Provided by: tcl9.0-doc_9.0.1+dfsg-1_all 
NAME
format - Format a string in the style of sprintf
SYNOPSIS
format formatString ?arg arg ...?
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INTRODUCTION
This command generates a formatted string in a fashion similar to the ANSI C sprintf procedure.
FormatString indicates how to format the result, using % conversion specifiers as in sprintf, and the
additional arguments, if any, provide values to be substituted into the result. The return value from
format is the formatted string.
DETAILS ON FORMATTING
The command operates by scanning formatString from left to right. Each character from the format string
is appended to the result string unless it is a percent sign. If the character is a % then it is not
copied to the result string. Instead, the characters following the % character are treated as a
conversion specifier. The conversion specifier controls the conversion of the next successive arg to a
particular format and the result is appended to the result string in place of the conversion specifier.
If there are multiple conversion specifiers in the format string, then each one controls the conversion
of one additional arg. The format command must be given enough args to meet the needs of all of the
conversion specifiers in formatString.
Each conversion specifier may contain up to six different parts: an XPG3 position specifier, a set of
flags, a minimum field width, a precision, a size modifier, and a conversion character. Any of these
fields may be omitted except for the conversion character. The fields that are present must appear in
the order given above. The paragraphs below discuss each of these fields in turn.
OPTIONAL POSITIONAL SPECIFIER
If the % is followed by a decimal number and a $, as in “%2$d”, then the value to convert 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 arg. If the conversion specifier requires multiple arguments because of *
characters in the specifier then successive arguments are used, starting with the argument given by the
number. This follows the XPG3 conventions for positional specifiers. If there are any positional
specifiers in formatString then all of the specifiers must be positional.
OPTIONAL FLAGS
The second portion of a conversion specifier may contain any of the following flag characters, in any
order:
- Specifies that the converted argument should be left-justified in its field (numbers are
normally right-justified with leading spaces if needed).
+ Specifies that a number should always be printed with a sign, even if positive.
space Specifies that a space should be added to the beginning of the number if the first character is
not a sign.
0 Specifies that the number should be padded on the left with zeroes instead of spaces.
# Requests an alternate output form. For o conversions, 0o will be added to the beginning of the
result unless it is zero. For x or X conversions, 0x will be added to the beginning of the
result unless it is zero. For b conversions, 0b will be added to the beginning of the result
unless it is zero. For d conversions, 0d there is no effect unless the 0 specifier is used as
well: In that case, 0d will be added to the beginning. For all floating-point conversions (e,
E, f, g, and G) it guarantees that the result always has a decimal point. For g and G
conversions it specifies that trailing zeroes should not be removed.
OPTIONAL FIELD WIDTH
The third portion of a conversion specifier is a decimal number giving a minimum field width for this
conversion. It is typically used to make columns line up in tabular printouts. If the converted
argument contains fewer characters than the minimum field width then it will be padded so that it is as
wide as the minimum field width. Padding normally occurs by adding extra spaces on the left of the
converted argument, but the 0 and - flags may be used to specify padding with zeroes on the left or with
spaces on the right, respectively. If the minimum field width is specified as * rather than a number,
then the next argument to the format command determines the minimum field width; it must be an integer
value.
OPTIONAL PRECISION/BOUND
The fourth portion of a conversion specifier is a precision, which consists of a period followed by a
number. The number is used in different ways for different conversions. For e, E, and f conversions it
specifies the number of digits to appear to the right of the decimal point. For g and G conversions it
specifies the total number of digits to appear, including those on both sides of the decimal point
(however, trailing zeroes after the decimal point will still be omitted unless the # flag has been
specified). For integer conversions, it specifies a minimum number of digits to print (leading zeroes
will be added if necessary). For s conversions it specifies the maximum number of characters to be
printed; if the string is longer than this then the trailing characters will be dropped. If the precision
is specified with * rather than a number then the next argument to the format command determines the
precision; it must be a numeric string.
