How to create xml from R objects, e.g., is there a 'listToXml' function?

Question

R's XML package has an xmlToList function, but does not have the reverse, is there a function for R that will convert a list to an XML object?

I would like something like

listToXML(list('a'))

that returns

<a></a>

but the closest I can find is

library(XML)
xmlNode(list('a'))

which returns

</a>

help on this question, and understanding the conversion of R objects to XML in general appreciated (the XML package appears more focused on the use of R to read XML, with less support for creating XML).

Update... One reason that I could not figure this out is because I did not realize that the trailing '/' in <node/> indicates an empty node, equivalent to <node></node>

x = newXMLNode("bob"); addChildren(x, newXMLNode("el", "Red", "Blue", "Green", attrs = c(lang ="en"))) does what you're looking for? — Roman Luštrik
– Roman Luštrik, Commented Jun 6, 2011 at 18:08
@Roman, that helps, but it does not do much to simplify the conversion of a list to XML. Mostly it did help me to figure out that <node/> = <node></node> and thereby better understand how the XML package works. — David LeBauer
– David LeBauer, Commented Jun 6, 2011 at 19:30

gung - Reinstate Monica · Accepted Answer · 2017-08-30 15:46:51Z

16

The function newXMLNode does what you need, i.e., write XML output. See the detailed help and examples in ?newXMLNode for more details. Here is a short extract:

library(XML)    
top = newXMLNode("a")
newXMLNode("b", attrs=c(x=1, y='abc'), parent=top)
newXMLNode("c", "With some text", parent=top)
top

Resulting in:

<a>
  <b x="1" y="abc"/>
  <c>With some text</c>
</a>

edited Aug 30, 2017 at 15:46

gung - Reinstate Monica

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answered Jun 6, 2011 at 18:11

Andrie

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Comments

Jeff Allen · Accepted Answer · 2012-01-05 20:56:40Z

16

I am surprised that no function already exists for this -- surely there's something already packaged out there.

Regardless, I use the following script to accomplish this:

root <- newXMLNode("root")
li <- list(a = list(aa = 1, ab=2), b=list(ba = 1, bb= 2, bc =3))
listToXML <- function(node, sublist){
    for(i in 1:length(sublist)){
        child <- newXMLNode(names(sublist)[i], parent=node);

        if (typeof(sublist[[i]]) == "list"){
            listToXML(child, sublist[[i]])
        }
        else{
            xmlValue(child) <- sublist[[i]]
        }
    } 
}
listToXML(root, li)

You can use the XML::saveXML() function to grab this as a character, if you prefer.

edited Jan 5, 2012 at 20:56

answered Jan 5, 2012 at 20:27

Jeff Allen

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2 Comments

symbolrush Over a year ago

This looks promising to me. Still I don't manage to get it up and running. If I run your example I end up with nothing printed on my console. If I replace the last line with xml <- listToXML(root, li) the code runs, but xml is NULL in the end. I'm not so much into the XML package, but it seems obviously to me, that you're function doesn't return much, because it doesn't keep track of the xml object as it fills up. Can you help me further on this one? Can you provide a minimal example that returns an xml object as @Andrie 's answer does? Thanks in Advance

HFBrowning Over a year ago

I can't believe after all this time no one caught the bug that made this code not work! In reply @symbolrush - there needed to be a return statement. The xml object you were looking for was node - it's almost as if it's overwritten or modified in place (not sure which one) as you go along and add child elements.

David LeBauer · Accepted Answer · 2015-01-09 17:11:38Z

6

Here is the listToXML function that we ended up creating

At first I adapted the answer by @Jeff

listToXml <- function(item, tag){
  if(typeof(item)!='list')
    return(xmlNode(tag, item))
  xml <- xmlNode(tag)
  for(name in names(item)){
    xml <- append.xmlNode(xml, listToXml(item[[name]], name))
  }
  return(xml)
}

But since the function has benefited from further development:

##' Convert List to XML
##'
##' Can convert list or other object to an xml object using xmlNode
##' @title List to XML
##' @param item 
##' @param tag xml tag
##' @return xmlNode
##' @export
##' @author David LeBauer, Carl Davidson, Rob Kooper
listToXml <- function(item, tag) {
  # just a textnode, or empty node with attributes
  if(typeof(item) != 'list') {
    if (length(item) > 1) {
      xml <- xmlNode(tag)
      for (name in names(item)) {
        xmlAttrs(xml)[[name]] <- item[[name]]
      }
      return(xml)
    } else {
      return(xmlNode(tag, item))
    }
  }

  # create the node
  if (identical(names(item), c("text", ".attrs"))) {
    # special case a node with text and attributes
    xml <- xmlNode(tag, item[['text']])
  } else {
    # node with child nodes
    xml <- xmlNode(tag)
    for(i in 1:length(item)) {
      if (names(item)[i] != ".attrs") {
        xml <- append.xmlNode(xml, listToXml(item[[i]], names(item)[i]))
      }
    }    
  }

  # add attributes to node
  attrs <- item[['.attrs']]
  for (name in names(attrs)) {
    xmlAttrs(xml)[[name]] <- attrs[[name]]
  }
  return(xml)
}

edited Jan 9, 2015 at 17:11

answered Jan 9, 2015 at 17:01

David LeBauer

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3 Comments

Chris S. Over a year ago

This is useful, but please check the xmlToList example. listToXml(xmlToList(xmlParse(tt)), "x") almost returns the same object as xmlParse(tt) except node b is 1 instead of .

David LeBauer Over a year ago

@ChrisS. thanks for your comment - in the specific application I wrote this for, the information is in the first format (1) which was determined by legacy Fortran code.

Julien Colomb Over a year ago

would be very nice to add it to the XML package... thanks

Julien Colomb · Accepted Answer · 2020-01-17 12:19:04Z

needed to make some changes to the function for cases where there are multiple elements (i.e. lists with no name entry):

##' Convert List to XML
##'
##' Can convert list or other object to an xml object using xmlNode
##' @title List to XML
##' @param item 
##' @param tag xml tag
##' @return xmlNode
##' @export
##' @author David LeBauer, Carl Davidson, Rob Kooper, julien colomb
listToXml <- function(item, tag) {
  # just a textnode, or empty node with attributes
  if(typeof(item) != 'list') {
    if (length(item) > 1) {
      xml <- xmlNode(tag)
      for (name in names(item)) {
        xmlAttrs(xml)[[name]] <- item[[name]]
      }
      return(xml)
    } else {
      return(xmlNode(tag, item))
    }
  }

  # create the node
  if (identical(names(item), c("text", ".attrs"))) {
    # special case a node with text and attributes
    xml <- xmlNode(tag, item[['text']])
  } else {
    # node with child nodes
    xml <- xmlNode(tag)
    for(i in 1:length(item)) {
      if (length (item[[i]]) == 0) {}
      else if (names(item)[i] != ".attrs") {
        print(i)
        if (is.null (names(item[[i]][1])) ){
          print(i)
          for (j in c(1:length (item[[i]]))){
            child <- xmlNode(names(item)[i])
            xmlValue(child) <- item[[i]][j]
            xml <- append.xmlNode(xml,child)
          }
        } else {
          xml <- append.xmlNode(xml, listToXml(item[[i]], names(item)[i]))
        }

      }
    }    
  }

  # add attributes to node
  attrs <- item[['.attrs']]
  for (name in names(attrs)) {
    xmlAttrs(xml)[[name]] <- attrs[[name]]
  }
  return(xml)
}

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