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fmt/doc/syntax.rst
Kevin Puetz 981b517ccf
nested replacement fields may omit arg_id (#1681) 
syntax.html already has examples like `fmt::format("{:.{}f}", 3.14, 1)` using this, 
and https://en.cppreference.com/w/cpp/utility/format/formatter#width_and_precision shows that this is the case for the C++20 std::format

The Format Specification Mini-Language grammar seems to be the only one not showing this; update it to match.
2020-05-13 17:20:05 -07:00

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.. _syntax:
********************
Format String Syntax
********************
Formatting functions such as :ref:`fmt::format() <format>` and
:ref:`fmt::print() <print>` use the same format string syntax described in this
section.
Format strings contain "replacement fields" surrounded by curly braces ``{}``.
Anything that is not contained in braces is considered literal text, which is
copied unchanged to the output. If you need to include a brace character in the
literal text, it can be escaped by doubling: ``{{`` and ``}}``.
The grammar for a replacement field is as follows:
.. productionlist:: sf
replacement_field: "{" [`arg_id`] [":" `format_spec`] "}"
arg_id: `integer` | `identifier`
integer: `digit`+
digit: "0"..."9"
identifier: `id_start` `id_continue`*
id_start: "a"..."z" | "A"..."Z" | "_"
id_continue: `id_start` | `digit`
In less formal terms, the replacement field can start with an *arg_id*
that specifies the argument whose value is to be formatted and inserted into
the output instead of the replacement field.
The *arg_id* is optionally followed by a *format_spec*, which is preceded
by a colon ``':'``. These specify a non-default format for the replacement value.
See also the :ref:`formatspec` section.
If the numerical arg_ids in a format string are 0, 1, 2, ... in sequence,
they can all be omitted (not just some) and the numbers 0, 1, 2, ... will be
automatically inserted in that order.
Named arguments can be referred to by their names or indices.
Some simple format string examples::
"First, thou shalt count to {0}" // References the first argument
"Bring me a {}" // Implicitly references the first argument
"From {} to {}" // Same as "From {0} to {1}"
The *format_spec* field contains a specification of how the value should be
presented, including such details as field width, alignment, padding, decimal
precision and so on. Each value type can define its own "formatting
mini-language" or interpretation of the *format_spec*.
Most built-in types support a common formatting mini-language, which is
described in the next section.
A *format_spec* field can also include nested replacement fields in certain
positions within it. These nested replacement fields can contain only an
argument id; format specifications are not allowed. This allows the formatting
of a value to be dynamically specified.
See the :ref:`formatexamples` section for some examples.
.. _formatspec:
Format Specification Mini-Language
==================================
"Format specifications" are used within replacement fields contained within a
format string to define how individual values are presented (see
:ref:`syntax`). Each formattable type may define how the format
specification is to be interpreted.
Most built-in types implement the following options for format specifications,
although some of the formatting options are only supported by the numeric types.
The general form of a *standard format specifier* is:
.. productionlist:: sf
format_spec: [[`fill`]`align`][`sign`]["#"]["0"][`width`]["." `precision`][`type`]
fill: <a character other than '{' or '}'>
align: "<" | ">" | "^"
sign: "+" | "-" | " "
width: `integer` | "{" [`arg_id`] "}"
precision: `integer` | "{" [`arg_id`] "}"
type: `int_type` | "a" | "A" | "c" | "e" | "E" | "f" | "F" | "g" | "G" | "L" | "p" | "s"
int_type: "b" | "B" | "d" | "o" | "x" | "X"
The *fill* character can be any Unicode code point other than ``'{'`` or
``'}'``. The presence of a fill character is signaled by the character following
it, which must be one of the alignment options. If the second character of
*format_spec* is not a valid alignment option, then it is assumed that both the
fill character and the alignment option are absent.
The meaning of the various alignment options is as follows:
+---------+----------------------------------------------------------+
| Option | Meaning |
+=========+==========================================================+
| ``'<'`` | Forces the field to be left-aligned within the available |
| | space (this is the default for most objects). |
+---------+----------------------------------------------------------+
| ``'>'`` | Forces the field to be right-aligned within the |
| | available space (this is the default for numbers). |
+---------+----------------------------------------------------------+
| ``'^'`` | Forces the field to be centered within the available |
| | space. |
+---------+----------------------------------------------------------+
Note that unless a minimum field width is defined, the field width will always
be the same size as the data to fill it, so that the alignment option has no
meaning in this case.
