format - Format a string in the style of sprintf
format formatString ?
arg arg ...?
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.
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.
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.
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
it guarantees that the first digit is always 0. For x or
X conversions, 0x or 0X (respectively) 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 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.
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.
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.
The fifth part of a conversion specifier is a size modifier, which must be
ll,
h, 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 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 neither
h nor
l are present, the
integer value is truncated to the same range as that produced by the
int() function of the
expr command (at least a 32-bit range, but
determined by the value of the
wordSize element of the
tcl_platform array).
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.yyy e±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.
- %
- No conversion: just insert %.
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]
-
%p and %n specifiers are 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 ll modifier has no sprintf
counterpart. The b specifier has no sprintf
counterpart.
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
scan(3tcl),
sprintf(3),
string(3tcl)
conversion specifier, format, sprintf, string, substitution