Scanf(3) OCaml library Scanf(3)NAME
Scanf - Formatted input functions.
Module
Module Scanf
Documentation
Module Scanf
: sig end
Formatted input functions.
=== Introduction ===
=== Functional input with format strings ===
=== The module Scanf provides formatted input functions or scanners.
The formatted input functions can read from any kind of input, includ‐
ing strings, files, or anything that can return characters. The more
general source of characters is named a formatted input channel (or
scanning buffer) and has type Scanf.Scanning.in_channel. The more gen‐
eral formatted input function reads from any scanning buffer and is
named bscanf. Generally speaking, the formatted input functions have 3
arguments: - the first argument is a source of characters for the
input, - the second argument is a format string that specifies the val‐
ues to read, - the third argument is a receiver function that is
applied to the values read. Hence, a typical call to the formatted
input function Scanf.bscanf is bscanf ic fmt f, where: - ic is a source
of characters (typically a formatted input channel with type
Scanf.Scanning.in_channel), - fmt is a format string (the same format
strings as those used to print material with module Printf or Format),
- f is a function that has as many arguments as the number of values to
read in the input. ===
=== A simple example ===
=== As suggested above, the expression bscanf ic %d f reads a decimal
integer n from the source of characters ic and returns f n. For
instance, - if we use stdin as the source of characters (Scanf.Scan‐
ning.stdin is the predefined formatted input channel that reads from
standard input), - if we define the receiver f as let f x = x + 1, then
bscanf Scanning.stdin %d f reads an integer n from the standard input
and returns f n (that is n + 1). Thus, if we evaluate bscanf stdin %d
f, and then enter 41 at the keyboard, we get 42 as the final result.
===
=== Formatted input as a functional feature ===
=== The OCaml scanning facility is reminiscent of the corresponding C
feature. However, it is also largely different, simpler, and yet more
powerful: the formatted input functions are higher-order functionals
and the parameter passing mechanism is just the regular function appli‐
cation not the variable assignment based mechanism which is typical for
formatted input in imperative languages; the OCaml format strings also
feature useful additions to easily define complex tokens; as expected
within a functional programming language, the formatted input functions
also support polymorphism, in particular arbitrary interaction with
polymorphic user-defined scanners. Furthermore, the OCaml formatted
input facility is fully type-checked at compile time. ===
=== Formatted input channel ===
module Scanning : sig end
=== Type of formatted input functions ===
type ('a, 'b, 'c, 'd) scanner = ('a, Scanning.in_channel, 'b, 'c, 'a ->
'd, 'd) format6 -> 'c
The type of formatted input scanners: ('a, 'b, 'c, 'd) scanner is the
type of a formatted input function that reads from some formatted input
channel according to some format string; more precisely, if scan is
some formatted input function, then scan ic fmt f applies f to the
arguments specified by the format string fmt , when scan has read those
arguments from the formatted input channel ic .
For instance, the scanf function below has type ('a, 'b, 'c, 'd) scan‐
ner , since it is a formatted input function that reads from Scan‐
ning.stdin : scanf fmt f applies f to the arguments specified by fmt ,
reading those arguments from Pervasives.stdin as expected.
If the format fmt has some %r indications, the corresponding input
functions must be provided before the receiver f argument. For
instance, if read_elem is an input function for values of type t , then
bscanf ic %r; read_elem f reads a value v of type t followed by a ';'
character, and returns f v .
Since 3.10.0
exception Scan_failure of string
The exception that formatted input functions raise when the input can‐
not be read according to the given format.
=== The general formatted input function ===
val bscanf : Scanning.in_channel -> ('a, 'b, 'c, 'd) scanner
bscanf ic fmt r1 ... rN f reads arguments for the function f , from the
formatted input channel ic , according to the format string fmt , and
applies f to these values. The result of this call to f is returned as
the result of the entire bscanf call. For instance, if f is the func‐
tion fun s i -> i + 1 , then Scanf.sscanf x= 1 %s = %i f returns 2 .
Arguments r1 to rN are user-defined input functions that read the argu‐
ment corresponding to the %r conversions specified in the format
string.
=== Format string description ===
=== The format string is a character string which contains three types
of objects: - plain characters, which are simply matched with the char‐
acters of the input (with a special case for space and line feed, see
Scanf.space), - conversion specifications, each of which causes reading
and conversion of one argument for the function f (see Scanf.conver‐
sion), - scanning indications to specify boundaries of tokens (see
scanning Scanf.indication). ===
=== The space character in format strings ===
=== As mentioned above, a plain character in the format string is just
matched with the next character of the input; however, two characters
are special exceptions to this rule: the space character (' ' or ASCII
code 32) and the line feed character ('\n' or ASCII code 10). A space
does not match a single space character, but any amount of 'whitespace'
in the input. More precisely, a space inside the format string matches
any number of tab, space, line feed and carriage return characters.
Similarly, a line feed character in the format string matches either a
single line feed or a carriage return followed by a line feed. Match‐
ing any amount of whitespace, a space in the format string also matches
no amount of whitespace at all; hence, the call bscanf ib Price = %d $
(fun p -> p) succeeds and returns 1 when reading an input with various
whitespace in it, such as Price = 1 $, Price = 1 $, or even Price=1$.
