R dataframe from XML when values are multiple or missing

Question

This question is similar to a previous question, Import all fields (and subfields) of XML as dataframe, but I want to pull out only a subset of the XML data and want to incl

Answer 1

Assuming the XML data is in a file called world.xml read it in and iterate over the cities extracting the city name and the bname of any associated landmarks :

library(XML)
doc <- xmlParse("world.xml", useInternalNodes = TRUE)

do.call(rbind, xpathApply(doc, "/world/city", function(node) {

   city <- xmlValue(node[["name"]])

   xp <- "./buildings/building[./type/text()='landmark']/bname"
   landmark <- xpathSApply(node, xp, xmlValue)
   if (is.null(landmark)) landmark <- NA

   data.frame(city, landmark, stringsAsFactors = FALSE)

}))

The result is:

      city     landmark
1   London Tower Bridge
2 New York         
3    Paris Eiffel Tower
4    Paris       Louvre