| readChar | R Documentation |
Transfer character strings to and from connections, without assuming they are null-terminated on the connection.
readChar(con, nchars, useBytes = FALSE)
writeChar(object, con, nchars = nchar(object, type = "chars"),
eos = "", useBytes = FALSE)
con |
A connection object, or a character string naming a file, or a raw vector. |
nchars |
integer vector, giving the lengths in characters of
(unterminated) character strings to be read or written. Elements
must be >= 0 and not |
useBytes |
logical: For |
object |
A character vector to be written to the connection, at
least as long as |
eos |
‘end of string’: character string. The terminator
to be written after each string, followed by an ASCII |
These functions complement readBin and
writeBin which read and write C-style zero-terminated
character strings. They are for strings of known length, and
can optionally write an end-of-string mark. They are intended only
for character strings valid in the current locale.
These functions are intended to be used with binary-mode connections.
If con is a character string, the functions call
file to obtain a binary-mode file connection which is
opened for the duration of the function call.
If the connection is open it is read/written from its current position. If it is not open, it is opened for the duration of the call in an appropriate mode (binary read or write) and then closed again. An open connection must be in binary mode.
If readChar is called with con a raw vector, the data in
the vector is used as input. If writeChar is called with
con a raw vector, it is just an indication that a raw vector
should be returned.
Character strings containing ASCII nul(s) will be read
correctly by readChar but truncated at the first
nul with a warning.
If the character length requested for readChar is longer than
the data available on the connection, what is available is
returned. For writeChar if too many characters are requested
the output is zero-padded, with a warning.
Missing strings are written as NA.
For readChar, a character vector of length the number of
items read (which might be less than length(nchars)).
For writeChar, a raw vector (if con is a raw vector) or
invisibly NULL.
Earlier versions of R allowed embedded nul bytes within character
strings, but not R >= 2.8.0. readChar was commonly used to
read fixed-size zero-padded byte fields for which readBin was
unsuitable. readChar can still be used for such fields if
there are no embedded nuls: otherwise readBin(what = "raw")
provides an alternative.
nchars will be interpreted in bytes not characters in a
non-UTF-8 multi-byte locale, with a warning.
There is little validity checking of UTF-8 reads.
Using these functions on a text-mode connection may work but should
not be mixed with text-mode access to the connection, especially if
the connection was opened with an encoding argument.
The ‘R Data Import/Export’ manual.
connections, readLines,
writeLines, readBin
## test fixed-length strings
zzfil <- tempfile("testchar")
zz <- file(zzfil, "wb")
x <- c("a", "this will be truncated", "abc")
nc <- c(3, 10, 3)
writeChar(x, zz, nc, eos = NULL)
writeChar(x, zz, eos = "\r\n")
close(zz)
zz <- file(zzfil, "rb")
readChar(zz, nc)
readChar(zz, nchar(x)+3) # need to read the terminator explicitly
close(zz)
unlink(zzfil)
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