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Module `std.format`

This package provides string formatting functionality using printf style format strings.


Submodule	Function Name	Description
package	`format`	Converts its arguments according to a format string into a string.
package	`sformat`	Converts its arguments according to a format string into a buffer.
package	`FormatException`	Signals a problem while formatting.
`write`	`formattedWrite`	Converts its arguments according to a format string and writes the result to an output range.
`write`	`formatValue`	Formats a value of any type according to a format specifier and writes the result to an output range.
`read`	`formattedRead`	Reads an input range according to a format string and stores the read values into its arguments.
`read`	`unformatValue`	Reads a value from the given input range and converts it according to a format specifier.
`spec`	`FormatSpec`	A general handler for format strings.
`spec`	`singleSpec`	Helper function that returns a `FormatSpec` for a single format specifier.

Limitation

This package does not support localization, but adheres to the rounding mode of the floating point unit, if available.

Format Strings

The functions contained in this package use format strings. A format string describes the layout of another string for reading or writing purposes. A format string is composed of normal text interspersed with format specifiers. A format specifier starts with a percentage sign '%', optionally followed by one or more parameters and ends with a format indicator. A format indicator may be a simple format character or a compound indicator.

Format strings are composed according to the following grammar:

FormatString:
    FormatStringItem FormatString
FormatStringItem:
    Character
    FormatSpecifier
FormatSpecifier:
    '%' Parameters FormatIndicator
FormatIndicator:
    FormatCharacter
    CompoundIndicator
FormatCharacter:
    see remark below
CompoundIndicator:
    '(' FormatString '%)'
    '(' FormatString '%|' Delimiter '%)'
Delimiter
    empty
    Character Delimiter
Parameters:
    Position Flags Width Precision Separator
Position:
    empty
    Integer '$'
    Integer ':' Integer '$'
    Integer ':' '$'
Flags:
    empty
    Flag Flags
Flag:
    '-'|'+'|' '|'0'|'#'|'='
Width:
    OptionalPositionalInteger
Precision:
    empty
    '.' OptionalPositionalInteger
Separator:
    empty
    ',' OptionalInteger
    ',' OptionalInteger '?'
OptionalInteger:
    empty
    Integer
    '*'
OptionalPositionalInteger:
    OptionalInteger
    '*' Integer '$'
Character
    '%%'
    AnyCharacterExceptPercent
Integer:
    NonZeroDigit Digits
Digits:
    empty
    Digit Digits
NonZeroDigit:
    '1'|'2'|'3'|'4'|'5'|'6'|'7'|'8'|'9'
Digit:
    '0'|'1'|'2'|'3'|'4'|'5'|'6'|'7'|'8'|'9'

Note

FormatCharacter is unspecified. It can be any character that has no other purpose in this grammar, but it is recommended to assign (lower- and uppercase) letters.

Note

The Parameters of a CompoundIndicator are currently limited to a '-' flag.

Format Indicator

The format indicator can either be a single character or an expression surrounded by %\() and %\. It specifies the basic manner in which a value will be formatted and is the minimum requirement to format a value.

The following characters can be used as format characters:


FormatCharacter	Semantics
's'	To be formatted in a human readable format. Can be used with all types.
'c'	To be formatted as a character.
'd'	To be formatted as a signed decimal integer.
'u'	To be formatted as a decimal image of the underlying bit representation.
'b'	To be formatted as a binary image of the underlying bit representation.
'o'	To be formatted as an octal image of the underlying bit representation.
'x' / 'X'	To be formatted as a hexadecimal image of the underlying bit representation.
'e' / 'E'	To be formatted as a real number in decimal scientific notation.
'f' / 'F'	To be formatted as a real number in decimal natural notation.
'g' / 'G'	To be formatted as a real number in decimal short notation. Depending on the number, a scientific notation or a natural notation is used.
'a' / 'A'	To be formatted as a real number in hexadecimal scientific notation.
'r'	To be formatted as raw bytes. The output may not be printable and depends on endianness.

The compound indicator can be used to describe compound types like arrays or structs in more detail. A compound type is enclosed within '%\(') and '%\'. The enclosed sub-format string is applied to individual elements. The trailing portion of the sub-format string following the specifier for the element is interpreted as the delimiter, and is therefore omitted following the last element. The '%|' specifier may be used to explicitly indicate the start of the delimiter, so that the preceding portion of the string will be included following the last element.