The *sign* option is only valid for number types, and can be one of the
following:
+---------+----------------------------------------------------------+
| Option | Meaning |
+=========+==========================================================+
| ``'+'`` | indicates that a sign should be used for both |
| | positive as well as negative numbers. |
+---------+----------------------------------------------------------+
| ``'-'`` | indicates that a sign should be used only for negative |
| | numbers (this is the default behavior). |
+---------+----------------------------------------------------------+
| space | indicates that a leading space should be used on |
| | positive numbers, and a minus sign on negative numbers. |
+---------+----------------------------------------------------------+
The ``'#'`` option causes the "alternate form" to be used for the
conversion. The alternate form is defined differently for different
types. This option is only valid for integer and floating-point types.
For integers, when binary, octal, or hexadecimal output is used, this
option adds the prefix respective ``"0b"`` (``"0B"``), ``"0"``, or
``"0x"`` (``"0X"``) to the output value. Whether the prefix is
lower-case or upper-case is determined by the case of the type
specifier, for example, the prefix ``"0x"`` is used for the type ``'x'``
and ``"0X"`` is used for ``'X'``. For floating-point numbers the
alternate form causes the result of the conversion to always contain a
decimal-point character, even if no digits follow it. Normally, a
decimal-point character appears in the result of these conversions
only if a digit follows it. In addition, for ``'g'`` and ``'G'``
conversions, trailing zeros are not removed from the result.
.. ifconfig:: False
The ``','`` option signals the use of a comma for a thousands separator.
For a locale aware separator, use the ``'L'`` integer presentation type
instead.
*width* is a decimal integer defining the minimum field width. If not
specified, then the field width will be determined by the content.
Preceding the *width* field by a zero (``'0'``) character enables sign-aware
zero-padding for numeric types. It forces the padding to be placed after the
sign or base (if any) but before the digits. This is used for printing fields in
the form '+000000120'. This option is only valid for numeric types and it has no
effect on formatting of infinity and NaN.
The *precision* is a decimal number indicating how many digits should be
displayed after the decimal point for a floating-point value formatted with
``'f'`` and ``'F'``, or before and after the decimal point for a floating-point
value formatted with ``'g'`` or ``'G'``. For non-number types the field
indicates the maximum field size - in other words, how many characters will be
used from the field content. The *precision* is not allowed for integer,
character, Boolean, and pointer values.
Finally, the *type* determines how the data should be presented.
The available string presentation types are:
+---------+----------------------------------------------------------+
| Type | Meaning |
+=========+==========================================================+
| ``'s'`` | String format. This is the default type for strings and |
| | may be omitted. |
+---------+----------------------------------------------------------+
| none | The same as ``'s'``. |
+---------+----------------------------------------------------------+
The available character presentation types are:
+---------+----------------------------------------------------------+
| Type | Meaning |
+=========+==========================================================+
| ``'c'`` | Character format. This is the default type for |
| | characters and may be omitted. |
+---------+----------------------------------------------------------+
| none | The same as ``'c'``. |
+---------+----------------------------------------------------------+
The available integer presentation types are:
+---------+----------------------------------------------------------+
| Type | Meaning |
+=========+==========================================================+
| ``'b'`` | Binary format. Outputs the number in base 2. Using the |
| | ``'#'`` option with this type adds the prefix ``"0b"`` |
| | to the output value. |
+---------+----------------------------------------------------------+
| ``'B'`` | Binary format. Outputs the number in base 2. Using the |
| | ``'#'`` option with this type adds the prefix ``"0B"`` |
| | to the output value. |
+---------+----------------------------------------------------------+
| ``'d'`` | Decimal integer. Outputs the number in base 10. |
+---------+----------------------------------------------------------+
| ``'o'`` | Octal format. Outputs the number in base 8. |
+---------+----------------------------------------------------------+
| ``'x'`` | Hex format. Outputs the number in base 16, using |
| | lower-case letters for the digits above 9. Using the |
| | ``'#'`` option with this type adds the prefix ``"0x"`` |
| | to the output value. |
+---------+----------------------------------------------------------+
| ``'X'`` | Hex format. Outputs the number in base 16, using |
| | upper-case letters for the digits above 9. Using the |
| | ``'#'`` option with this type adds the prefix ``"0X"`` |
| | to the output value. |
+---------+----------------------------------------------------------+
| ``'L'`` | Locale-specific format. This is the same as ``'d'``, |
| | except that it uses the current locale setting to insert |
| | the appropriate number separator characters. |
+---------+----------------------------------------------------------+
| none | The same as ``'d'``. |
+---------+----------------------------------------------------------+
Integer presentation types can also be used with character and Boolean values.
