[R] How to check for existence url from within a function?

[Duncan Murdoch]

On 26/05/2007 7:13 AM, Heinz Tuechler wrote:
> Dear All,
> 
> To check if an url exists, I can use try(). This works, as I expected, if I
> do it directly, as in the first part of the following example, but I could
> not find a way to do it from within a function, as in the second part.
> 
> Where could I find information on how to do this?
> 
> Thanks,
> Heinz
> 
> 
> ## set nonexisting url
> url.string <- 'http://www.google.at/nonexist.html'
> 
> ## first part
> 1 # to start with defined .Last.value
> try(con.url <- url(url.string, open='rb'))
> class.try.res <- class(.Last.value)
> try.error <- class.try.res== 'try-error'
> print(try.error)      # TRUE
> try(close(con.url))
> 
> ## try() within a function
> url.error <- function(url.string) {
>   1 # to start with defined .Last.value
>   try(con.url <- url(url.string, open='rb'))
>   class.try.res <- class(.Last.value)
>   try.error <- class.try.res== 'try-error'

.Last.value isn't set until your function returns.  You should write this as

con.url <- try(url(url.string, open='rb'))
try.error <- inherits(con.url, "try-error")

Notice that I used "inherits", rather than testing for equality.  It's 
documented that the result of try() will be "of class 'try-error'" if an 
error occurs, but there may be circumstances (in the future?) where 
different types of errors are signalled by using a more complicated class.

Duncan Murdoch

>   print(try.error)
>   try(close(con.url))
>   invisible(try.error)
> }
> 
> ## call the function
> url.error(url.string)   # result -> FALSE
> 
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