scan {base}R Documentation

Read Data Values

Description

Read data into a vector or list from the console or file.

Usage

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 = TRUE)

Arguments

file the name of a file to read data values from. If the specified file is "", then input is taken from the keyboard (or stdin if input is redirected). (In this case input can be terminated by a blank line or an EOF signal, Ctrl-D on Unix and Ctrl-Z on Windows.)
Otherwise, the file name is interpreted relative to the current working directory (given by getwd()), unless it specifies an absolute path. Tilde-expansion is performed where supported.
Alternatively, file can be a 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 sep = "\n".
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 file connection setting the encoding argument.
what the type of what gives the type of data to be read. If what is a list, it is assumed that the lines of the data file are records each containing length(what) items (“fields”). The supported types are logical, integer, numeric, complex, character, raw and list: list values should have elements which are one of the first six types listed or NULL.
nmax the maximum number of data values to be read, or if what is a list, the maximum number of records to be read. If omitted or not positive (and nlines is not set to a positive value), scan will read to the end of file.
n the maximum number of data values to be read, defaulting to no limit.
sep by default, scan expects to read white-space delimited input fields. Alternatively, 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 NULL or a character string containing just one single-byte character.
quote the set of quoting characters as a single character string or NULL. In a multibyte locale the quoting characters must be ASCII (single-byte).
dec decimal point character. This should be a character string containing just one single-byte character. (NULL and a zero-length character vector are also accepted, and taken as the default.)
skip the number of lines of the input file to skip before beginning to read data values.
nlines if positive, the maximum number of lines of data to be read.
na.strings character vector. Elements of this vector are to be interpreted as missing (NA) values.
flush logical: if TRUE, 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.
fill logical: if TRUE, scan will implicitly add empty fields to any lines with fewer fields than implied by what.
strip.white vector of logical value(s) corresponding to items in the what argument. It is used only when sep has been specified, and allows the stripping of leading and trailing white space from character fields (numeric fields are always stripped).
If strip.white is of length 1, it applies to all fields; otherwise, if strip.white[i] is TRUE and the i-th field is of mode character (because what[i] is) then the leading and trailing white space from field i is stripped.
quiet logical: if FALSE (default), scan() will print a line, saying how many items have been read.
blank.lines.skip logical: if TRUE blank lines in the input are ignored, except when counting skip and nlines.
multi.line logical. Only used if what is a list. If FALSE, all of a record must appear on one line (but more than one record can appear on a single line). Note that using fill = TRUE implies that a record will terminated at the end of a line.
comment.char character: a character vector of length one containing a single character or an empty string. Use "" to turn off the interpretation of comments altogether (the default).
allowEscapes logical. Should C-style escapes such as \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).

Details

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

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

‘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, vertical tab or other non-ASCII space characters.

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

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

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

Quoting is only interpreted in character fields, and as from R 1.8.0 in NULL fields (which might be skipping character fields).

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

If 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.

As from R 2.1.0, scan attempts to share storage with character strings that have already been read in the call. If an upper bound on the number of character strings cannot be deduced from nmax or n, sharing is used for the first 10000 unique strings which are read in.

Value

if what is a list, a list of the same length and same names (as any) as what.
Otherwise, a vector of the type of what.

Note

The default for multi.line differs from S. To read one record per line, use flush = TRUE and multi.line = FALSE.

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 n or nmax whenever inputting a large vector, and nmax or nlines when inputting a large list.

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

References

Becker, R. A., Chambers, J. M. and Wilks, A. R. (1988) The New S Language. Wadsworth & Brooks/Cole.

See Also

read.table for more user-friendly reading of data matrices; readLines to read a file a line at a time. write.

Examples

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

[Package base version 2.1.0 Index]