Function std.format.formattedWrite

The variadic arguments are normally consumed in order. POSIX-style positional parameter syntax is also supported. Each argument is formatted into a sequence of chars according to the format specification, and the characters are passed to w. As many arguments as specified in the format string are consumed and formatted. If there are fewer arguments than format specifiers, a FormatException is thrown. If there are more remaining arguments than needed by the format specification, they are ignored but only if at least one argument was formatted.

The format string supports the formatting of array and nested array elements via the grouping format specifiers %( and %). Each matching pair of %( and %) corresponds with a single array argument. The enclosed sub-format string is applied to individual array elements. The trailing portion of the sub-format string following the conversion specifier for the array element is interpreted as the array delimiter, and is therefore omitted following the last array 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 array element. (See below for explicit examples.)

Parameters

Returns

Formatted number of arguments.

Throws

Mismatched arguments and formats result in a FormatException being thrown.

Format String

Format strings consist of characters interspersed with format specifications. Characters are simply copied to the output (such as putc) after any necessary conversion to the corresponding UTF-8 sequence.

The format string has the following grammar:

FormatString:
    FormatStringItem*
FormatStringItem:
    '%%'
    '%' Position Flags Width Separator Precision FormatChar
    '%(' FormatString '%)'
    OtherCharacterExceptPercent
Position:
    empty
    Integer '$'
Flags:
    empty
    '-' Flags
    '+' Flags
    '#' Flags
    '0' Flags
    ' ' Flags
Width:
    empty
    Integer
    '*'
Separator:
    empty
    ','
    ',' '?'
    ',' '*' '?'
    ',' Integer '?'
    ',' '*'
    ',' Integer
Precision:
    empty
    '.'
    '.' Integer
    '.*'
Integer:
    Digit
    Digit Integer
Digit:
    '0'|'1'|'2'|'3'|'4'|'5'|'6'|'7'|'8'|'9'
FormatChar:
    's'|'c'|'b'|'d'|'o'|'x'|'X'|'e'|'E'|'f'|'F'|'g'|'G'|'a'|'A'|'|'

Width

Specifies the minimum field width. If the width is a *, an additional argument of type int, preceding the actual argument, is taken as the width. If the width is negative, it is as if the - was given as a Flags character.

Precision

Gives the precision for numeric conversions. If the precision is a *, an additional argument of type int, preceding the actual argument, is taken as the precision. If it is negative, it is as if there was no Precision specifier.

Separator

Inserts the separator symbols ',' every X digits, from right to left, into numeric values to increase readability. The fractional part of floating point values inserts the separator from left to right. Entering an integer after the ',' allows to specify X. If a '*' is placed after the ',' then X is specified by an additional parameter to the format function. Adding a '?' after the ',' or X specifier allows to specify the separator character as an additional parameter.

FormatChar

's'

The corresponding argument is formatted in a manner consistent with its type:

bool

The result is "true" or "false".

integral types

The %d format is used.

floating point types

The %g format is used.

string types

The result is the string converted to UTF-8. A Precision specifies the maximum number of characters to use in the result.

structs

If the struct defines a toString() method the result is the string returned from this function. Otherwise the result is StructName(field₀, field₁, ...) where field_n is the nth element formatted with the default format.

classes derived from Object

The result is the string returned from the class instance's .toString() method. A Precision specifies the maximum number of characters to use in the result.

unions

If the union defines a toString() method the result is the string returned from this function. Otherwise the result is the name of the union, without its contents.

non-string static and dynamic arrays

The result is [s₀, s₁, ...] where s_n is the nth element formatted with the default format.

associative arrays

The result is the equivalent of what the initializer would look like for the contents of the associative array, e.g.: ["red" : 10, "blue" : 20].

'c'

The corresponding argument must be a character type.

'b','d','o','x','X'

The corresponding argument must be an integral type and is formatted as an integer. If the argument is a signed type and the FormatChar is d it is converted to a signed string of characters, otherwise it is treated as unsigned. An argument of type bool is formatted as '1' or '0'. The base used is binary for b, octal for o, decimal for d, and hexadecimal for x or X. x formats using lower case letters, X uppercase. If there are fewer resulting digits than the Precision, leading zeros are used as necessary. If the Precision is 0 and the number is 0, no digits result.

