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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 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:



  

  
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.

  

  
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.

  

  
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.

  

  
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.

  

  
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.

  

  
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.

  

  
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.

  

  
The input substring consists of all the characters up to the next
      white-space character; the characters are copied to the variable.

  

  
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.

  

  
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(n), sscanf(3)






KEYWORDS


conversion specifier, parse, scan







  

    8.4
    Tcl