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