format(3tcl)

📦
version 9.0.4 · 4 versions
, in ubuntu resolute, ubuntu stonking

✨ format — bestie, let's make a string 💅

okay so real talk, this is the format page from the Tcl Built-In Commands manual, and it's genuinely one of the most useful little commands in the whole language. let's get into it 🫶

🌸 NAME 🌸

format — format a string in the style of sprintf. that's it. that's the vibe. ✨

🎀 SYNOPSIS 🎀

format formatString ?arg arg ...?

like… she's simple. she's elegant. she ate. 👑

🦋 INTRODUCTION 🦋

okay so this command generates a formatted string, and it works a lot like the ANSI C sprintf procedure — like, spiritually they're sisters. 💖

formatString is the blueprint: it tells you how to format the result, using % conversion specifiers, exactly like in sprintf. and then the additional arguments — if there are any — are the values that get substituted into the result. 🌈

the return value from format is the formatted string. periodt. that's what you get back. ✨

🔥 DETAILS ON FORMATTING 🔥

so here's the tea ☕ — the command works by scanning formatString from left to right, one character at a time. each character from the format string gets appended to the result string… unless it's a percent sign. 🙈

if the character is a %, it does not get copied to the result string. instead, the characters following the % are treated as a conversion specifier. that conversion specifier controls how the next successive arg gets converted into a particular format, and the result gets 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. and here's the rule bestie, no cap: the format command must be given enough args to meet the needs of all of the conversion specifiers in formatString. if you don't, that's on you. 😭

each conversion specifier may contain up to six different parts:

  1. an XPG3 position specifier
  2. a set of flags
  3. a minimum field width
  4. a precision
  5. a size modifier
  6. a conversion character

any of these fields may be omitted — except for the conversion character. she's mandatory, she's the main character, she must be there. 👑

and the fields that are present must appear in the order given above. the paragraphs below discuss each of these fields in turn. let's goooo ⭐

💜 OPTIONAL POSITIONAL SPECIFIER 💜

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. nope! instead, it's 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. ✨

⚠️ and listen: if there are any positional specifiers in formatString, then all of the specifiers must be positional. no mixing, bestie, that's the rule. 💀

🌸 OPTIONAL FLAGS 🌸

the second portion of a conversion specifier may contain any of the following flag characters, 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. bold of her but okay! ✨
  • space — specifies that a space should be added to the beginning of the number if the first character is not a sign.
  • 0 — specifies that the number should be padded on the left with zeroes instead of spaces. slay 💅
  • # — requests an alternate output form. okay here's the breakdown, because she's specific:
    • for o conversions, 0o will be added to the beginning of the result unless it is zero.
    • for x or X conversions, 0x will be added to the beginning of the result unless it is zero.
    • for b conversions, 0b will be added to the beginning of the result unless it is zero.
    • for d conversions, 0d will be added to the beginning of the result unless it is zero.
    • for all floating-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.

🌈 OPTIONAL FIELD WIDTH 🌈

the third portion of a conversion specifier is a decimal number giving a minimum field width for this conversion. it's typically used to make columns line up in tabular printouts — you know, that cute organized aesthetic. 📊

if the converted argument contains fewer characters than the minimum field width, then it will be padded so that it's as wide as the minimum field width. padding normally happens 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 — and it must be an integer value. no exceptions bestie. 🔥

⭐ OPTIONAL PRECISION/BOUND ⭐

the fourth portion of a conversion specifier is a precision, which consists of a period followed by a number. and the number means different things for different conversions — she contains multitudes. 🌸

  • 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. bye felicia ✂️

if the precision is specified with * rather than a number, then the next argument to the format command determines the precision — and it must be a numeric string. 🎯

💅 OPTIONAL SIZE MODIFIER 💅

the fifth part of a conversion specifier is a size modifier, which must be ll, h, l, z, t, or L. okay here's the rundown:

  • 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. like, she's giving "why is she here" energy. 🙈
  • if it is l (or j or q) 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 it is z or t it specifies that the integer value is truncated to the range determined by the value of the pointerSize element of the tcl_platform array. 🖥️
  • if it is L it specifies that an integer or double value is taken without truncation for conversion to a formatted substring. ✨

and if neither of those are present, the integer value is truncated to a 32-bit range. that's the default, bestie, that's just how it is. 💀

👑 MANDATORY CONVERSION TYPE 👑

the last thing in a conversion specifier is an alphabetic character that determines what kind of conversion to perform. she's the finale, the closing number, the moment. 🎤

the following conversion characters are currently supported:

  • d — convert integer to signed decimal string.
  • u — convert integer to unsigned decimal string.
  • i — convert integer to signed decimal string (equivalent to d). twins! 👯
  • 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).
  • b — convert integer to unsigned binary string, using digits 0 and 1. 🖥️
  • c — convert integer to the Unicode character it represents. 🦋
  • s — no conversion; just insert string. lowkey the chillest one. 😌
  • f — convert 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 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 number as for %e or %E. otherwise convert as for %f. trailing zeroes and a trailing decimal point are omitted.
  • a or A — convert double to hexadecimal notation in the form 0x1.yyyp±zz, where the number of y's is determined by the precision (default: 13). if the A form is used then the hex characters are printed in uppercase. 💜
  • % — no conversion: just insert %. 🎀
  • p — shorthand form for 0x%zx, so it outputs the integer in hexadecimal form with 0x prefix.

💀 DIFFERENCES FROM ANSI SPRINTF 💀

the behavior of the format command is the same as the ANSI C sprintf procedure except for the following differences. read carefully bestie, this part matters:

  1. Tcl guarantees that it will be working with UNICODE characters. 🌈
  2. %n specifier is not supported. sorry, she's not coming. 🙅
  3. for %c conversions the argument must be an integer value, which will then be converted to the corresponding character value.
  4. the size modifiers are ignored when formatting floating-point values. the b specifier has no sprintf counterpart. (she's unique, she's one of a kind 💅)

🌟 EXAMPLES 🌟

okay bestie, this is the fun part. let's see her in action 🎬

convert the numeric value of a UNICODE character to the character itself:

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

🫶 SEE ALSO 🫶

scan(3tcl), sprintf(3), string(3tcl)

🎀 KEYWORDS 🎀

conversion specifier, format, sprintf, string, substitution

and that's the whole page bestie!! 💖 this one's from the Tcl manual, version 8.1, and she's been out here since the format(3tcl) days, still serving, still iconic. go format something cute, you got this 👑✨🌸

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