R help - Jul 2010 - Command that is conditional upon file retrieval: is it possible?

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 at 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????  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,
Andrew
-- 
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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 at 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