path: root/doc/scan.n

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.so man.macros
.TH scan n 8.4 Tcl "Tcl Built-In Commands"
.BS
'\" Note:  do not modify the .SH NAME line immediately below!
.SH NAME
scan \- Parse string using conversion specifiers in the style of sscanf
.SH SYNOPSIS
\fBscan \fIstring format \fR?\fIvarName varName ...\fR?
.BE
.SH INTRODUCTION
.PP
This command parses substrings from an input string in a fashion similar
to the ANSI C \fBsscanf\fR 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.  \fIString\fR gives the input
to be parsed and \fIformat\fR indicates how to parse it, using \fB%\fR
conversion specifiers as in \fBsscanf\fR.  Each \fIvarName\fR gives the
name of a variable; when a substring is scanned from \fIstring\fR that
matches a conversion specifier, the substring is assigned to the
corresponding variable.
If no \fIvarName\fR variables are specified, then \fBscan\fR 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.
.SH "DETAILS ON SCANNING"
.PP
\fBScan\fR operates by scanning \fIstring\fR and \fIformat\fR together.
If the next character in \fIformat\fR is a blank or tab then it
matches any number of white space characters in \fIstring\fR (including
zero).
Otherwise, if it is not a \fB%\fR character then it 
must match the next character of \fIstring\fR.
When a \fB%\fR is encountered in \fIformat\fR, it indicates
the start of a conversion specifier.
A conversion specifier contains up to four fields after the \fB%\fR:
a XPG3 position specifier (or a \fB*\fR to indicate the converted
value is to be discarded 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.
.PP
When \fBscan\fR finds a conversion specifier in \fIformat\fR, it
first skips any white-space characters in \fIstring\fR (unless the
conversion character is \fB[\fR or \fBc\fR).
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 \fBscan\fR.
.SS "OPTIONAL POSITIONAL SPECIFIER"
.PP
If the \fB%\fR is followed by a decimal number and a \fB$\fR, as in
.QW \fB%2$d\fR ,
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 \fIvarName\fR.  If
there are any positional specifiers in \fIformat\fR then all of the
specifiers must be positional.  Every \fIvarName\fR 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.
.SS "OPTIONAL SIZE MODIFIER"
.PP
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 \fBh\fR, \fBL\fR,
\fBl\fR, and \fBll\fR.  The \fBh\fR 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 \fBint()\fR function of the \fBexpr\fR
command.  The \fBL\fR size modifier is equivalent to the \fBl\fR size
modifier. Either one indicates the integer range to be stored is
limited to the same range produced by the \fBwide()\fR function of
the \fBexpr\fR command.  The \fBll\fR size modifier indicates that
the integer range to be stored is unlimited.
.SS "MANDATORY CONVERSION CHARACTER"
.PP
The following conversion characters are supported:
.TP
\fBd\fR
.
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.
.TP
\fBo\fR
.
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.
.TP
\fBx\fR
.
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.
.TP
\fBb\fR
.
The input substring must be a binary integer.
It is read in and the integer value is stored in the variable,
truncated as required by the size modifier value.
.TP
\fBu\fR
.
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 \fBll\fR is not permitted in combination
with conversion character \fBu\fR.
.TP
\fBi\fR
.
The input substring must be an integer.  The base (i.e. decimal, binary,
octal, or hexadecimal) is determined in the same fashion as described in
\fBexpr\fR.  The integer value is stored in the variable,
truncated as required by the size modifier value.
.TP
\fBc\fR
.
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.
.TP
\fBs\fR
.
The input substring consists of all the characters up to the next 
white-space character; the characters are copied to the variable.
.TP
\fBe\fR or \fBf\fR or \fBg\fR
.
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 consisting 
of an \fBe\fR or \fBE\fR 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.
.TP
\fB[\fIchars\fB]\fR
.
The input substring consists of one or more characters in \fIchars\fR.
The matching string is stored in the variable.
If the first character between the brackets is a \fB]\fR then
it is treated as part of \fIchars\fR rather than the closing
bracket for the set.
