format(n) Tcl format(n)
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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 the same way as
the ANSI C sprintf procedure (it uses sprintf in its
implementation). 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 length 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
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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
isn't 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 and 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 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 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 a
numeric string.
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
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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 length
modifier, which must be h or l. If it is h it specifies
that the numeric value should be truncated to a 16-bit value
before converting. This option is rarely useful. The l
modifier is ignored.
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; the
integer may either be in decimal, in octal (with a
leading 0) or in hexadecimal (with a leading 0x).
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).
c Convert integer to the 8-bit character it
represents.
s No conversion; just insert string.
f Convert floating-point 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 floating-point 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.
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format(n) Tcl format(n)
g or G If the exponent is less than -4 or greater than or
equal to the precision, then convert floating-
point number as for %e or %E. Otherwise convert
as for %f. Trailing zeroes and a trailing decimal
point are omitted.
% No conversion: just insert %.
For the numerical conversions the argument being converted
must be an integer or floating-point string; format converts
the argument to binary and then converts it back to a string
according to the conversion specifier.
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] %p and %n specifiers are not currently supported.
[2] For %c conversions the argument must be a decimal
string, which will then be converted to the
corresponding character value.
[3] The l modifier is ignored; integer values are always
converted as if there were no modifier present and real
values are always converted as if the l modifier were
present (i.e. type double is used for the internal
representation). If the h modifier is specified then
integer values are truncated to short before
conversion.
KEYWORDS
conversion specifier, format, sprintf, string, substitution
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