scan(n) Tcl Built-In Commands scan(n)
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NAME
scan - Parse string using conversion specifiers in the style of sscanf
SYNOPSIS
scan string format ?varName varName ...?
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INTRODUCTION
This command parses fields from an input string in the same fashion as the ANSI C sscanf procedure
and returns a count of the number of conversions performed, or -1 if the end of the input string is
reached before any conversions have been performed. String gives the input to be parsed and format
indicates how to parse it, using % conversion specifiers as in sscanf. Each varName gives the name
of a variable; when a field is scanned from string the result is converted back into a string and
assigned to the corresponding variable. If no varName variables are specified, then scan works in an
inline manner, returning the data that would otherwise be stored in the variables as a list. In the
inline case, an empty string is returned when the end of the input string is reached before any con-versions conversions
versions have been performed.
DETAILS ON SCANNING
Scan operates by scanning string and format together. If the next character in format is a blank or
tab then it matches any number of white space characters in string (including zero). Otherwise, if
it isn't a % character then it must match the next character of string. When a % is encountered in
format, it indicates the start of a conversion specifier. A conversion specifier contains up to four |
fields after the %: a *, which indicates that the converted value is to be discarded instead of |
assigned to a variable; a XPG3 position specifier; a number indicating a maximum field width; a field |
size modifier; and a conversion character. All of these fields are optional except for the conver-sion conversion
sion character. The fields that are present must appear in the order given above.
When scan finds a conversion specifier in format, it first skips any white-space characters in string
(unless the specifier is [ or c). Then it converts the next input characters according to the con-version conversion
version specifier and stores the result in the variable given by the next argument to scan.
If the % is followed by a decimal number and a $, as in ``%2$d'', then the variable to use is not
taken from the next sequential argument. Instead, it is taken from the argument indicated by the
number, where 1 corresponds to the first varName. If there are any positional specifiers in format
then all of the specifiers must be positional. Every varName on the argument list must correspond to
exactly one conversion specifier or an error is generated, or in the inline case, any position can be
specified at most once and the empty positions will be filled in with empty strings.
The following conversion characters are supported:
d The input field must be a decimal integer. It is read in and the value is stored in the
variable as a decimal string. If the l or L field size modifier is given, the scanned |
value will have an internal representation that is at least 64-bits in size.
o The input field must be an octal integer. It is read in and the value is stored in the
variable as a decimal string. If the l or L field size modifier is given, the scanned |
value will have an internal representation that is at least 64-bits in size. If the value |
exceeds MAX_INT (017777777777 on platforms using 32-bit integers when the l and L modifiers |
are not given), it will be truncated to a signed integer. Hence, 037777777777 will appear |
as -1 on a 32-bit machine by default.
x The input field must be a hexadecimal integer. It is read in and the value is stored in the
variable as a decimal string. If the l or L field size modifier is given, the scanned |
value will have an internal representation that is at least 64-bits in size. If the value |
exceeds MAX_INT (0x7FFFFFFF on platforms using 32-bit integers when the l and L modifiers |
are not given), it will be truncated to a signed integer. Hence, 0xFFFFFFFF will appear as |
-1 on a 32-bit machine.
u The input field must be a decimal integer. The value is stored in the variable as an
unsigned decimal integer string. If the l or L field size modifier is given, the scanned |
value will have an internal representation that is at least 64-bits in size.
i The input field must be an integer. The base (i.e. decimal, octal, or hexadecimal) is
determined in the same fashion as described in expr. The value is stored in the variable
as a decimal string. If the l or L field size modifier is given, the scanned value will |
have an internal representation that is at least 64-bits in size.
c A single character is read in and its binary value is stored in the variable as a decimal
string. Initial white space is not skipped in this case, so the input field may be a
white-space character. This conversion is different from the ANSI standard in that the
input field always consists of a single character and no field width may be specified.
s The input field consists of all the characters up to the next white-space character; the
characters are copied to the variable.
e or f or g
The input field must be a floating-point number consisting of an optional sign, a string of
decimal digits possibly containing a decimal point, and an optional exponent consisting of
an e or E followed by an optional sign and a string of decimal digits. It is read in and
stored in the variable as a floating-point string.
[chars] The input field consists of any number of characters in chars. The matching string is
stored in the variable. If the first character between the brackets is a ] then it is
treated as part of chars rather than the closing bracket for the set. If chars contains a
sequence of the form a-b then any character between a and b (inclusive) will match. If the
first or last character between the brackets is a -, then it is treated as part of chars
rather than indicating a range.
[^chars] The input field consists of any number of characters not in chars. The matching string is
stored in the variable. If the character immediately following the ^ is a ] then it is
treated as part of the set rather than the closing bracket for the set. If chars contains
a sequence of the form a-b then any character between a and b (inclusive) will be excluded
from the set. If the first or last character between the brackets is a -, then it is
treated as part of chars rather than indicating a range.
n No input is consumed from the input string. Instead, the total number of characters
scanned from the input string so far is stored in the variable.
The number of characters read from the input for a conversion is the largest number that makes sense
for that particular conversion (e.g. as many decimal digits as possible for %d, as many octal digits
as possible for %o, and so on). The input field for a given conversion terminates either when a
white-space character is encountered or when the maximum field width has been reached, whichever
comes first. If a * is present in the conversion specifier then no variable is assigned and the next
scan argument is not consumed.
DIFFERENCES FROM ANSI SSCANF
The behavior of the scan command is the same as the behavior of the ANSI C sscanf procedure except
for the following differences:
[1] %p conversion specifier is not currently supported.
[2] For %c conversions a single character value is converted to a decimal string, which is then
assigned to the corresponding varName; no field width may be specified for this conversion.
[3] The h modifier is always ignored and the l and L modifiers are ignored when converting real |
values (i.e. type double is used for the internal representation).
[4] If the end of the input string is reached before any conversions have been performed and no
variables are given, an empty string is returned.
SEE ALSO
format(n), sscanf(3)
KEYWORDS
conversion specifier, parse, scan
Tcl 8.4 scan(n)
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