===
=== Conversion specifications in format strings ===
=== Conversion specifications consist in the % character, followed by
an optional flag, an optional field width, and followed by one or two
conversion characters. The conversion characters and their meanings
are: - d: reads an optionally signed decimal integer. - i: reads an
optionally signed integer (usual input conventions for decimal (0-9+),
hexadecimal (0x[0-9a-f]+ and 0X[0-9A-F]+), octal (0o[0-7]+), and binary
(0b[0-1]+) notations are understood). - u: reads an unsigned decimal
integer. - x or X: reads an unsigned hexadecimal integer
([0-9a-fA-F]+). - o: reads an unsigned octal integer ([0-7]+). - s:
reads a string argument that spreads as much as possible, until the
following bounding condition holds: - a whitespace has been found (see
Scanf.space), - a scanning indication (see scanning Scanf.indication)
has been encountered, - the end-of-input has been reached. Hence, this
conversion always succeeds: it returns an empty string if the bounding
condition holds when the scan begins. - S: reads a delimited string
argument (delimiters and special escaped characters follow the lexical
conventions of OCaml). - c: reads a single character. To test the cur‐
rent input character without reading it, specify a null field width,
i.e. use specification %0c. Raise Invalid_argument, if the field width
specification is greater than 1. - C: reads a single delimited charac‐
ter (delimiters and special escaped characters follow the lexical con‐
ventions of OCaml). - f, e, E, g, G: reads an optionally signed float‐
ing-point number in decimal notation, in the style dddd.ddd e/E+-dd. -
F: reads a floating point number according to the lexical conventions
of OCaml (hence the decimal point is mandatory if the exponent part is
not mentioned). - B: reads a boolean argument (true or false). - b:
reads a boolean argument (for backward compatibility; do not use in new
programs). - ld, li, lu, lx, lX, lo: reads an int32 argument to the
format specified by the second letter for regular integers. - nd, ni,
nu, nx, nX, no: reads a nativeint argument to the format specified by
the second letter for regular integers. - Ld, Li, Lu, Lx, LX, Lo:
reads an int64 argument to the format specified by the second letter
for regular integers. - [ range ]: reads characters that matches one
of the characters mentioned in the range of characters range (or not
mentioned in it, if the range starts with ^). Reads a string that can
be empty, if the next input character does not match the range. The set
of characters from c1 to c2 (inclusively) is denoted by c1-c2. Hence,
%[0-9] returns a string representing a decimal number or an empty
string if no decimal digit is found; similarly,
%[\\048-\\057\\065-\\070] returns a string of hexadecimal digits. If a
closing bracket appears in a range, it must occur as the first charac‐
ter of the range (or just after the ^ in case of range negation); hence
[]] matches a ] character and [^]] matches any character that is not ].
Use %% and %@ to include a % or a @ in a range. - r: user-defined
reader. Takes the next ri formatted input function and applies it to
the scanning buffer ib to read the next argument. The input function ri
must therefore have type Scanning.in_channel -> 'a and the argument
read has type 'a. - { fmt %}: reads a format string argument. The for‐
mat string read must have the same type as the format string specifica‐
tion fmt. For instance, %{ %i %} reads any format string that can read
a value of type int; hence, if s is the string fmt:\ number is %u\"",
then Scanf.sscanf s fmt: %{%i%} succeeds and returns the format string
number is %u . - \( fmt %\): scanning sub-format substitution. Reads
a format string rf in the input, then goes on scanning with rf instead
of scanning with fmt. The format string rf must have the same type as
the format string specification fmt that it replaces. For instance, %(
%i %) reads any format string that can read a value of type int. The
conversion returns the format string read rf, and then a value read
using rf. Hence, if s is the string \ %4d\"1234.00", then Scanf.sscanf
s %(%i%) (fun fmt i -> fmt, i) evaluates to ("%4d", 1234). This behav‐
iour is not mere format substitution, since the conversion returns the
format string read as additional argument. If you need pure format sub‐
stitution, use special flag _ to discard the extraneous argument: con‐
version %_\( fmt %\) reads a format string rf and then behaves the same
as format string rf. Hence, if s is the string \ %4d\"1234.00", then
Scanf.sscanf s %_(%i%) is simply equivalent to Scanf.sscanf 1234.00 %4d
. - l: returns the number of lines read so far. - n: returns the num‐
ber of characters read so far. - N or L: returns the number of tokens
read so far. - !: matches the end of input condition. - %: matches
one % character in the input. - @: matches one @ character in the
input. - ,: does nothing. Following the % character that introduces a
conversion, there may be the special flag _: the conversion that fol‐
lows occurs as usual, but the resulting value is discarded. For
instance, if f is the function fun i -> i + 1, and s is the string x =
1 , then Scanf.sscanf s %_s = %i f returns 2. The field width is com‐
posed of an optional integer literal indicating the maximal width of
the token to read. For instance, %6d reads an integer, having at most
6 decimal digits; %4f reads a float with at most 4 characters; and
%8[\\000-\\255] returns the next 8 characters (or all the characters
still available, if fewer than 8 characters are available in the
input). Notes: - as mentioned above, a %s conversion always succeeds,
even if there is nothing to read in the input: in this case, it simply
returns . - in addition to the relevant digits, '_' characters may
appear inside numbers (this is reminiscent to the usual OCaml lexical
conventions). If stricter scanning is desired, use the range conversion
facility instead of the number conversions. - the scanf facility is
not intended for heavy duty lexical analysis and parsing. If it appears
not expressive enough for your needs, several alternative exists: regu‐
lar expressions (module Str), stream parsers, ocamllex-generated lex‐
ers, ocamlyacc-generated parsers. ===
=== Scanning indications in format strings ===
=== Scanning indications appear just after the string conversions %s
and %[ range ] to delimit the end of the token. A scanning indication
is introduced by a @ character, followed by some plain character c. It
means that the string token should end just before the next matching c
(which is skipped). If no c character is encountered, the string token
spreads as much as possible. For instance, %s@\t reads a string up to
the next tab character or to the end of input. If a @ character appears
anywhere else in the format string, it is treated as a plain character.
