How can I determine if try returned an error or not?

Question

I am trying to do the following:

try(htmlParse(ip[1], T),

where I define a as:

ip[1] = paste('http://en.wikipedia.org/wiki/George_Clooney')

I want to check if the htmlParse worked or not. For many names in my list, there will be no wikipedia sites and thus I need to be able to check and replace ip[1] with NA if the wiki pages does not exist.

Can someone please advise how I can do that. I tried using the command geterrmessage(), however I am not sure how to flush that everytime I change the name of the celebrity.

Currently I have the following:

if(!isTRUE(as.logical(grep(ip[1],err)))) {
    ip[1] = NA
    }
    else {

This is definately incorrect as it is not running the logical statement I want.

Thanks

Amar

Answer 1

This simple example should help you out, I think:

res <- try(log("a"),silent = TRUE)
class(res) == "try-error"
[1] TRUE

The basic idea is the try returns (invisibly) an object of class "try-error" when there's an error. Otherwise, res will contain the result of the expression you pass to try. i.e.

res <- try(log(2),silent = TRUE)
res
[1] 0.6931472

Spend some time reading ?try carefully, including the examples (which aren't as simple as they could be, I guess). As GSee notes below, a more idiomatic way to check if an error is thrown is to use inherits(res,'try-error').

Answer 2

I would try to download all the names (existing or not) from wiki and save it in separate files.I would then grep the following string Wikipedia does not have an article with this exact name and for the non-existing ones I would get a TRUE value. In this way I believe you'll make sure whether the parser worked or the name didn't exist. Additionally you can sort the downloaded files based on their size in case you are suspecting that something went wrong. Corrupted ones have smaller size.

Additionally I would use tryCatch function in order to treat the logical status:

x<-3
tryCatch(x>5,error=print("this is an error"))