r - Using tryCatch and rvest to deal with 404 and other crawling errors

Question

When retrieving the h1 title using rvest, I sometimes run into 404 pages. This stop the process and returns this error.

Error in open.connection(x, "rb") : HTTP error 404.

See the example below

Data<-data.frame(Pages=c(
"http://boingboing.net/2016/06/16/spam-king-sanford-wallace.html",
"http://boingboing.net/2016/06/16/omg-the-japanese-trump-commer.html",
"http://boingboing.net/2016/06/16/omar-mateen-posted-to-facebook.html",
"http://boingboing.net/2016/06/16/omar-mateen-posted-to-facdddebook.html"))

Code used to retrieve h1

library (rvest)
sapply(Data$Pages, function(url){
 url %>%
 as.character() %>% 
 read_html() %>% 
 html_nodes('h1') %>% 
 html_text()
 })

Is there a way to include an argument to ignore errors and continue the process ?

Answer 1

You're looking for try or tryCatch, which are how R handles error catching.

With try, you just need to wrap the thing that might fail in try(), and it will return the error and keep running:

library(rvest)

sapply(Data$Pages, function(url){
  try(
    url %>%
      as.character() %>% 
      read_html() %>% 
      html_nodes('h1') %>% 
      html_text()
  )
})

# [1] "'Spam King' Sanford Wallace gets 2.5 years in prison for 27 million Facebook scam messages"
# [2] "OMG, this Japanese Trump Commercial is everything"                                         
# [3] "Omar Mateen posted to Facebook during Orlando mass shooting"                               
# [4] "Error in open.connection(x, "rb") : HTTP error 404.
"  

However, while that will get everything, it will also insert bad data into our results. tryCatch allows you to configure what happens when an error is called by passing it a function to run when that condition arises:

sapply(Data$Pages, function(url){
  tryCatch(
    url %>%
      as.character() %>% 
      read_html() %>% 
      html_nodes('h1') %>% 
      html_text(), 
    error = function(e){NA}    # a function that returns NA regardless of what it's passed
  )
})

# [1] "'Spam King' Sanford Wallace gets 2.5 years in prison for 27 million Facebook scam messages"
# [2] "OMG, this Japanese Trump Commercial is everything"                                         
# [3] "Omar Mateen posted to Facebook during Orlando mass shooting"                               
# [4] NA  

There we go; much better.

---

Update

In the tidyverse, the purrr package offers two functions, safely and possibly, which work like try and tryCatch. They are adverbs, not verbs, meaning they take a function, modify it so as to handle errors, and return a new function (not a data object) which can then be called. Example:

library(tidyverse)
library(rvest)

df <- Data %>% rowwise() %>%     # Evaluate each row (URL) separately
    mutate(Pages = as.character(Pages),    # Convert factors to character for read_html
           title = possibly(~.x %>% read_html() %>%    # Try to take a URL, read it,
                                html_nodes('h1') %>%    # select header nodes,
                                html_text(),    # and collect text inside.
                            NA)(Pages))    # If error, return NA. Call modified function on URLs.

df %>% select(title)
## Source: local data frame [4 x 1]
## Groups: <by row>
## 
## # A tibble: 4 × 1
##                                                                                        title
##                                                                                        <chr>
## 1 'Spam King' Sanford Wallace gets 2.5 years in prison for 27 million Facebook scam messages
## 2                                          OMG, this Japanese Trump Commercial is everything
## 3                                Omar Mateen posted to Facebook during Orlando mass shooting
## 4                                                                                       <NA>