Boolean values are formatted using textual representation, either ``true`` or
``false``, if the presentation type is not specified.
The available presentation types for floating-point values are:
+---------+----------------------------------------------------------+
| Type | Meaning |
+=========+==========================================================+
| ``'a'`` | Hexadecimal floating point format. Prints the number in |
| | base 16 with prefix ``"0x"`` and lower-case letters for |
| | digits above 9. Uses ``'p'`` to indicate the exponent. |
+---------+----------------------------------------------------------+
| ``'A'`` | Same as ``'a'`` except it uses upper-case letters for |
| | the prefix, digits above 9 and to indicate the exponent. |
+---------+----------------------------------------------------------+
| ``'e'`` | Exponent notation. Prints the number in scientific |
| | notation using the letter 'e' to indicate the exponent. |
+---------+----------------------------------------------------------+
| ``'E'`` | Exponent notation. Same as ``'e'`` except it uses an |
| | upper-case ``'E'`` as the separator character. |
+---------+----------------------------------------------------------+
| ``'f'`` | Fixed point. Displays the number as a fixed-point |
| | number. |
+---------+----------------------------------------------------------+
| ``'F'`` | Fixed point. Same as ``'f'``, but converts ``nan`` to |
| | ``NAN`` and ``inf`` to ``INF``. |
+---------+----------------------------------------------------------+
| ``'g'`` | General format. For a given precision ``p >= 1``, |
| | this rounds the number to ``p`` significant digits and |
| | then formats the result in either fixed-point format |
| | or in scientific notation, depending on its magnitude. |
| | |
| | A precision of ``0`` is treated as equivalent to a |
| | precision of ``1``. |
+---------+----------------------------------------------------------+
| ``'G'`` | General format. Same as ``'g'`` except switches to |
| | ``'E'`` if the number gets too large. The |
| | representations of infinity and NaN are uppercased, too. |
+---------+----------------------------------------------------------+
| ``'L'`` | Locale-specific format. This is the same as ``'g'``, |
| | except that it uses the current locale setting to insert |
| | the appropriate number separator characters. |
+---------+----------------------------------------------------------+
| none | Similar to ``'g'``, except that fixed-point notation, |
| | when used, has at least one digit past the decimal |
| | point. The default precision is as high as needed to |
| | represent the particular value. |
+---------+----------------------------------------------------------+
.. ifconfig:: False
+---------+----------------------------------------------------------+
| | The precise rules are as follows: suppose that the |
| | result formatted with presentation type ``'e'`` and |
| | precision ``p-1`` would have exponent ``exp``. Then |
| | if ``-4 <= exp < p``, the number is formatted |
| | with presentation type ``'f'`` and precision |
| | ``p-1-exp``. Otherwise, the number is formatted |
| | with presentation type ``'e'`` and precision ``p-1``. |
| | In both cases insignificant trailing zeros are removed |
| | from the significand, and the decimal point is also |
| | removed if there are no remaining digits following it. |
| | |
| | Positive and negative infinity, positive and negative |
| | zero, and nans, are formatted as ``inf``, ``-inf``, |
| | ``0``, ``-0`` and ``nan`` respectively, regardless of |
| | the precision. |
| | |
+---------+----------------------------------------------------------+
The available presentation types for pointers are:
+---------+----------------------------------------------------------+
| Type | Meaning |
+=========+==========================================================+
| ``'p'`` | Pointer format. This is the default type for |
| | pointers and may be omitted. |
+---------+----------------------------------------------------------+
| none | The same as ``'p'``. |
+---------+----------------------------------------------------------+
.. _formatexamples:
Format Examples
===============
This section contains examples of the format syntax and comparison with
the printf formatting.
