End of File for binary files
On 24/01/2010 12:15 PM, Henrik Bengtsson wrote:
On Sun, Jan 24, 2010 at 3:56 AM, Duncan Murdoch <murdoch at stats.uwo.ca> wrote:
On 23/01/2010 10:40 PM, rn00b wrote:
I am using readBin to continuously read characters from the binary file.
I'm
trying to figure out how many characters are in the file. What I would
like
to do is something like
(while! EOF)
{
charRead <-.Internal(readBin(con,"character",1L,NA,TRUE,swap))
i++
}
I'm not clear on how to determine the EOF condition in this case.
You should not be calling .Internal. It's for internal use, subject to change, etc. Using readBin(...) you can detect EOF by reading fewer than n items when you ask for n.
Is this safe? Is EOF the only case where readBin() returns fewer
elements than you requested? Does it depend on the type of connection
you are reading from? Does it depend on OS?
The help("readBin") does not say much about this, but it says:
"If readBin(what = character()) is used incorrectly on a file which
does not contain C-style character strings, warnings (usually many)
are given. From a file or connection, the input will be broken into
pieces of length 10000 with any final part being discarded."
which seems to suggest that you (at least in special cases) can get
fewer items than requested without hitting EOF. EOF behavior might be
document elsewhere in R.
I read the passage above to say I might get more than n strings from a file containing n of them if they are too long; I don't see it saying that I would ever get fewer than n if there are n properly terminated strings remaining in the file. EOF is not the only case in general where you'd get fewer than n strings (e.g. a non-blocking connection will only return what's in the buffer), but in the usual case of reading from a file, it should be safe. If you are worried about some particular case, check the source code. It's always the final arbiter. Duncan Murdoch
/Henrik
So the loop would be something like
while (length(charRead <- readBin(con, "character")) > 0) {
i <- i + 1
}
Duncan Murdoch
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