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scan(n) Tcl Built-In Commands scan(n)
NAME
scan - Parse string using conversion specifiers in the style of sscanf
SYNOPSIS
scan string format ?varName varName ...?
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 con-
versions 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 vari-
ables 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 conversions 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 conver-
sion 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
conversion 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 conversion
specifier and stores the result in the variable given by the next argu-
ment 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 speci-
fiers 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 plat-
forms 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 plat-
forms 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 vari-
able.
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 con-
taining 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 pos-
sible for %o, and so on). The input field for a given conversion ter-
minates 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 var-
Name; 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.
EXAMPLES
Parse a simple color specification of the form #RRGGBB using hexadeci-
mal conversions with field sizes: set string "#08D03F" scan $string
"#%2x%2x%2x" r g b
Parse a HH:MM time string, noting that this avoids problems with octal
numbers by forcing interpretation as decimals (if we did not care, we
would use the %i conversion instead): set string "08:08" ;# *Not*
octal! if {[scan $string "%d:%d" hours minutes] != 2} {
error "not a valid time string" } # We have to understand numeric
ranges ourselves... if {$minutes < 0 || $minutes > 59} {
error "invalid number of minutes" }
Break a string up into sequences of non-whitespace characters (note the
use of the %n conversion so that we get skipping over leading white-
space correct): set string " a string {with braced words} + leading
space " set words {} while {[scan $string %s%n word length] == 2} {
lappend words $word
set string [string range $string $length end] }
Parse a simple coordinate string, checking that it is complete by look-
ing for the terminating character explicitly: set string "(5.2,-4e-2)"
# Note that the spaces before the literal parts of # the scan pattern
are significant, and that ")" is # the Unicode character \u0029 if {
[scan $string " (%f ,%f %c" x y last] != 3
|| $last != 0x0029 } then {
error "invalid coordinate string" } puts "X=$x, Y=$y"
SEE ALSO
format(n), sscanf(3)
KEYWORDS
conversion specifier, parse, scan
Tcl 8.4 scan(n)
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