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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 substrings from an input string in a fashion similar to the ANSI C sscanf proce-dure procedure
dure 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 for-mat format
mat indicates how to parse it, using % conversion specifiers as in sscanf. Each varName gives the
name of a variable; when a substring is scanned from string that matches a conversion specifier, the
substring is 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 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 is not 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 XPG3 position specifier (or a * to indicate the converted value is to be dis-carded discarded
carded instead of assigned to any variable); a number indicating a maximum substring width; a 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 conversion character 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 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 speci-fied specified
fied at most once and the empty positions will be filled in with empty strings.
The size modifier field is used only when scanning a substring into one of Tcl's integer values. The |
size modifier field dictates the integer range acceptable to be stored in a variable, or, for the |
inline case, in a position in the result list. The syntactically valid values for the size modifier |
are h, L, l, and ll. The h size modifier value is equivalent to the absence of a size modifier in |
the the conversion specifier. Either one indicates the integer range to be stored is limited to the |
same range produced by the int() function of the expr command. The L size modifier is equivalent to |
the l size modifier. Either one indicates the integer range to be stored is limited to the same range |
produced by the wide() function of the expr command. The ll size modifier indicates that the integer |
range to be stored is unlimited.
The following conversion characters are supported:
d The input substring must be a decimal integer. It is read in and the integer value is
stored in the variable, truncated as required by the size modifier value.
o The input substring must be an octal integer. It is read in and the integer value is stored
in the variable, truncated as required by the size modifier value.
x The input substring must be a hexadecimal integer. It is read in and the integer value is
stored in the variable, truncated as required by the size modifier value.
u The input substring must be a decimal integer. The integer value is truncated as required
by the size modifier value, and the corresponding unsigned value for that truncated range
is computed and stored in the variable as a decimal string. The conversion makes no sense
without reference to a truncation range, so the size modifier ll is not permitted in combi-nation combination
nation with conversion character u.
i The input substring must be an integer. The base (i.e. decimal, binary, octal, or hexadec-imal) hexadecimal)
imal) is determined in the same fashion as described in expr. The integer value is stored
in the variable, truncated as required by the size modifier value.
c A single character is read in and its Unicode value is stored in the variable as an integer
value. Initial white space is not skipped in this case, so the input substring may be a
white-space character.
s The input substring 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 substring 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 con-sisting consisting
sisting 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 value.
[chars] The input substring consists of one or more 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 substring consists of one or more 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 value.
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 substring for a given conversion terminates either when a
white-space character is encountered or when the maximum substring 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 supported.
[2] For %c conversions a single character value is converted to a decimal string, which is then
assigned to the corresponding varName; no substring width may be specified for this conver-sion. conversion.
sion.
[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). The ll modifier has no
sscanf counterpart.
[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
Convert a UNICODE character to its numeric value:
set char "x"
set value [scan $char %c]
Parse a simple color specification of the form #RRGGBB using hexadecimal conversions with substring
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 interpreta-tion interpretation
tion 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 whitespace 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 looking for the terminating charac-ter character
ter 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"
An interactive session demonstrating the truncation of integer values determined by size modifiers: |
% set tcl_platform(wordSize) |
4 |
% scan 20000000000000000000 %d |
2147483647 |
% scan 20000000000000000000 %ld |
9223372036854775807 |
% scan 20000000000000000000 %lld |
20000000000000000000 |
SEE ALSO
format(n), sscanf(3)
KEYWORDS
conversion specifier, parse, scan
Tcl 8.4 scan(n)
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