** Summary **

Format a string in the style of sprintf

** Synopsis **

    :   '''format''' ''formatString'' ?''arg arg ...''?


** Description **

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.


** Documentation **

   [http://www.tcl.tk/man/tcl/TclCmd/format.htm%|%man page]:   


** Maximum Width for Numbers ** 

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
   }
}
======


** 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:

======none
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:

======none
% fixform 12345678
11.7M
======


** 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]
            }
        }
    }
}
======

======none
% 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:

======none
% strictformat %5.2f 12.345
12.35
% strictformat %5.2f 123.45
*****
% strictformat %5.2f 12345.67
*****
======


** 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
======


** 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<<br>>
> it's currently limited to 64 bits...

======none
% 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])

** Digit Grouping **

Digit grouping can make numbers with many digits easier to read.

[ET]: While I never liked the language ADA, it did have an idea that I wish had
caught on, the optional use of an underscore character in large numerical
constants, to make the numbers readable. (And trival for a compiler or
interpreter to scan/parse).

So, 1_234_567 is a number that is as readable as 1,234,567 and is much better
than 1234567. I have a handy little converter, (which I stole from somewhere on
this wiki and modifed):

======
proc {commas} {var {num 3} {char ","}} {
    set len   [string length $var]
    set first [expr $len - $num]
    set x     ""
    while { $len > 0} {
        # grab left num chars
        set lef [string range $var $first end] 
        if {[string length $x] > 0} {
            set x   "${lef}$char${x}"
        } else {
            set x   ${lef}
        }
        # grab everything except left num chars
        set var [string range  $var 0 [expr $first -1]]
        set len   [string length $var]
        set first [expr $len - $num]
    }
    return $x
}
======

Here are some examples of its use:

======none
dec2bin 987654
11110001001000000110

% commas [dec2bin 987654] 4 _
1111_0001_0010_0000_0110

% commas [dec2bin 987654] 4 { }
1111 0001 0010 0000 0110

% commas [dec2bin 987654] 1 { }
1 1 1 1 0 0 0 1 0 0 1 0 0 0 0 0 0 1 1 0

%commas 123456789     ;# naturally, it defaults for use with large decimal integers
123,456,789

% commas 123456789 3 _  ;# and here's how I wish numbers could be entered, in tcl and in C etc.
123_456_789

% puts "0x[commas [format %08X 123456789] 4 _]"    ;# and for hex numbers as well
0x075B_CD15   
======


** Misc **

[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?



''[[TODO: Explain XPG positional format specifiers.]]''

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