The format string inside of the compound indicator should contain exactly one format specifier (two in case of associative arrays), which specifies the formatting mode of the elements of the compound type. This format specifier can be a compound indicator itself.

Note

Inside a compound indicator, strings and characters are escaped automatically. To avoid this behavior, use "%-(" instead of "%(".

Flags

There are several flags that affect the outcome of the formatting.


Flag	Semantics
'-'	When the formatted result is shorter then the value given by the width parameter, the output is right justified. With the '-' flag this is changed to left justification. There are two exceptions where the '-' flag has a different meaning: (1) with 'r' it denotes to use little endian and (2) in case of a compound indicator it means that no special handling of the members is applied.
'='	When the formatted result is shorter then the value given by the width parameter, the output is centered. If the central position is not possible it is moved slightly to the right. In this case, if '-' flag is present in addition to the '=' flag, it is moved slightly to the left.
'+' / ' '	Applies to numerical values. By default, positive numbers are not formatted to include the `+` sign. With one of these two flags present, positive numbers are preceded by a plus sign or a space. When both flags are present, a plus sign is used. In case of 'r', a big endian format is used.
'0'	Is applied to numerical values that are printed right justified. If the zero flag is present, the space left to the number is filled with zeros instead of spaces.
'#'	Denotes that an alternative output must be used. This depends on the type to be formatted and the format character used. See the sections below for more information.

Width, Precision and Separator

Precision and Separator

The width parameter specifies the minimum width of the result.

The meaning of precision depends on the format indicator. For integers it denotes the minimum number of digits printed, for real numbers it denotes the number of fractional digits and for strings and compound types it denotes the maximum number of elements that are included in the output.

A separator is used for formatting numbers. If it is specified, the output is divided into chunks of three digits, separated by a ','. The number of digits in a chunk can be given explicitly by providing a number or a '*' after the ','.

In all three cases the number of digits can be replaced by a '*'. In this scenario, the next argument is used as the number of digits. If the argument is a negative number, the precision and separator parameters are considered unspecified. For width, the absolute value is used and the '-' flag is set.

The separator can also be followed by a '?'. In that case, an additional argument is used to specify the symbol that should be used to separate the chunks.

Position

By default, the arguments are processed in the provided order. With the position parameter it is possible to address arguments directly. It is also possible to denote a series of arguments with two numbers separated by ':', that are all processed in the same way. The second number can be omitted. In that case the series ends with the last argument.

It's also possible to use positional arguments for width, precision and separator by adding a number and a '$' after the '*'.

Types

This section describes the result of combining types with format characters. It is organized in 2 subsections: a list of general information regarding the formatting of types in the presence of format characters and a table that contains details for every available combination of type and format character.

When formatting types, the following rules apply:

If the format character is upper case, the resulting string will be formatted using upper case letters.
The default precision for floating point numbers is 6 digits.
Rounding of floating point numbers adheres to the rounding mode of the floating point unit, if available.
The floating point values NaN and Infinity are formatted as nan and inf, possibly preceded by '+' or '-' sign.
Formatting reals is only supported for 64 bit reals and 80 bit reals. All other reals are cast to double before they are formatted. This will cause the result to be inf for very large numbers.
Characters and strings formatted with the 's' format character inside of compound types are surrounded by single and double quotes and unprintable characters are escaped. To avoid this, a '-' flag can be specified for the compound specifier (e.g. "%-(%s%)" instead of "%(%s%)" ).
Structs, unions, classes and interfaces are formatted by calling a toString method if available. See module std.format.write for more details.
Only part of these combinations can be used for reading. See module std.format.read for more detailed information.