In most of the cases the syntax is similar to the printf formatting, with the
addition of the ``{}`` and with ``:`` used instead of ``%``.
For example, ``"%03.2f"`` can be translated to ``"{:03.2f}"``.
The new format syntax also supports new and different options, shown in the
following examples.
Accessing arguments by position::
fmt::format("{0}, {1}, {2}", 'a', 'b', 'c');
// Result: "a, b, c"
fmt::format("{}, {}, {}", 'a', 'b', 'c');
// Result: "a, b, c"
fmt::format("{2}, {1}, {0}", 'a', 'b', 'c');
// Result: "c, b, a"
fmt::format("{0}{1}{0}", "abra", "cad"); // arguments' indices can be repeated
// Result: "abracadabra"
Aligning the text and specifying a width::
fmt::format("{:<30}", "left aligned");
// Result: "left aligned "
fmt::format("{:>30}", "right aligned");
// Result: " right aligned"
fmt::format("{:^30}", "centered");
// Result: " centered "
fmt::format("{:*^30}", "centered"); // use '*' as a fill char
// Result: "***********centered***********"
Dynamic width::
fmt::format("{:<{}}", "left aligned", 30);
// Result: "left aligned "
Dynamic precision::
fmt::format("{:.{}f}", 3.14, 1);
// Result: "3.1"
Replacing ``%+f``, ``%-f``, and ``% f`` and specifying a sign::
fmt::format("{:+f}; {:+f}", 3.14, -3.14); // show it always
// Result: "+3.140000; -3.140000"
fmt::format("{: f}; {: f}", 3.14, -3.14); // show a space for positive numbers
// Result: " 3.140000; -3.140000"
fmt::format("{:-f}; {:-f}", 3.14, -3.14); // show only the minus -- same as '{:f}; {:f}'
// Result: "3.140000; -3.140000"
Replacing ``%x`` and ``%o`` and converting the value to different bases::
fmt::format("int: {0:d}; hex: {0:x}; oct: {0:o}; bin: {0:b}", 42);
// Result: "int: 42; hex: 2a; oct: 52; bin: 101010"
// with 0x or 0 or 0b as prefix:
fmt::format("int: {0:d}; hex: {0:#x}; oct: {0:#o}; bin: {0:#b}", 42);
// Result: "int: 42; hex: 0x2a; oct: 052; bin: 0b101010"
Padded hex byte with prefix and always prints both hex characters::
fmt::format("{:#04x}", 0);
// Result: "0x00"
Box drawing using Unicode fill::
fmt::print(
"┌{0:─^{2}}┐\n"
"│{1: ^{2}}│\n"
"└{0:─^{2}}┘\n", "", "Hello, world!", 20);
prints::
┌────────────────────┐
│ Hello, world! │
└────────────────────┘
Using type-specific formatting::
#include <fmt/chrono.h>
auto t = tm();
t.tm_year = 2010 - 1900;
t.tm_mon = 6;
t.tm_mday = 4;
t.tm_hour = 12;
t.tm_min = 15;
t.tm_sec = 58;
fmt::print("{:%Y-%m-%d %H:%M:%S}", t);
// Prints: 2010-08-04 12:15:58
Using the comma as a thousands separator::
#include <fmt/locale.h>
auto s = fmt::format(std::locale("en_US.UTF-8"), "{:L}", 1234567890);
// s == "1,234,567,890"
.. ifconfig:: False
Nesting arguments and more complex examples::
>>> for align, text in zip('<^>', ['left', 'center', 'right']):
... '{0:{fill}{align}16}") << text, fill=align, align=align)
...
'left<<<<<<<<<<<<'
'^^^^^center^^^^^'
'>>>>>>>>>>>right'
>>>
>>> octets = [192, 168, 0, 1]
Format("{:02X}{:02X}{:02X}{:02X}") << *octets)
'C0A80001'
>>> int(_, 16)
3232235521
>>>
>>> width = 5
>>> for num in range(5,12):
... for base in 'dXob':
... print('{0:{width}{base}}") << num, base=base, width=width), end=' ')
... print()
...
5 5 5 101
6 6 6 110
7 7 7 111
8 8 10 1000
9 9 11 1001
10 A 12 1010
11 B 13 1011
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