format(n) Tcl Built-In Commands format(n)
format - Format a string in the style of sprintf
format formatString ?arg arg ...?
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
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 char-
acter is a % then it is not copied to the result string. Instead, the characters follow-
ing the % character are treated as a conversion specifier. The conversion specifier con-
trols 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 speci-
fier, a set of flags, a minimum field width, a precision, a length modifier, and a conver-
sion 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 con-
vert is not taken from the next sequential argument. Instead, it is taken from the argu-
ment 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 suc-
cessive 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 charac-
ters, 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
# 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 float-
ing-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 be omitted unless the # flag has been specified). For inte-
ger 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. If it is l it specifies that the numeric value |
should be (at least) a 64-bit value. If neither h or l are present, numeric values are |
interpreted as being values of the width of the native machine word, as described by |
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 sup-
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). |
Convert integer to the Unicode 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.
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:
 %p and %n specifiers are not currently supported.
 For %c conversions the argument must be a decimal string, which will then be con-
verted to the corresponding character value.
 The l modifier is ignored for real values and on 64-bit platforms, which are always |
converted as if the l modifier were present (i.e. the types double and long are |
used for the internal representation of real and integer values, respectively). If
the h modifier is specified then integer values are truncated to short before con-
version. Both h and l modifiers are ignored on all other conversions.
conversion specifier, format, sprintf, string, substitution
Tcl 8.1 format(n)