readLines: Read Text Lines from a Connection

Description

Read some or all text lines from a connection.

Usage

readLines(con = stdin(), n = -1L, ok = TRUE, warn = TRUE,
          encoding = "unknown", skipNul = FALSE)

Arguments

con

a connection object or a character string.

integer. The (maximal) number of lines to read. Negative values indicate that one should read up to the end of input on the connection.

logical. Is it OK to reach the end of the connection before n > 0 lines are read? If not, an error will be generated.

warn

logical. Warn if a text file is missing a final EOL or if there are embedded nuls in the file.

encoding

encoding to be assumed for input strings. It is used to mark character strings as known to be in Latin-1 or UTF-8: it is not used to re-encode the input. To do the latter, specify the encoding as part of the connection con or via options(encoding=): see the examples. See also ‘Details’.

skipNul

logical: should nuls be skipped?

Value

A character vector of length the number of lines read.

The elements of the result have a declared encoding if encoding is "latin1" or "UTF-8",

Details

If the con is a character string, the function calls file to obtain a file connection which is opened for the duration of the function call. This can be a compressed file.

If the connection is open it is read from its current position. If it is not open, it is opened in "rt" mode for the duration of the call and then closed (but not destroyed; one must call close to do that).

If the final line is incomplete (no final EOL marker) the behaviour depends on whether the connection is blocking or not. For a non-blocking text-mode connection the incomplete line is pushed back, silently. For all other connections the line will be accepted, with a warning.

Whatever mode the connection is opened in, any of LF, CRLF or CR will be accepted as the EOL marker for a line.

Embedded nuls in the input stream will terminate the line currently being read, with a warning (unless skipNul = TRUE or warn = FALSE).

If con is a not-already-open connection with a non-default encoding argument, the text is converted to UTF-8 and declared as such (and the encoding argument to readLines is ignored). See the examples.

Examples

Run this code

# NOT RUN {
fil <- tempfile(fileext = ".data")
cat("TITLE extra line", "2 3 5 7", "", "11 13 17", file = fil,
    sep = "\n")
readLines(fil, n = -1)
unlink(fil) # tidy up

## difference in blocking
fil <- tempfile("test")
cat("123\nabc", file = fil)
readLines(fil) # line with a warning

con <- file(fil, "r", blocking = FALSE)
readLines(con) # empty
cat(" def\n", file = fil, append = TRUE)
readLines(con) # gets both
close(con)

unlink(fil) # tidy up

# }
# NOT RUN {
# read a 'Windows Unicode' file
A <- readLines(con <- file("Unicode.txt", encoding = "UCS-2LE"))
close(con)
unique(Encoding(A)) # will most likely be UTF-8
# }

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