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Scanf(3o) OCaml library Scanf(3o)
NAME
Scanf - Formatted input functions.
Module
Module Scanf
Documentation
Module Scanf
: sig end
Formatted input functions.
=== Functional input with format strings. ===
=== The formatted input functions provided by module Scanf are functionals that apply
their function argument to the values they read in the input. The specification of the
values to read is simply given by a format string (the same format strings as those used
to print material using module Printf or module Format). As an example, consider the for-
matted input function scanf that reads from standard input; a typical call to scanf is
simply scanf fmt f, meaning that f should be applied to the arguments read according to
the format string fmt. For instance, if f is defined as let f x = x + 1, then scanf %d f
will read a decimal integer i from stdin and return f i; thus, if we enter 41 at the key-
board, scanf %d f evaluates to 42. This module provides general formatted input functions
that read from any kind of input, including strings, files, or anything that can return
characters. Hence, a typical call to a formatted input function bscan is bscan ib fmt f,
meaning that f should be applied to the arguments read from input ib, according to the
format string fmt. The Caml scanning facility is reminiscent of the corresponding C fea-
ture. However, it is also largely different, simpler, and yet more powerful: the format-
ted input functions are higher-order functionals and the parameter passing mechanism is
simply the regular function application not the variable assigment based mechanism which
is typical of formatted input in imperative languages; the format strings also feature
useful additions to easily define complex tokens; as expected of a functional programming
language feature, the formatted input functions support polymorphism, in particular arbi-
trary interaction with polymorphic user-defined scanners. Furthermore, the Caml formatted
input facility is fully type-checked at compile time. ===
module Scanning : sig end
Scanning buffers
exception Scan_failure of string
The exception raised by formatted input functions when the input cannot be read according
to the given format.
type ('a, 'b, 'c, 'd) scanner = ('a, Scanning.scanbuf, '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 scanning buffer according to some format string; more
precisely, if scan is some formatted input function, then scan ib fmt f applies f to the
arguments specified by the format string fmt , when scan has read those arguments from
some scanning buffer ib .
For instance, the scanf function below has type ('a, 'b, 'c, 'd) scanner , since it is a
formatted input function that reads from stdib : 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 input functions must be pro-
vided before the f argument. For instance, if read_elem is an input function for values of
type t , then bscanf ib %r; read_elem f reads a value of type t followed by a ';' charac-
ter.
=== Formatted input functions ===
val bscanf : Scanning.scanbuf -> ('a, 'b, 'c, 'd) scanner
bscanf ib fmt r1 ... rN f reads arguments for the function f from the scanning buffer ib
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 bscanf . 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 a %r conversion.
The format is a character string which contains three types of objects:
-plain characters, which are simply matched with the characters of the input,
-conversion specifications, each of which causes reading and conversion of one argument
for f ,
-scanning indications to specify boundaries of tokens.
Among plain characters the space character (ASCII code 32) has a special meaning: it
matches ``whitespace'', that is any number of tab, space, line feed and carriage return
characters. Hence, a space in the format matches any amount of whitespace in the input.
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 formats for hexadecimal ( 0x[d]+ and
0X[d]+ ), octal ( 0o[d]+ ), and binary 0b[d]+ notations are understood).
- u : reads an unsigned decimal integer.
- x or X : reads an unsigned hexadecimal integer.
- o : reads an unsigned octal integer.
- s : reads a string argument that spreads as much as possible, until the next white
space, the next scanning indication, or the end-of-input is reached. Hence, this conver-
sion 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 Caml).
- 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 Caml).
- f , e , E , g , G : reads an optionally signed floating-point number in decimal nota-
tion, in the style dddd.ddd e/E+-dd .
- F : reads a floating point number according to the lexical conventions of Caml (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 (decimal, hexadecimal, etc).
- nd , ni , nu , nx , nX , no : reads a nativeint argument to the format specified by the
second letter.
- Ld , Li , Lu , Lx , LX , Lo : reads an int64 argument to the format specified by the
second letter.
- [ 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 char-
acters 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 ] .
- 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.scanbuf -> 'a and the argument read has type 'a .
- { fmt %} : reads a format string argument to the format specified by the internal format
fmt . The format string to be read must have the same type as the internal format fmt .
For instance, "%{%i%}" reads any format string that can read a value of type int ; hence
Scanf.sscanf fmt:\\\ number is %u\\\"" fmt:%{%i%} succeeds and returns the format string
number is %u .
- \( fmt %\) : scanning format substitution. Reads a format string to replace fmt . The
format string read must have the same type as fmt .
- 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.
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 , then Scanf.sscanf x = 1 %_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 char-
acters (or all the characters still available, if fewer than 8 characters are available in
the input).
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 constant 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 scanning indication @c does not follow a string conversion, it is
treated as a plain c character.
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. As a consequence, scanning a %s conversion
never raises exception End_of_file : if the end of input is reached the conversion suc-
ceeds and simply returns the characters read so far, or if none were read.
Raise Invalid_argument if the format string is invalid.
Notes:
-the scanning indications introduce slight differences in the syntax of Scanf format
strings compared to those used by the Printf module. However, scanning indications are
similar to those of the Format module; hence, when producing formatted text to be scanned
by !Scanf.bscanf , it is wise to use printing functions from Format (or, if you need to
use functions from Printf , banish or carefully double check the format strings that con-
tain '@' characters).
-in addition to relevant digits, '_' characters may appear inside numbers (this is remi-
niscent to the usual Caml 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 expres-
sions (module Str ), stream parsers, ocamllex -generated lexers, ocamlyacc -generated
parsers.
val fscanf : Pervasives.in_channel -> ('a, 'b, 'c, 'd) scanner
Same as Scanf.bscanf , but reads from the given channel.
Warning: since all formatted input functions operate from a scanning buffer, be aware that
each fscanf invocation must allocate a new fresh scanning buffer (unless you make careful
use of partial application). Hence, there are chances that some characters seem to be
skipped (in fact they are pending in the previously used scanning buffer). This happens in
particular when calling fscanf again after a scan involving a format that necessitated
some look ahead (such as a format that ends by skipping whitespace in the input).
To avoid confusion, consider using bscanf with an explicitly created scanning buffer. Use
for instance Scanning.from_file f to allocate the scanning buffer reading from file f .
This method is not only clearer it is also faster, since scanning buffers to files are
optimized for fast buffered reading.
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 scanning buffer Scanf.Scanning.stdib
that is connected to stdin .
val kscanf : Scanning.scanbuf -> (Scanning.scanbuf -> exn -> 'a) -> ('b, 'c, 'd, 'a) scan-
ner
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 scanning buffer and the exception that
aborted the scanning process.
val bscanf_format : Scanning.scanbuf -> ('a, 'b, 'c, 'd, 'e, 'f) format6 -> (('a, 'b, 'c,
'd, 'e, 'f) format6 -> 'g) -> 'g
bscanf_format ib fmt f reads a format string token from the scannning buffer ib , accord-
ing to the given format string fmt , and applies f to the resulting format string value.
Raise Scan_failure if the format string value read doesn't 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.
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,
doesn't have the same type as fmt .
OCamldoc 2008-05-19 Scanf(3o)
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On Apache
Under GNU General Public License
2012-05-25 07:54 @38.107.179.236 Crawled by CCBot/1.0 (+http://www.commoncrawl.org/bot.html)