Yeah of course you should close the file when done. I didn't give a
complete code snippet.
In any case, a quick glance at the documentation seems to imply that
opening a file as file('filename') will defer the choice of mode (i.e.
is it 'r', 'w', etc.?) until it is first used. In my case the first use
is a read, so it should presumably be set to "r". However as shown in my
examples this does work differently than opening it as
file('filename','r') in the first place.
I do agree that it is reasonable that the default behavior of
readLines/writeLines might be to reset the file offset each time, but I
certainly do not agree that that should be happening dependent upon
whether the original file is opened without 'r' and then read from
versus being opened with 'r' in the first place. That kind of a
side-effect really makes no sense to me and is entirely unintuitive. If
the goal was to have the default behavior to reset the file offset, a
reasonable thing would be to have a flag in readLines like
reset_fileoffset = TRUE or something like that.
In any case, thanks so much for the help!
Cheers,
Thomas
On 10/29/2014 12:59 PM, William Dunlap wrote:
I meant you should close the file when you are done with it, not after
every few lines.
File descriptors are a limited resource.
As for the rationale for the default behavior, there is a common use
pattern of reading
and parsing an entire file (or url, etc.), examining the results, and trying
again with a different parsing scheme. In that case the default
behavior works well.
In any case, I assume the behavior is documented in help("file").
Bill Dunlap
TIBCO Software
wdunlap tibco.com
On Wed, Oct 29, 2014 at 9:51 AM, Thomas Nyberg <tomnyb...@gmail.com> wrote:
Thanks for the response! I'd rather keep the file open than close it, since
it would flush the internal buffer. The whole reason I'm doing this is to
take advantage of the buffering and closing it would defeat the purpose.
I actually just found a solution which is to open the files with the "r"
flag explicitly. I.e. the following is what I want.
-----
bash $ echo 1 > testfile
bash $ echo 2 >> testfile
bash $ cat testfile
1
2
bash $ R
R > f <- file('testfile', 'r')
R > readLines(f, n = 1)
[1] "1"
R > readLines(f, n = 1)
[1] "2"
R > readLines(f, n = 1)
character(0)
-----
If you want to use writeLines in this same fashion you'll also need to open
the original file with the "w" as well.
It's very odd that file('filename') will let you read from it, but will not
act the same as file('filename', 'r') when it comes to readLines. Is this a
bug or is there some reasoning behind this? Regardless, it's certainly
extremely unintuitive.
Thanks again for the response!
Cheers,
Thomas
On 10/29/2014 12:22 PM, William Dunlap wrote:
Open your file object before calling readLines and close it when you
are done with
a sequence of calls to readLines.
> tf <- tempfile()
> cat(sep="\n", letters[1:10], file=tf)
> f <- file(tf)
> open(f)
> # or f <- file(tf, "r") instead of previous 2 lines
> readLines(f, n=1)
[1] "a"
> readLines(f, n=1)
[1] "b"
> readLines(f, n=2)
[1] "c" "d"
> close(f)
I/O operations on an unopened connection generally open it, do the
operation,
then close it.
Bill Dunlap
TIBCO Software
wdunlap tibco.com
On Wed, Oct 29, 2014 at 8:23 AM, Thomas Nyberg <tomnyb...@gmail.com>
wrote:
Hi everyone,
I would like to read a file line by line, but I would rather not load all
lines into memory first. I've tried using readLines with n = 1, but that
seems to reset the internal file descriptor's file offset after each
call.
I.e. this is the current behavior:
-------
bash $ echo 1 > testfile
bash $ echo 2 >> testfile
bash $ cat testfile
1
2
bash > R
R > f <- file('testfile')
R > readLines(f, n = 1)
[1] "1"
R > readLines(f, n = 1)
[1] "1"
-------
I would like the behavior to be:
-------
bash > R
R > f <- file('testfile')
R > readLines(f, n = 1)
[1] "1"
R > readLines(f, n = 1)
[1] "2"
-------
I'm coming to R from a python background, where the default behavior is
exactly the opposite. I.e. when you read a line from a file it is your
responsibility to use seek explicitly to get back to the original
position
in the file (this is rarely necessary though). Is there some flag to turn
off the default behavior of resetting the file offset in R?
Cheers,
Thomas
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