Data Mashups in R

The Many Ways to Philly (Latitude)

Using Data Structures

Using the indexing list notation from R we can get to the nodes we need

  > lat<-xmlResult[['doc']][['ResultSet']][['Result']][['Latitude']][['text']]
  > long<-xmlResult[['doc']][['ResultSet']][['Result']][['Longitude']][['text']]

  > lat
  39.951405 

looks good, but if we examine this further

  > str(lat)
  list()
  - attr(*, "class")= chr [1:5] "XMLTextNode" "XMLNode" "RXMLAbstractNode" "XML...

Although it has a decent display value this variable still considers itself an XMLNode and contains no index to obtain raw leaf value we want—the descriptor just says list() instead of something we can use (like $lat). We’re not quite there yet.

Using Helper Methods

Fortunately, the XML package offers a method to access the leaf value: xmlValue

  > lat<-xmlValue(xmlResult[['doc']][['ResultSet']][['Result']][['Latitude']]) 

  > str(lat)
  chr "39.951405" 

Using Internal Class Methods

There are usually multiple ways to accomplish the same task in R. Another means to get to this our character lat/long data is to use the “value” method provided by the node itself

  > lat<-xmlResult[['doc']][['ResultSet']][['Result']][['Latitude']][['text']]$value

If we were really clever we would have understood that XML doc class provided us with useful methods all the way down! Try neurotically holding down the tab key after typing

  > lat<-xmlResult$ (now hold down the tab key)

  xmlResult$doc  xmlResult$dtd  
  (let's go with doc and start looking for more methods using $)

 > lat<-xmlResult$doc$ ...