Scanf(3o) OCaml library Scanf(3o)
Scanf - Formatted input functions.
Module Scanf
Module Scanf
: sig end
Formatted input functions.
=== Functional input with format strings. ===
=== The formatted input functions provided by module Scanf are func‐
tionals 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 mate‐
rial using module Printf or module Format). As an example, consider
the formatted 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 keyboard, 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 typi‐
cal 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 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 simply
the regular function application not the variable assigment based mech‐
anism 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 arbitrary
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 for‐
matted 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) scan‐
ner , 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 provided 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 ’;’ character.
=== 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 argu‐
ment 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 spe‐
cial 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 con‐
version 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 hex‐
adecimal ( 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 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 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 notation, in the style dddd.ddd e/E+-dd .
- F : reads a floating point number according to the lexical conven‐
tions 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 for‐
mat 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 men‐
tioned 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 ] .
- 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 characters
(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 succeeds
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. How‐
ever, 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 contain ’@’ characters).
-in addition to relevant digits, ’_’ characters may appear inside num‐
bers (this is reminiscent 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 expressions (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) 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 scanning buffer and the exception that
aborted the scanning process.
val bscanf_format : Scanning.scanbuf -> (’a, ’b, ’c, ’d, ’e, ’f) for‐
mat6 -> ((’a, ’b, ’c, ’d, ’e, ’f) format6 -> ’g) -> ’g
bscanf_format ib fmt f reads a format string token from the scannning
buffer ib , 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 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)