scan

scan(3tcl)                    Tcl Built-In Commands                   scan(3tcl)



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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 substrings from an input string in a fashion similar
       to 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 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
       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.

       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.

   OPTIONAL POSITIONAL SPECIFIER
       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.

   OPTIONAL SIZE MODIFIER
       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.

   MANDATORY CONVERSION CHARACTER
       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 or 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.

       b      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.

       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 combination with conversion
              character u.

       i      The input substring must be an integer.  The base (i.e. decimal,
              octal, or hexadecimal) is determined by the C convention (leading
              0 for octal; prefix 0x for hexadecimal).  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 or E 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 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 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 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).  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 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 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 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"

       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(3tcl), sscanf(3)

KEYWORDS
       conversion specifier, parse, scan



Tcl                                    8.4                            scan(3tcl)