's returning and the 4 is the # of bytes per element.

You may have to add endian="little" as a parameter to readBin depending on your architecture (at least I think that might be the case…I found readBin to be less than deterministic per-OS when using it to read color swatch files but I'm sure the CRAN guaRdians would berate me for questioning the behavior of it).