\HeaderA{scan}{Read Data Values}{scan}
\keyword{file}{scan}
\keyword{connection}{scan}
\begin{Description}\relax
Read data into a vector or list from the console or file.
\end{Description}
\begin{Usage}
\begin{verbatim}
scan(file = "", what = double(0), nmax = -1, n = -1, sep = "",
     quote = if(identical(sep, "\n")) "" else "'\"", dec = ".",
     skip = 0, nlines = 0, na.strings = "NA",
     flush = FALSE, fill = FALSE, strip.white = FALSE,
     quiet = FALSE, blank.lines.skip = TRUE, multi.line = TRUE,
     comment.char = "", allowEscapes = FALSE,
     encoding = "unknown")
\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{file}] the name of a file to read data values from.  If the
specified file is \code{""}, then input is taken from the keyboard
(or whatever \code{\LinkA{stdin}{stdin}()} reads if input is redirected or
\R{} is embedded).
(In this case input can be terminated by a blank line or an EOF
signal, \samp{Ctrl-D} on Unix and \samp{Ctrl-Z} on Windows.)

Otherwise, the file name is interpreted \emph{relative} to the
current working directory (given by \code{\LinkA{getwd}{getwd}()}),
unless it specifies an \emph{absolute} path.
Tilde-expansion is performed where supported.
When running \R{} from a script, \code{file="stdin"} can be used to
refer to the process's \code{stdin} file stream.

Alternatively, \code{file} can be a \code{\LinkA{connection}{connection}},
which will be opened if necessary, and if so closed at the end of
the function call.  Whatever mode the connection is opened in,
any of LF, CRLF or CR will be accepted as the EOL marker for a line
and so will match \code{sep = "\bsl{}n"}.

\code{file} can also be a complete URL.

To read a data file not in the current encoding (for example a
Latin-1 file in a UTF-8 locale or conversely) use a
\code{\LinkA{file}{file}} connection setting the \code{encoding} argument.

\item[\code{what}] the type of \code{what} gives the type of data to be read.
The supported types are \code{logical}, \code{integer}, \code{numeric},
\code{complex}, \code{character}, \code{raw} and \code{\LinkA{list}{list}}.
If \code{what} is a list, it is assumed that the lines of the data file
are records each containing \code{length(what)} items
(\sQuote{fields}) and the list components should have elements which
are one of the first six types listed or \code{NULL}, see section
\sQuote{Details} below.
\item[\code{nmax}] integer: the maximum number of data values to be read, or if
\code{what} is a list, the maximum number of records to be read.  If
omitted or not positive or an invalid value for an integer
(and \code{nlines} is not set to a positive value), \code{scan} will
read to the end of \code{file}.
\item[\code{n}] integer: the maximum number of data values to be read, defaulting to
no limit.  Invalid values will be ignored.
\item[\code{sep}] by default, scan expects to read white-space delimited
input fields.  Alternatively, \code{sep} can be used to specify a
character which delimits fields.  A field is always delimited by an
end-of-line marker unless it is quoted.

If specified this should be the empty character string (the default)
or \code{NULL} or a character string containing just one single-byte
character.

\item[\code{quote}] the set of quoting characters as a single character
string or \code{NULL}.  In a multibyte locale the quoting characters
must be ASCII (single-byte).
\item[\code{dec}] decimal point character.  This should be a character string
containing just one single-byte character.  (\code{NULL} and a
zero-length character vector are also accepted, and taken as the
default.)
\item[\code{skip}] the number of  lines of the input file to skip before
beginning to read data values.
\item[\code{nlines}] if positive, the maximum number of lines of data to be read.
\item[\code{na.strings}] character vector.  Elements of this vector are to be
interpreted as missing (\code{\LinkA{NA}{NA}}) values. Blank fields are
also considered to be missing values in logical, integer, numeric
and complex fields.
\item[\code{flush}] logical: if \code{TRUE}, \code{scan} will flush to the
end of the line after reading the last of the fields requested.
This allows putting comments after the last field, but precludes
putting more that one record on a line.
\item[\code{fill}] logical: if \code{TRUE}, \code{scan} will implicitly add
empty fields to any lines with fewer fields than implied by
\code{what}.
\item[\code{strip.white}] vector of logical value(s) corresponding to items
in the \code{what} argument. It is used only when \code{sep} has
been specified, and allows the stripping of leading and trailing
white space from \code{character} fields (\code{numeric} fields
are always stripped).

If \code{strip.white} is of length 1, it applies to all fields;
otherwise, if \code{strip.white[i]} is \code{TRUE} \emph{and} the
\code{i}-th field is of mode character (because \code{what[i]} is)
then the leading and trailing white space from field \code{i} is
stripped.
\item[\code{quiet}] logical: if \code{FALSE} (default), scan() will print a
line, saying how many items have been read.
\item[\code{blank.lines.skip}] logical: if \code{TRUE} blank lines in the
input are ignored, except when counting \code{skip} and \code{nlines}.
\item[\code{multi.line}] logical. Only used if \code{what} is a list.
If \code{FALSE}, all of a record must appear on one line (but more
than one record can appear on a single line).  Note that using
\code{fill = TRUE} implies that a record will terminated at the end
of a line.
\item[\code{comment.char}] character: a character vector of length one
containing a single character or an empty string.  Use \code{""} to
turn off the interpretation of comments altogether (the default).
\item[\code{allowEscapes}] logical.  Should C-style escapes such as
\samp{\bsl{}n} be processed (the default) or read verbatim?   Note that if
not within quotes these could be interpreted as a delimiter (but not
as a comment character).

