Version 25 of format

Updated 2007-12-19 13:19:43 by LV

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 string.

Tcl 8.4 doc at: http://purl.org/tcl/home/man/tcl8.4/TclCmd/format.htm

Tcl 8.5 doc at: http://purl.org/tcl/home/man/tcl8.5/TclCmd/format.htm


LV The man page for 8.4 is missing examples. 8.5 is better, but I'm looking for an example of the following. I have a report line that I am trying to fill out. It consists of a time stamp, a date stamp, and 2 text strings. each of these items must begin in a specific column.

 set g "OHIO"
 set fmtg [format "%-25.25s" $g]
 puts [string length "$fmtg"]

The man page is complex enough that I want to be certain that I am not missing something. This seems to ensure that if g is longer than 25 characters, it is truncated, and if it is shorter than 25 characters, that it is left justified and blank padded. Are there any gotchas of which I need to be aware?


Note that format, by default, doesn't provide a way to use one of the number formats (like %d, etc.) AND specify a maximum number of digits. You could specify the format as a %s and then provide a maximum number of characters, or you could write tcl code to check for maximum. Jonathan Bromley posted, on comp.lang.tcl during early Sep 2007, this code, which provides a first cut at a "-strict" initial arguement to format.

  proc strictformat {fmt value} {
    set f [format $fmt $value]
    regexp {%(\d+)} $fmt -> maxwidth
    if {[string length $f]>$maxwidth} {
       return [string repeat * $maxwidth]
    } else {return $f}
 }
 rename format _format
 proc format {args} {
  if {[string equal [lindex $args 0] -strict]} {
    eval strictformat $args
  } else {
    eval _format $args
  }
 }

Tips and Tricks with format

You can use [format] to produce unsigned integers for display (but don't reckon with them - for expr they're still signed!):

 % format %u -1
 4294967295

See floating-point formatting for discussion on how to write format strings to handle floats...


To make numbers look nice:

 set fah [format "%0.2f" [expr $temperature_cel * 9 / 5 + 32]]

Color formatting:

 set color [format #%02x%02x%02x $r $g $b]

A limited formatting of decimals to characters is available in other languages, e.g. CHR() in Basic. If you use that more often, here's a cute shortcut:

 interp alias {} chr {} format %c
 % set a [chr 49][chr 48]
 10

See Narrow formatting for short rendering of big integers, with powers of 1024:

 % fixform 12345678
 11.7M

This method should get format string and explain the format structure. This is a fast scatch:

proc explainFormat {formatStr vars} {
    set index 1
    foreach frm [split $formatStr "%"] {
        set extra ""
        set size 0
        regexp {([0-9]+)([duioxXcsfegG])(.*)} $frm => size type extra
        if {$size==0} {
            set size [string length $frm]
        } else {
            set frm "%$size$type [lindex $vars 0]"
            set vars [lrange $vars 1 end]
            set size [string trimleft $size 0]
        }
        for {set i 0} {$i<2} {incr i} {
            set newIndex [expr {$size +$index -1}]
            puts "$index-$newIndex '$frm'"
            set index [expr {$newIndex +1}]
            if {$extra==""} {
                break
            } else {
                set frm $extra
                set size [string length $extra]
            }
        }
    }
}

% explainFormat hello%02s000%3d $a $b
1-5 'hello'
6-7 '%02s $a'
8-10 '000'
11-13 '%3d $b'

RS 2007-09-04: Here's emulating the FORTRAN behavior that numbers too large for the format are marked as an asterisk string:

 proc strictformat {fmt value} {
    set f [format $fmt $value]
    regexp {%(\d+)} $fmt -> maxwidth
    if {[string length $f]>$maxwidth} {
       return [string repeat * $maxwidth]
    } else {return $f}
 }

Testing:

 % strictformat %5.2f 12.345
 12.35
 % strictformat %5.2f 123.45
 *****
 % strictformat %5.2f 12345.67
 *****

While using Tcl 8.5, you will begin to see strings like 0.0052499999999999995 where before you were seeing values like 0.00525. To round the value to a shorter value, try something like:

 format %.3g 0.0052499999999999995


[TODO: Explain XPG positional format specifiers.]