If \fIchars\fR
contains a sequence of the form \fIa\fB\-\fIb\fR then any
character between \fIa\fR and \fIb\fR (inclusive) will match.
If the first or last character between the brackets is a \fB\-\fR, then
it is treated as part of \fIchars\fR rather than indicating a range.
.TP
\fB[^\fIchars\fB]\fR
.
The input substring consists of one or more characters not in \fIchars\fR.
The matching string is stored in the variable.
If the character immediately following the \fB^\fR is a \fB]\fR then it is 
treated as part of the set rather than the closing bracket for 
the set.
If \fIchars\fR
contains a sequence of the form \fIa\fB\-\fIb\fR then any
character between \fIa\fR and \fIb\fR (inclusive) will be excluded
from the set.
If the first or last character between the brackets is a \fB\-\fR, then
it is treated as part of \fIchars\fR rather than indicating a range value.
.TP
\fBn\fR
.
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.
.PP
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 \fB%d\fR, as 
many octal digits as possible for \fB%o\fR, 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 \fB*\fR is present in the conversion specifier 
then no variable is assigned and the next scan argument is not consumed.
.SH "DIFFERENCES FROM ANSI SSCANF"
.PP
The behavior of the \fBscan\fR command is the same as the behavior of
the ANSI C \fBsscanf\fR procedure except for the following differences:
.IP [1]
\fB%p\fR conversion specifier is not supported.
.IP [2]
For \fB%c\fR conversions a single character value is
converted to a decimal string, which is then assigned to the
corresponding \fIvarName\fR;
no substring width may be specified for this conversion.
.IP [3]
The \fBh\fR modifier is always ignored and the \fBl\fR and \fBL\fR
modifiers are ignored when converting real values (i.e. type
\fBdouble\fR is used for the internal representation).  The \fBll\fR
modifier has no \fBsscanf\fR counterpart.
.IP [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.
.SH EXAMPLES
.PP
Convert a UNICODE character to its numeric value:
.PP
.CS
set char "x"
set value [\fBscan\fR $char %c]
.CE
.PP
Parse a simple color specification of the form \fI#RRGGBB\fR using
hexadecimal conversions with substring sizes:
.PP
.CS
set string "#08D03F"
\fBscan\fR $string "#%2x%2x%2x" r g b
.CE
.PP
Parse a \fIHH:MM\fR time string, noting that this avoids problems with
octal numbers by forcing interpretation as decimals (if we did not
care, we would use the \fB%i\fR conversion instead):
.PP
.CS
set string "08:08"   ;# *Not* octal!
if {[\fBscan\fR $string "%d:%d" hours minutes] != 2} {
    error "not a valid time string"
}
# We have to understand numeric ranges ourselves...
if {$minutes < 0 || $minutes > 59} {
    error "invalid number of minutes"
}
.CE
.PP
Break a string up into sequences of non-whitespace characters (note
the use of the \fB%n\fR conversion so that we get skipping over
leading whitespace correct):
.PP
.CS
set string " a string {with braced words} + leading space "
set words {}
while {[\fBscan\fR $string %s%n word length] == 2} {
    lappend words $word
    set string [string range $string $length end]
}
.CE
.PP
Parse a simple coordinate string, checking that it is complete by
looking for the terminating character explicitly:
.PP
.CS
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 \eu0029
if {
    [\fBscan\fR $string " (%f ,%f %c" x y last] != 3
    || $last != 0x0029
} then {
    error "invalid coordinate string"
}
puts "X=$x, Y=$y"
.CE
.PP
An interactive session demonstrating the truncation of integer
values determined by size modifiers:
.PP
.CS
\fI%\fR set tcl_platform(wordSize)
4
\fI%\fR scan 20000000000000000000 %d
2147483647
\fI%\fR scan 20000000000000000000 %ld
9223372036854775807
\fI%\fR scan 20000000000000000000 %lld
20000000000000000000
.CE
.SH "SEE ALSO"
format(n), sscanf(3)
.SH KEYWORDS
conversion specifier, parse, scan
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