Clarification for readChar man page
Duncan Murdoch wrote:
On 6/14/2007 10:49 AM, Jeffrey Horner wrote:
Hi, Here's a patch to the readChar manual page (R-trunk as of today) that better clarifies readChar's return value.
Your update is not right. For example:
x <- as.raw(32:96)
readChar(x, nchars=rep(2,100))
This returns a character vector of length 100, of which the first 32
elements have 2 chars, the next one has 1, and the rest are "".
So the length of nchars really does affect the length of the value.
Now, I haven't looked at the code, but it's possible we could delete the
"(which might be less than \code{length(nchars)})" remark, and if not,
it would be useful to explain the situations in which the return value
could be shorter than the nchars vector.
Well, this is rather a misunderstanding on my part; I completely forgot about vectorization. The manual page makes sense to me now. But the situation about the return value possibly being less than length(nchars) isn't clear. Consider a 101 byte text file in a non-multibyte character locale: f <- tempfile() writeChar(paste(rep(seq(0,9),10),collapse=''),con=f) and calling readChar() to read 100 bytes with length(nchar)=10: > readChar(f,nchar=rep(10,10)) [1] "0123456789" "0123456789" "0123456789" "0123456789" "0123456789" [6] "0123456789" "0123456789" "0123456789" "0123456789" "0123456789" and readChar() reading the entire file with length(nchar)=11: > readChar(f,nchar=rep(10,11)) [1] "0123456789" "0123456789" "0123456789" "0123456789" "0123456789" [6] "0123456789" "0123456789" "0123456789" "0123456789" "0123456789" [11] "\0" but the following two outputs are confusing. readchar() with length(nchar)>=12 returns a character vector length 12: > readChar(f,nchar=rep(10,12)) [1] "0123456789" "0123456789" "0123456789" "0123456789" "0123456789" [6] "0123456789" "0123456789" "0123456789" "0123456789" "0123456789" [11] "\0" "" > readChar(f,nchar=rep(10,13)) [1] "0123456789" "0123456789" "0123456789" "0123456789" "0123456789" [6] "0123456789" "0123456789" "0123456789" "0123456789" "0123456789" [11] "\0" "" It seems that the first time EOF is encountered on a read operation, an empty string is returned, but on subsequent reads nothing is returned. Is this intended behavior? Jeff
Duncan Murdoch It could use some work as I'd
also like to add some text about using nchar() to find the length of
the string that readchar() returns, but I'm unsure which of
type="bytes" or type="chars" to mention. Is it type="chars"?
Index: src/library/base/man/readChar.Rd
===================================================================
--- src/library/base/man/readChar.Rd (revision 41943)
+++ src/library/base/man/readChar.Rd (working copy)
@@ -57,8 +57,8 @@
}
\value{
- For \code{readChar}, a character vector of length the number of
- items read (which might be less than \code{length(nchars)}).
+ For \code{readChar}, a character vector of length 1 with the number
+ of characters less than or equal to nchars.
For \code{writeChar}, a raw vector (if \code{con} is a raw vector) or
invisibly \code{NULL}.
Jeff