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

It's easier to select the enclosing tag (the div here) for each, and look for each tag inside. With rvest and purrr, which I find simpler,

library(rvest)
library(purrr)

html %>% read_html() %>% 
    html_nodes('.line') %>% 
    map_df(~list(number = .x %>% html_node('.number') %>% html_text(), 
                 surface = .x %>% html_node('.surface') %>% html_text()))

#> # A tibble: 2 × 2
#>     number   surface
#>           
#> 1 Number 1 Surface 1
#> 2      Surface 2