format(n) Tcl Built-In Commands format(n) ______________________________________________________________________________
NAME
format - Format a string in the style of sprintf
SYNOPSIS
format formatString ?arg arg ...?
______________________________________________________________________________
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 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.
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, 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).
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. 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 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.
% No conversion: just insert %.
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] %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.
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(n), sprintf(3), string(n)
KEYWORDS
conversion specifier, format, sprintf, string, substitution
Tcl 8.1 format(n)
tcl 8.6.15 - Generated Tue Dec 3 18:13:33 CST 2024