This table contains descriptions for every possible combination of type and format character:


Type	Format Character	Formatted as...
`null`	's'	`null`
`bool`	's'	`false` or `true`
	'b', 'd', 'o', 'u', 'x', 'X'	As the integrals 0 or 1 with the same format character. Please note, that 'o'* and 'x' with '#' flag might produce unexpected results due to special handling of the value 0.*
	'r'	`\0` or `\1`
Integral	's', 'd'	A signed decimal number. The '#' flag is ignored.
	'b', 'o', 'u', 'x', 'X'	An unsigned binary, decimal, octal or hexadecimal number. In case of 'o' and 'x', the '#' flag denotes that the number must be preceded by `0` and `0x`, with the exception of the value 0, where this does not apply. For 'b' and 'u' the '#' flag has no effect.
	'e', 'E', 'f', 'F', 'g', 'G', 'a', 'A'	As a floating point value with the same specifier. Default precision is large enough to add all digits of the integral value. In case of ($B 'a') and 'A', the integral digit can be any hexadecimal digit.
	'r'	Characters taken directly from the binary representation.
Floating Point	'e', 'E'	Scientific notation: Exactly one integral digit followed by a dot and fractional digits, followed by the exponent. The exponent is formatted as 'e' followed by a '+' or '-' sign, followed by at least two digits. When there are no fractional digits and the '#' flag is not present, the dot is omitted.
	'f', 'F'	Natural notation: Integral digits followed by a dot and fractional digits. When there are no fractional digits and the '#' flag is not present, the dot is omitted. Please note: the difference between 'f'* and 'F' is only visible for `NaN` and `Infinity`.*
	's', 'g', 'G'	Short notation: If the absolute value is larger than `10 ^^ precision` or smaller than `0.0001`, the scientific notation is used. If not, the natural notation is applied. In both cases precision denotes the count of all digits, including the integral digits. Trailing zeros (including a trailing dot) are removed. If '#' flag is present, trailing zeros are not removed.
	'a', 'A'	Hexadecimal scientific notation: `0x` followed by `1` (or `0` in case of value zero or denormalized number) followed by a dot, fractional digits in hexadecimal notation and an exponent. The exponent is build by `p`, followed by a sign and the exponent in decimal notation. When there are no fractional digits and the '#' flag is not present, the dot is omitted.
	'r'	Characters taken directly from the binary representation.
Character	's', 'c'	As the character. Inside of a compound indicator 's' is treated differently: The character is surrounded by single quotes and non printable characters are escaped. This can be avoided by preceding the compound indicator with a '-' flag (e.g. `"%-(%s%)"`).
	'b', 'd', 'o', 'u', 'x', 'X'	As the integral that represents the character.
	'r'	Characters taken directly from the binary representation.
String	's'	The sequence of characters that form the string. Inside of a compound indicator the string is surrounded by double quotes and non printable characters are escaped. This can be avoided by preceding the compound indicator with a '-' flag (e.g. `"%-(%s%)"`).
	'r'	The sequence of characters, each formatted with 'r'.
	compound	As an array of characters.
Array	's'	When the elements are characters, the array is formatted as a string. In all other cases the array is surrounded by square brackets and the elements are separated by a comma and a space. If the elements are strings, they are surrounded by double quotes and non printable characters are escaped.
	'r'	The sequence of the elements, each formatted with 'r'.
	compound	The sequence of the elements, each formatted according to the specifications given inside of the compound specifier.
Associative Array	's'	As a sequence of the elements in unpredictable order. The output is surrounded by square brackets. The elements are separated by a comma and a space. The elements are formatted as `key:value`.
Associative Array	compound	As a sequence of the elements in unpredictable order. Each element is formatted according to the specifications given inside of the compound specifier. The first specifier is used for formatting the key and the second specifier is used for formatting the value. The order can be changed with positional arguments. For example `"%(%2$s (%1$s), %)"` will write the value, followed by the key in parenthesis.
Enum	's'	The name of the value. If the name is not available, the base value is used, preceeded by a cast.
Enum	All, but 's'	Enums can be formatted with all format characters that can be used with the base value. In that case they are formatted like the base value.
Input Range	's'	When the elements of the range are characters, they are written like a string. In all other cases, the elements are enclosed by square brackets and separated by a comma and a space.
	'r'	The sequence of the elements, each formatted with 'r'.
	compound	The sequence of the elements, each formatted according to the specifications given inside of the compound specifier.
Struct	's'	When the struct has neither an applicable `toString` nor is an input range, it is formatted as follows: `StructType(field1, field2, ...)`.
Class	's'	When the class has neither an applicable `toString` nor is an input range, it is formatted as the fully qualified name of the class.
Union	's'	When the union has neither an applicable `toString` nor is an input range, it is formatted as its base name.
Pointer	's'	A null pointer is formatted as 'null'. All other pointers are formatted as hexadecimal numbers with the format character 'X'.
Pointer	'x', 'X'	Formatted as a hexadecimal number.
SIMD vector	's'	The array is surrounded by square brackets and the elements are separated by a comma and a space.
	'r'	The sequence of the elements, each formatted with 'r'.
	compound	The sequence of the elements, each formatted according to the specifications given inside of the compound specifier.
Delegate	's', 'r', compound	As the `.stringof` of this delegate treated as a string. Please note: The implementation is currently buggy and its use is discouraged.

