[R] Command that is conditional upon file retrieval: is it possible?

[David Winsemius]

On Jul 21, 2010, at 5:33 PM, AndrewPage wrote:

>
> Hi all,
>
> I'm currently working on an R program where I have to access an FTP  
> server
> to download some of the data I need.  However, the people who post  
> up the
> files I access are at times inconsistent with regards to time  
> posted, if
> they post at all, etc....  Here's some of the code I use:
>
> library(RCurl)
>
> url1 = paste("ftp://user:password@a.great.website.com/", "file", num1,
> ".csv", sep = "")
>
> data1 = getURL(url1)
>
> write(data1, file = paste("inMyFolder", num1, ".csv", sep = ""))
>
>
> Sometimes this process works perfectly, and sometimes I get an error  
> message
> like this attached to "data1 = getURL(url1)":
>
> Error in curlPerform(curl = curl, .opts = opts, .encoding  
> = .encoding) :
>  RETR response: 550
>
> That's because that particular file isn't on the FTP server yet.   
> Now...
> let's just imagine that there's another way for me to access the file
> elsewhere, and I can drag it into the working directory with the  
> same name
> as the file I'm telling R to write immediately after finding it on  
> the FTP
> server.
>
> So here's my question: is it possible to write a command that will  
> write the
> file if there isn't an error message going along with "data1 =
> getURL(url1)", but won't write the file if it can't find it????

?try

Untested in absence of workable example...

Return <- try(getURL(url1), silent=TRUE) # silent keeps error from  
showing up on console
if(grep("550", Return[1]) { # not sure about exact structure of error  
message
                             # but could do nothing
                  } else { # assign data
                          data1 <- Return
if(("550", Return[1]) { # go to the other site
           Return2 <- try( getURL(url2), silent =TRUE ) # and so on



> As of right
> now, if I got the error message, dragged the file into the working  
> directory
> and ran the program again, R will overwrite my good file with an  
> empty one
> because in all cases, I'm telling it to write a file with that name  
> that
> includes the information in "data1".
>
> Thanks in advance,

Search help archives for try examples or perhaps try-error or tryCatch  
which might be more specific.

> Andrew
> -- 
-- 

David Winsemius, MD
West Hartford, CT