How to return NA when nothing is found in an xpath?

Question

It is difficult to formulate the question, but with an example, it is simple to understand.

I use R to parse html code.

In the following, I have a html code call

Answer 1

library( 'XML' )  # load library
doc = htmlParse( html )  # parse html
# define xpath expression. div contains class = line, within which span has classes number and surface
xpexpr <- '//div[ @class = "line" ]'  

a1 <- lapply( getNodeSet( doc, xpexpr ), function( x ) { # loop through nodeset
      y <- xmlSApply( x, xmlValue, trim = TRUE )  # get xmlvalue
      names(y) <- xmlApply( x, xmlAttrs ) # get xmlattributes and assign it as names to y
      y   # return y
    } )

loop through a1 and extract values of number and surface and set names accordingly. Then column bind number and surface values

nm <- c( 'number', 'surface' )
do.call( 'cbind', lapply( a1, function( x ) setNames( x[ nm ], nm ) ) )
#                [,1]        [,2]       
# number  "Number 1"  NA         
# surface "Surface 1" "Surface 2"

Data:

html <- '

Number 1
Surface 1


Surface 2
'