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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, including 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 general
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
values 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.Scanning.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 application 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) scanner , since it is a
formatted input function that reads from Scanning.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 cannot 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 function 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 argument 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 characters 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.conversion), -
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. Matching 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 current 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 character (delimiters and special escaped characters follow the lexical
conventions of OCaml). - f, e, E, g, G: reads an optionally signed floating-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 character 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
format string read must have the same type as the format string specification 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 behaviour is not mere format substitution, since the
conversion returns the format string read as additional argument. If you need pure format
substitution, use special flag _ to discard the extraneous argument: conversion %_\( fmt
%\) 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 number 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 follows
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 composed 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: regular expressions (module Str), stream
parsers, ocamllex-generated lexers, 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 differences 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 current
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 conversion fails, the scanning function aborts
and calls the error handling 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 lexical 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