OPTIONAL SIZE MODIFIER
The fifth part of a conversion specifier is a size modifier, which must be ll, h, l, z, t, or L. If it
is ll it specifies that an integer value is taken without truncation for conversion to a formatted
substring. If it is h it specifies that an integer value is truncated to a 16-bit range before
converting. This option is rarely useful. If it is l (or j or q) it specifies that the integer value is
truncated to the same range as that produced by the wide() function of the expr command (at least a
64-bit range). If it is z or t it specifies that the integer value is truncated to the range determined
by the value of the pointerSize element of the tcl_platform array. If it is L it specifies that an
integer or double value is taken without truncation for conversion to a formatted substring. If neither
of those are present, the integer value is truncated to a 32-bit range.
MANDATORY CONVERSION TYPE
The last thing in a conversion specifier is an alphabetic character that determines what kind of
conversion to perform. The following conversion characters are currently supported:
d Convert integer to signed decimal string.
u Convert integer to unsigned decimal string.
i Convert integer to signed decimal string (equivalent to d).
o Convert integer to unsigned octal string.
x or X Convert integer to unsigned hexadecimal string, using digits “0123456789abcdef” for x and
“0123456789ABCDEF” for X).
b Convert integer to unsigned binary string, using digits 0 and 1.
c Convert integer to the Unicode character it represents.
s No conversion; just insert string.
f Convert number to signed decimal string of the form xx.yyy, where the number of y's is
determined by the precision (default: 6). If the precision is 0 then no decimal point is
output.
e or E Convert number to scientific notation in the form x.yyye±zz, where the number of y's is
determined by the precision (default: 6). If the precision is 0 then no decimal point is
output. If the E form is used then E is printed instead of e.
g or G If the exponent is less than -4 or greater than or equal to the precision, then convert number
as for %e or %E. Otherwise convert as for %f. Trailing zeroes and a trailing decimal point
are omitted.
a or A Convert double to hexadecimal notation in the form 0x1.yyyp±zz, where the number of y's is
determined by the precision (default: 13). If the A form is used then the hex characters are
printed in uppercase.
% No conversion: just insert %.
p Shorthand form for 0x%zx, so it outputs the integer in hexadecimal form with 0x prefix.
DIFFERENCES FROM ANSI SPRINTF
The behavior of the format command is the same as the ANSI C sprintf procedure except for the following
differences:
[1] Tcl guarantees that it will be working with UNICODE characters.
[2] %n specifier is not supported.
[3] For %c conversions the argument must be an integer value, which will then be converted to the
corresponding character value.
[4] The size modifiers are ignored when formatting floating-point values. The b specifier has no
sprintf counterpart.
EXAMPLES
Convert the numeric value of a UNICODE character to the character itself:
set value 120
set char [format %c $value]
Convert the output of time into seconds to an accuracy of hundredths of a second:
set us [lindex [time $someTclCode] 0]
puts [format "%.2f seconds to execute" [expr {$us / 1e6}]]
Create a packed X11 literal color specification:
# Each color-component should be in range (0..255)
set color [format "#%02x%02x%02x" $r $g $b]
Use XPG3 format codes to allow reordering of fields (a technique that is often used in localized message
catalogs; see msgcat) without reordering the data values passed to format:
set fmt1 "Today, %d shares in %s were bought at $%.2f each"
puts [format $fmt1 123 "Global BigCorp" 19.37]
set fmt2 "Bought %2\$s equity ($%3$.2f x %1\$d) today"
puts [format $fmt2 123 "Global BigCorp" 19.37]
Print a small table of powers of three:
# Set up the column widths
set w1 5
set w2 10
# Make a nice header (with separator) for the table first
set sep +-[string repeat - $w1]-+-[string repeat - $w2]-+
puts $sep
puts [format "| %-*s | %-*s |" $w1 "Index" $w2 "Power"]
puts $sep
# Print the contents of the table
set p 1
for {set i 0} {$i<=20} {incr i} {
puts [format "| %*d | %*ld |" $w1 $i $w2 $p]
set p [expr {wide($p) * 3}]
}
# Finish off by printing the separator again
puts $sep
SEE ALSO
scan(3tcl), sprintf(3), string(3tcl)
KEYWORDS
conversion specifier, format, sprintf, string, substitution
Tcl 8.1 format(3tcl)