'e','E'

A floating point number is formatted as one digit before the decimal point, Precision digits after, the FormatChar, ±, followed by at least a two digit exponent: d.dddddde±dd. If there is no Precision, six digits are generated after the decimal point. If the Precision is 0, no decimal point is generated.

'f','F'

A floating point number is formatted in decimal notation. The Precision specifies the number of digits generated after the decimal point. It defaults to six. At least one digit is generated before the decimal point. If the Precision is zero, no decimal point is generated.

'g','G'

A floating point number is formatted in either e or f format for g; E or F format for G. The f format is used if the exponent for an e format is greater than -5 and less than the Precision. The Precision specifies the number of significant digits, and defaults to six. Trailing zeros are elided after the decimal point, if the fractional part is zero then no decimal point is generated.

'a','A'

A floating point number is formatted in hexadecimal exponential notation 0xh.hhhhhhp±d. There is one hexadecimal digit before the decimal point, and as many after as specified by the Precision. If the Precision is zero, no decimal point is generated. If there is no Precision, as many hexadecimal digits as necessary to exactly represent the mantissa are generated. The exponent is written in as few digits as possible, but at least one, is in decimal, and represents a power of 2 as in h.hhhhhh*2^±d. The exponent for zero is zero. The hexadecimal digits, x and p are in upper case if the FormatChar is upper case.

Floating point NaN's are formatted as nan if the FormatChar is lower case, or NAN if upper. Floating point infinities are formatted as inf or infinity if the FormatChar is lower case, or INF or INFINITY if upper.

The positional and non-positional styles can be mixed in the same format string. (POSIX leaves this behavior undefined.) The internal counter for non-positional parameters tracks the next parameter after the largest positional parameter already used.

Example using array and nested array formatting:

import std.stdio;

void main()
{
    writefln("My items are %(%s %).", [1,2,3]);
    writefln("My items are %(%s, %).", [1,2,3]);
}

The output is:

My items are 1 2 3.
My items are 1, 2, 3.

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. 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:

import std.stdio;

void main()
{
    writefln("My items are %(-%s-%|, %).", [1,2,3]);
}

which gives the output:

My items are -1-, -2-, -3-.

These compound format specifiers may be nested in the case of a nested array argument:

import std.stdio;
void main() {
     auto mat = [[1, 2, 3],
                 [4, 5, 6],
                 [7, 8, 9]];

     writefln("%(%(%d %)\n%)", mat);
     writeln();

     writefln("[%(%(%d %)\n %)]", mat);
     writeln();

     writefln("[%([%(%d %)]%|\n %)]", mat);
     writeln();
}

The output is:

1 2 3
4 5 6
7 8 9
[1 2 3
 4 5 6
 7 8 9]
[[1 2 3]
 [4 5 6]
 [7 8 9]]

Inside a compound format specifier, strings and characters are escaped automatically. To avoid this behavior, add '-' flag to "%(".

import std.stdio;

void main()
{
    writefln("My friends are %s.", ["John", "Nancy"]);
    writefln("My friends are %(%s, %).", ["John", "Nancy"]);
    writefln("My friends are %-(%s, %).", ["John", "Nancy"]);
}

which gives the output:

My friends are ["John", "Nancy"].
My friends are "John", "Nancy".
My friends are John, Nancy.

Example

The format string can be checked at compile-time (see format for details):

import std.array : appender;

auto writer = appender!string();
writer.formattedWrite!"%s is the ultimate %s."(42, "answer");
writeln(writer.data); // "42 is the ultimate answer."

// Clear the writer
writer = appender!string();
formattedWrite(writer, "Date: %2$s %1$s", "October", 5);
writeln(writer.data); // "Date: 5 October"

Example

writeln(format("%,d", 1000)); // "1,000"
writeln(format("%,f", 1234567.891011)); // "1,234,567.891,011"
writeln(format("%,?d", '?', 1000)); // "1?000"
writeln(format("%,1d", 1000)); // "1,0,0,0"
writeln(format("%,*d", 4, -12345)); // "-1,2345"
writeln(format("%,*?d", 4, '_', -12345)); // "-1_2345"
writeln(format("%,6?d", '_', -12345678)); // "-12_345678"
assert(format("%12,3.3f", 1234.5678) == "   1,234.568", "'" ~
        format("%12,3.3f", 1234.5678) ~ "'");

Authors

Walter Bright, Andrei Alexandrescu, and Kenji Hara

License

Boost License 1.0.