Example

Simple use:

// Easiest way is to use `%s` everywhere:
// "I got 30 eggs for 5.27 euros."
writeln(format("I got %s %s for %s euros.", 30, "eggs", 5.27));

// Other format characters provide more control:
// "I got 11110 65676773 for 5.270000 euros."
writeln(format("I got %b %(%X%) for %f euros.", 30, "eggs", 5.27));

Example

Compound specifiers allow formatting arrays and other compound types:

/*
The trailing end of the sub-format string following the specifier for
each item is interpreted as the array delimiter, and is therefore
omitted following the last array item:
 */
    writeln(format("My items are %(%s %).", [1, 2, 3])); // "My items are 1 2 3."
    writeln(format("My items are %(%s, %).", [1, 2, 3])); // "My items are 1, 2, 3."

/*
The "%|" delimiter specifier may be used to indicate where the
delimiter begins, so that the portion of the format string prior to
it will be retained in the last array element:
 */
    writeln(format("My items are %(-%s-%|, %).", [1, 2, 3])); // "My items are -1-, -2-, -3-."

/*
These compound format specifiers may be nested in the case of a
nested array argument:
 */
    auto mat = [[1, 2, 3],
                [4, 5, 6],
                [7, 8, 9]];

    assert(format("%(%(%d %) - %)", mat), "1 2 3 - 4 5 6 - 7 8 9");
    assert(format("[%(%(%d %) - %)]", mat), "[1 2 3 - 4 5 6 - 7 8 9]");
    assert(format("[%([%(%d %)]%| - %)]", mat), "[1 2 3] - [4 5 6] - [7 8 9]");

/*
Strings and characters are escaped automatically inside compound
format specifiers. To avoid this behavior, use "%-(" instead of "%(":
 */
    // `My friends are ["John", "Nancy"].`
    writeln(format("My friends are %s.", ["John", "Nancy"]));
    // `My friends are "John", "Nancy".`
    writeln(format("My friends are %(%s, %).", ["John", "Nancy"]));
    // `My friends are John, Nancy.`
    writeln(format("My friends are %-(%s, %).", ["John", "Nancy"]));

Example

Using parameters:

// Flags can be used to influence to outcome:
writeln(format("%g != %+#g", 3.14, 3.14)); // "3.14 != +3.14000"

// Width and precision help to arrange the formatted result:
writeln(format(">%10.2f<", 1234.56789)); // ">   1234.57<"

// Numbers can be grouped:
writeln(format("%,4d", int.max)); // "21,4748,3647"

// It's possible to specify the position of an argument:
writeln(format("%3$s %1$s", 3, 17, 5)); // "5 3"

Example

Providing parameters as arguments:

// Width as argument
writeln(format(">%*s<", 10, "abc")); // ">       abc<"

// Precision as argument
writeln(format(">%.*f<", 5, 123.2)); // ">123.20000<"

// Grouping as argument
writeln(format("%,*d", 1, int.max)); // "2,1,4,7,4,8,3,6,4,7"

// Grouping separator as argument
writeln(format("%,3?d", '_', int.max)); // "2_147_483_647"

// All at once
writeln(format("%*.*,*?d", 20, 15, 6, '/', int.max)); // "   000/002147/483647"

Functions

Name	Description
`format(fmt, args)`	Converts its arguments according to a format string into a string.
`sformat(buf, fmt, args)`	Converts its arguments according to a format string into a buffer. The buffer has to be large enough to hold the formatted string.

Classes

Name	Description
`FormatException`	Signals an issue encountered while formatting.

Authors

Walter Bright, Andrei Alexandrescu, and Kenji Hara

License

Boost License 1.0.

API Documentation

Module std.format

Limitation

Format Strings

Note

Note

Format Indicator

Note

Flags

Width, Precision and Separator

Position

Types

Example

Example

Example

Example

Functions

Classes

Authors

License

Module `std.format`