Note: - As usual in format strings, % and @ characters must be escaped
using %% and %@; this rule still holds within range specifications and
scanning indications. For instance, %s@%% reads a string up to the
next % character. - The scanning indications introduce slight differ‐
ences in the syntax of Scanf format strings, compared to those used for
the Printf module. However, the scanning indications are similar to
those used in the Format module; hence, when producing formatted text
to be scanned by !Scanf.bscanf, it is wise to use printing functions
from the Format module (or, if you need to use functions from Printf,
banish or carefully double check the format strings that contain '@'
characters). ===
=== Exceptions during scanning ===
=== Scanners may raise the following exceptions when the input cannot
be read according to the format string: - Raise Scanf.Scan_failure if
the input does not match the format. - Raise Failure if a conversion
to a number is not possible. - Raise End_of_file if the end of input
is encountered while some more characters are needed to read the cur‐
rent conversion specification. - Raise Invalid_argument if the format
string is invalid. Note: - as a consequence, scanning a %s conversion
never raises exception End_of_file: if the end of input is reached the
conversion succeeds and simply returns the characters read so far, or
if none were ever read. ===
=== Specialised formatted input functions ===
val fscanf : Pervasives.in_channel -> ('a, 'b, 'c, 'd) scanner
Same as Scanf.bscanf , but reads from the given regular input channel.
Warning: since all formatted input functions operate from a formatted
input channel, be aware that each fscanf invocation will operate with a
formatted input channel reading from the given channel. This extra
level of bufferization can lead to a strange scanning behaviour if you
use low level primitives on the channel (reading characters, seeking
the reading position, and so on).
As a consequence, never mix direct low level reading and high level
scanning from the same regular input channel.
val sscanf : string -> ('a, 'b, 'c, 'd) scanner
Same as Scanf.bscanf , but reads from the given string.
val scanf : ('a, 'b, 'c, 'd) scanner
Same as Scanf.bscanf , but reads from the predefined formatted input
channel Scanf.Scanning.stdin that is connected to Pervasives.stdin .
val kscanf : Scanning.in_channel -> (Scanning.in_channel -> exn -> 'd)
-> ('a, 'b, 'c, 'd) scanner
Same as Scanf.bscanf , but takes an additional function argument ef
that is called in case of error: if the scanning process or some con‐
version fails, the scanning function aborts and calls the error han‐
dling function ef with the formatted input channel and the exception
that aborted the scanning process as arguments.
=== Reading format strings from input ===
val bscanf_format : Scanning.in_channel -> ('a, 'b, 'c, 'd, 'e, 'f)
format6 -> (('a, 'b, 'c, 'd, 'e, 'f) format6 -> 'g) -> 'g
bscanf_format ic fmt f reads a format string token from the formatted
input channel ic , according to the given format string fmt , and
applies f to the resulting format string value. Raise Scan_failure if
the format string value read does not have the same type as fmt .
Since 3.09.0
val sscanf_format : string -> ('a, 'b, 'c, 'd, 'e, 'f) format6 -> (('a,
'b, 'c, 'd, 'e, 'f) format6 -> 'g) -> 'g
Same as Scanf.bscanf_format , but reads from the given string.
Since 3.09.0
val format_from_string : string -> ('a, 'b, 'c, 'd, 'e, 'f) format6 ->
('a, 'b, 'c, 'd, 'e, 'f) format6
format_from_string s fmt converts a string argument to a format string,
according to the given format string fmt . Raise Scan_failure if s ,
considered as a format string, does not have the same type as fmt .
Since 3.10.0
val unescaped : string -> string
Return a copy of the argument with escape sequences, following the lex‐
ical conventions of OCaml, replaced by their corresponding special
characters. If there is no escape sequence in the argument, still
return a copy, contrary to String.escaped.
Since 4.00.0
OCamldoc 2013-10-24 Scanf(3)
