R help - Sep 2006 - 404 HTTP not found

Hi

I wrote a script which retrieves links from websites and loads them with scan:

...
website<-tolower(scan(current.pages[i], what="character",
sep="\n", quiet=TRUE))
...

However occasionally, the script finds broken links, such as
<http://www.google.com/test>. when the script tries to access such
websites, the repeat loop breaks and I get the error message

Error in file(file, "r") : unable to open connection
In addition: Warning message:
cannot open: HTTP status was '404 Not Found' 

Now my question: is there a way to test whether the target of a link exists that
does not result in an error and, thus, discontinues my loop? I looked at the
help files for files, scans, connections, and did a search for "404?'
in th archives but couldn't find anything. I work with R 2.3.1 patched on
Windows XP (both Home and Prof) and would appreciate any pointers ...
Thanks a lot,
STG

See ?try as in this example:

current.pages <- c("http://www.google.com",
"http://www.google.com/test",
  "http://www.yahoo.com")

for(i in seq(along = current.pages)) {
	website <- try(tolower(scan(current.pages[i],
		what="character", sep="\n", quiet=TRUE)))
	if (inherits(website, "try-error")) cat(current.pages[i],
"bad\n")
	else cat(current.pages[i], "ok\n")
}



On 9/18/06, Stefan Th. Gries <stgries_lists at arcor.de>
wrote:
> Hi
>
> I wrote a script which retrieves links from websites and loads them with
scan:
>
> ...
> website<-tolower(scan(current.pages[i], what="character",
sep="\n", quiet=TRUE))
> ...
>
> However occasionally, the script finds broken links, such as
<http://www.google.com/test>. when the script tries to access such
websites, the repeat loop breaks and I get the error message
>
> Error in file(file, "r") : unable to open connection
> In addition: Warning message:
> cannot open: HTTP status was '404 Not Found'
>
> Now my question: is there a way to test whether the target of a link exists
that does not result in an error and, thus, discontinues my loop? I looked at
the help files for files, scans, connections, and did a search for
"404?' in th archives but couldn't find anything. I work with R
2.3.1 patched on Windows XP (both Home and Prof) and would appreciate any
pointers ...
> Thanks a lot,
> STG
>
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

See

?try
?tryCatch

On Mon, 18 Sep 2006, Stefan Th. Gries wrote:
> Hi
>
> I wrote a script which retrieves links from websites and loads them with
scan:
>
> ...
> website<-tolower(scan(current.pages[i], what="character",
sep="\n", quiet=TRUE))
> ...
>
> However occasionally, the script finds broken links, such as
<http://www.google.com/test>. when the script tries to access such
websites, the repeat loop breaks and I get the error message
>
> Error in file(file, "r") : unable to open connection
> In addition: Warning message:
> cannot open: HTTP status was '404 Not Found'
>
> Now my question: is there a way to test whether the target of a link exists
that does not result in an error and, thus, discontinues my loop? I looked at
the help files for files, scans, connections, and did a search for
"404?' in th archives but couldn't find anything. I work with R
2.3.1 patched on Windows XP (both Home and Prof) and would appreciate any
pointers ...
> Thanks a lot,
> STG
-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595