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:
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 tcl_platform(wordSize)).
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:
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