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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
according to fmt .
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, the result we get is 42 .
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 all the arguments specified by format string fmt , when scan has
read those arguments from the Scanf.Scanning.in_channel formatted input channel ic .
For instance, the Scanf.scanf function below has type ('a, 'b, 'c, 'd) scanner , since it
is a formatted input function that reads from Scanf.Scanning.stdin : scanf fmt f applies f
to the arguments specified by fmt , reading those arguments from stdin as expected.
If the format fmt has some %r indications, the corresponding formatted input functions
must be provided before receiver function f . 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
When the input can not be read according to the format string specification, formatted
input functions typically raise exception Scan_failure .
The general formatted input function
val bscanf : Scanning.in_channel -> ('a, 'b, 'c, 'd) scanner
bscanf ic fmt r1 ... rN f reads characters from the Scanf.Scanning.in_channel formatted
input channel ic and converts them to values according to format string fmt . As a final
step, receiver function f is applied to the values read and gives the result of the 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 ( 0-9 +).
- 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 .
- h , H : reads an optionally signed floating-point number in hexadecimal notation.
- 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, %[0-9a-f] 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,
format "%s@%%" reads a string up to the next % character, and format "%s@%@" reads a
string up to the next @ .
-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 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 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.
val ksscanf : string -> (Scanning.in_channel -> exn -> 'd) -> ('a, 'b, 'c, 'd) scanner
Same as Scanf.kscanf but reads from the given string.
Since 4.02.0
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.
Since 3.09.0
Raises Scan_failure if the format string value read does not have the same type as fmt .
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 .
Since 3.10.0
Raises Scan_failure if s , considered as a format string, does not have the same type as
fmt .
val unescaped : string -> string
unescaped s return a copy of s with escape sequences (according to the lexical conventions
of OCaml) replaced by their corresponding special characters. More precisely,
Scanf.unescaped has the following property: for all string s , Scanf.unescaped
(String.escaped s) = s .
Always return a copy of the argument, even if there is no escape sequence in the argument.
Since 4.00.0
Raises Scan_failure if s is not properly escaped (i.e. s has invalid escape sequences or
special characters that are not properly escaped). For instance, Scanf.unescaped "\""
will fail.
Deprecated
val fscanf : in_channel -> ('a, 'b, 'c, 'd) scanner
Deprecated.
Scanf.fscanf is error prone and deprecated since 4.03.0.
This function violates the following invariant of the Scanf module: To preserve scanning
semantics, all scanning functions defined in Scanf must read from a user defined
Scanf.Scanning.in_channel formatted input channel.
If you need to read from a in_channel input channel ic , simply define a
Scanf.Scanning.in_channel formatted input channel as in let ib = Scanning.from_channel ic
, then use Scanf.bscanf ib as usual.
val kfscanf : in_channel -> (Scanning.in_channel -> exn -> 'd) -> ('a, 'b, 'c, 'd) scanner
Deprecated.
Scanf.kfscanf is error prone and deprecated since 4.03.0.