The escapes which are interpreted are the control characters
\samp{\bsl{}a, \bsl{}b, \bsl{}f, \bsl{}n, \bsl{}r, \bsl{}t, \bsl{}v} and octal and
hexadecimal representations like \samp{\bsl{}040} and \samp{\bsl{}0x2A}.  Any
other escaped character is treated as itself, including backslash.

\item[\code{encoding}] encoding to be assumed for input strings.  If the
value is \code{"latin1"} or \code{"UTF-8"} 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 \code{con} or \emph{via}
\code{\LinkA{options}{options}(encoding=)}: see the example under
\code{\LinkA{file}{file}}.

\end{ldescription}
\end{Arguments}
\begin{Details}\relax
The value of \code{what} can be a list of types, in which case
\code{scan} returns a list of vectors with the types given by the
types of the elements in \code{what}.  This provides a way of reading
columnar data.  If any of the types is \code{NULL}, the corresponding
field is skipped (but a \code{NULL} component appears in the result).

The type of \code{what} or its components can be one of the six
atomic vector types or \code{NULL} (see \code{\LinkA{is.atomic}{is.atomic}}).

\sQuote{White space} is defined for the purposes of this function as
one or more contiguous characters from the set space, horizontal tab,
carriage return and line feed.  It does not include form feed or
vertical tab, but in Latin-1 and Windows 8-bit locales 'space'
includes non-breaking space.

Empty numeric fields are always regarded as missing values.
Empty character fields are scanned as empty character vectors, unless
\code{na.strings} contains \code{""} when they are regarded as missing
values.

The allowed input for a numeric field is optional whitespace followed
either \code{NA} or an optional sign followed by a decimal or
hexadecimal constant (see \LinkA{NumericConstants}{NumericConstants}), or \code{NaN},
\code{Inf} or \code{infinity} (ignoring case).  Out-of-range values
are recorded as \code{Inf}, \code{-Inf} or \code{0}.

For an integer field the allowed input is optional whitespace,
followed by either \code{NA} or an optional sign and one or more
digits (\samp{0-9}): all out-of-range values are converted to
\code{NA\_integer\_}.

If \code{sep} is the default (\code{""}), the character \samp{\bsl{}}
in a quoted string escapes the following character, so quotes may be
included in the string by escaping them.

If \code{sep} is non-default, the fields may be quoted in the style of
\file{.csv} files where separators inside quotes (\code{''} or
\code{""}) are ignored and quotes may be put inside strings by
doubling them.  However, if \code{sep = "\bsl{}n"} it is assumed
by default that one wants to read entire lines verbatim.

Quoting is only interpreted in character fields and in \code{NULL}
fields (which might be skipping character fields).

Note that since \code{sep} is a separator and not a terminator,
reading a file by \code{scan("foo", sep="\bsl{}n", blank.lines.skip=FALSE)}
will give an empty final line if the file ends in a linefeed and not if
it does not.  This might not be what you expected; see also
\code{\LinkA{readLines}{readLines}}.

If \code{comment.char} occurs (except inside a quoted character
field), it signals that the rest of the line
should be regarded as a comment and be discarded.  Lines beginning
with a comment character (possibly after white space with the default
separator) are treated as blank lines.

There is a check for a user interrupt every 1000 lines if \code{what}
is a list, otherwise every 10000 items.
\end{Details}
\begin{Value}
if \code{what} is a list, a list of the same length and same names (as
any) as \code{what}.

Otherwise, a vector of the type of \code{what}.

Character strings in the result will have a declared encoding if
\code{encoding} is \code{"latin1"} or \code{"UTF-8"}.
\end{Value}
\begin{Note}\relax
The default for \code{multi.line} differs from S.  To read one record
per line, use \code{flush = TRUE} and \code{multi.line = FALSE}.
(Note that quoted character strings can still include embedded newlines.)

If number of items is not specified, the internal
mechanism re-allocates memory in powers of two and so could use up
to three times as much memory as needed.  (It needs both old and new
copies.)  If you can, specify either \code{n} or \code{nmax} whenever
inputting a large vector, and \code{nmax} or \code{nlines} when
inputting a large list.

Using \code{scan} on an open connection to read partial lines can lose
chars: use an explicit separator to avoid this.

Having \code{nul} bytes in fields (including \samp{\bsl{}0} if
\code{allowEscapes = TRUE}) may lead to interpretation of the
field being terminated at the \code{nul}.  They not normally present
in text files -- see \code{\LinkA{readBin}{readBin}}.
\end{Note}
\begin{References}\relax
Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988)
\emph{The New S Language}.
Wadsworth \& Brooks/Cole.
\end{References}
\begin{SeeAlso}\relax
\code{\LinkA{read.table}{read.table}} for more user-friendly reading of data
matrices;
\code{\LinkA{readLines}{readLines}} to read a file a line at a time.
\code{\LinkA{write}{write}}.

\code{Quotes} for the details of C-style escape sequences.

\code{\LinkA{readChar}{readChar}} and \code{\LinkA{readBin}{readBin}} to read fixed or
variable length character strings or binary representations of numbers
a few at a time from a connection.
\end{SeeAlso}
\begin{Examples}
\begin{ExampleCode}
cat("TITLE extra line", "2 3 5 7", "11 13 17", file="ex.data", sep="\n")
pp <- scan("ex.data", skip = 1, quiet= TRUE)
scan("ex.data", skip = 1)
scan("ex.data", skip = 1, nlines=1) # only 1 line after the skipped one
scan("ex.data", what = list("","","")) # flush is F -> read "7"
scan("ex.data", what = list("","",""), flush = TRUE)
unlink("ex.data") # tidy up
\end{ExampleCode}
\end{Examples}

