R help - Oct 2009 - User input when running R code in batch mode

Hi

I've been stumbling over a simple issue that undoubtedly has an easy
solution.  I need to have some way for a user to enter some values
into a data frame which R will then work on.  I know that data entry
should ideally be done otherwise and I should use R only for the
computation, but R's data manipulation abilities makes it efficient
for me to write the entire code in it.

When I ran R line-by-line, I did:
----
> a <- scan (what='character',n=1)
1: abcd
Read 1 item
> a
[1] "abcd"
----
Everything works fine.

But if I save the line "a <- scan (what='character', n=1);
a" as
`test.r' and run the file in command line, I get:
----
$ r --vanilla < test.r
> a <- scan(what='character',n=1); a
1: Read 0 items
character(0)
----
Now it's not working.

I read what others had suggested on earlier threads and tried
different functions, including readLines().  In line-by-line, I wrote
the same lines as earlier, except using `readLines(con=stdin(),n=1)'
instead of `scan(what='character', n=1)'.  It worked fine.

Yet, when I tried the modified lines (with readLines() instead of
scan()) in batch mode, R didn't wait for user input and said
`character(0)'.

Is there any way to make R stop for the user to enter values when
running in batch mode either by changing the way I invoke scan() or
readLines() or by using any other function?

Thanks in advance
-- 
Kaushik Krishnan
(kaushik.s.krishnan at gmail.com)

On Tue, Oct 27, 2009 at 9:20 AM, Kaushik Krishnan
<kaushik.s.krishnan at gmail.com> wrote:
> $ r --vanilla < test.r
>> a <- scan(what='character',n=1); a
> 1: Read 0 items
> character(0)
> ----
> Now it's not working.
 Assuming this is a unix environment, the syntax '< test.r' means
'my
standard input stream is the file test.r'. That's not what you want.
Give R the file name as an argument and let the standard input stream
remain user input:

$ r --vanilla  test.r
1: hello
Read 1 item
[1] "hello"

 Note that this is 'r' and not 'R'. For me this comes from the
'littler' package in Ubuntu Linux. The same thing with 'R'
doesn't
work:

$ R --vanilla  test.r
ARGUMENT 'test.r' __ignored__
[banner]
 >  [the R prompt appears]

 Maybe there's a way of doing this with big R, but I think littler is
designed for this kind of thing.
> Is there any way to make R stop for the user to enter values when
> running in batch mode either by changing the way I invoke scan() or
> readLines() or by using any other function?
 An alternative is to use the tcltk package to make a dialog for user input.

Barry

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Kaushik Krishnan wrote:
> Is there any way to make R stop for the user to enter values when
> running in batch mode either by changing the way I invoke scan() or
> readLines() or by using any other function?
At least on linux this works:

bernd at t40:~/r-test $ cat test.R

readline2 = function(prompt=""){
        cat(prompt)
        readLines("stdin", 1)
}

print(readline2("enter a number: "))

bernd at t40:~/r-test $ Rscript test.R
enter a number: 42
[1] "42"


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