trycatch only returns warning and not error in R

Question

I am running the exact same code as in the accepted answer here: How to write trycatch in R. However, I only get a warning message as output for the third link ("xxxxx"). In the quoted answer, the error and the warning message are shown. Also NULL is returned in my case instead of NA.

What is going on here? What am I doing wrong? I tried it in R Version 4.1.2 and 4.0.2.

The code is:

urls <- c(
  "http://stat.ethz.ch/R-manual/R-devel/library/base/html/connections.html",
  "http://en.wikipedia.org/wiki/Xz",
  "xxxxx"
)
readUrl <- function(url) {
  out <- tryCatch(
    {
      # Just to highlight: if you want to use more than one 
      # R expression in the "try" part then you'll have to 
      # use curly brackets.
      # 'tryCatch()' will return the last evaluated expression 
      # in case the "try" part was completed successfully
      
      message("This is the 'try' part")
      
      readLines(con=url, warn=FALSE) 
      # The return value of `readLines()` is the actual value 
      # that will be returned in case there is no condition 
      # (e.g. warning or error). 
      # You don't need to state the return value via `return()` as code 
      # in the "try" part is not wrapped inside a function (unlike that
      # for the condition handlers for warnings and error below)
    },
    error=function(cond) {
      message(paste("URL does not seem to exist:", url))
      message("Here's the original error message:")
      message(cond)
      # Choose a return value in case of error
      return(NA)
    },
    warning=function(cond) {
      message(paste("URL caused a warning:", url))
      message("Here's the original warning message:")
      message(cond)
      # Choose a return value in case of warning
      return(NULL)
    },
    finally={
      # NOTE:
      # Here goes everything that should be executed at the end,
      # regardless of success or error.
      # If you want more than one expression to be executed, then you 
      # need to wrap them in curly brackets ({...}); otherwise you could
      # just have written 'finally=<expression>' 
      message(paste("Processed URL:", url))
      message("Some other message at the end")
    }
  )    
  return(out)
}

y <- lapply(urls, readUrl)

The console output is:

> y <- lapply(urls, readUrl)
This is the 'try' part
Processed URL: http://stat.ethz.ch/R-manual/R-devel/library/base/html/connections.html
Some other message at the end
This is the 'try' part
Processed URL: http://en.wikipedia.org/wiki/Xz
Some other message at the end
This is the 'try' part
URL caused a warning: xxxxx
Here's the original warning message:
kann Datei 'xxxxx' nicht öffnen: No such file or directoryProcessed URL: xxxxx
Some other message at the end

Investigating y:

> head(y[[1]])
[1] "<!DOCTYPE html PUBLIC \"-//W3C//DTD XHTML 1.0 Strict//EN\" \"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd\"><html xmlns=\"http://www.w3.org/1999/xhtml\"><head><title>R: Functions to Manipulate Connections (Files, URLs, ...)</title>"
[2] "<meta http-equiv=\"Content-Type\" content=\"text/html; charset=utf-8\" />"                                                                                                                                                                   
[3] "<meta name=\"viewport\" content=\"width=device-width, initial-scale=1.0, user-scalable=yes\" />"                                                                                                                                             
[4] "<link rel=\"stylesheet\" type=\"text/css\" href=\"R.css\" />"                                                                                                                                                                                
[5] "</head><body><div class=\"container\">"                                                                                                                                                                                                      
[6] ""                                                                                                                                                                          
> length(y)
[1] 3
> y[[3]]
NULL

trycatch only returns warning and not error in R

0 Answers0