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?

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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
  }
 }

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Tips and Tricks with format edit

Make Unsigned Values:

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...

DKF: Note that 8.5 makes this sort of thing much less necessary as we can now handle arbitrary width integers.

---

Nice Looking 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]

---

Converting Characters:

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

---

Abbreviating Integers:

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

 % fixform 12345678
 11.7M

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Understanding Formats:

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'

---

Emulating Fortran:

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
 *****

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Restricting Floats:

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

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Rebasing:

On comp.lang.tcl, Don Porter writes, in reply to a question about how to go from base 10 to another base, such as 2 or 16, using arbitrarily large numbers in Tcl 8.5:

> I would have guessed that "format %x" should do the job, but apparently
> it's currently limited to 64 bits...


% format %llx 1234567890123456789012345
1056e0f36a6443de2df79

(Note: Those are ELLs above in '%11x'. Not {percent eleven lower-case-x}, but rather {percent, ell, ell, lower-case-x}. DrASK)

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[TODO: Explain XPG positional format specifiers.]