format_specifications - Documentation for Ruby 4.0
Description: format_specifications: Argument +format_string+, which has zero or more embedded _format_ _specifications_ (see below). tt>, which are zero or more objects to be formatted
Keywords: ruby,documentation,format_specifications

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# Format Specifications

Several Ruby core classes have instance method `printf` or `sprintf`:

- `ARGF#printf`

- `IO#printf`

- `Kernel#printf`

- `Kernel#sprintf`

Each of these methods takes:

- Argument `format_string`, which has zero or more embedded format specifications (see below).

- Arguments `*arguments`, which are zero or more objects to be formatted.

Each of these methods prints or returns the string resulting from replacing each format specification embedded in `format_string` with a string form of the corresponding argument among `arguments`.

A simple example:

    sprintf('Name: %s; value: %d', 'Foo', 0) # => "Name: Foo; value: 0"

A format specification has the form:

    %[flags][width][.precision]type

It consists of:

- A leading percent character.

- Zero or more flags (each is a character).

- An optional width specifier (an integer, or `*`).

- An optional precision specifier (a period followed by a non-negative integer, or `*`).

- A type specifier (a character).

Except for the leading percent character, the only required part is the type specifier, so we begin with that.

## Type Specifiers

This section provides a brief explanation of each type specifier. The links lead to the details and examples.

### Integer Type Specifiers

- `b` or `B`: Format `argument` as a binary integer. See Specifiers b and B.

- `d`, `i`, or `u` (all are identical): Format `argument` as a decimal integer. See Specifier d.

- `o`: Format `argument` as an octal integer. See Specifier o.

- `x` or `X`: Format `argument` as a hexadecimal integer. See Specifiers x and X.

### Floating-Point Type Specifiers

- `a` or `A`: Format `argument` as hexadecimal floating-point number. See Specifiers a and A.

- `e` or `E`: Format `argument` in scientific notation. See Specifiers e and E.

- `f`: Format `argument` as a decimal floating-point number. See Specifier f.

- `g` or `G`: Format `argument` in a “general” format. See Specifiers g and G.

### Other Type Specifiers

- `c`: Format `argument` as a character. See Specifier c.

- `p`: Format `argument` as a string via `argument.inspect`. See Specifier p.

- `s`: Format `argument` as a string via `argument.to_s`. See Specifier s.

- `%`: Format `argument` (`'%'`) as a single percent character. See Specifier %.

## Flags

The effect of a flag may vary greatly among type specifiers. These remarks are general in nature. See type-specific details.

Multiple flags may be given with single type specifier; order does not matter.

### `' '` Flag

Insert a space before a non-negative number:

    sprintf('%d', 10)  # => "10"
    sprintf('% d', 10) # => " 10"

Insert a minus sign for negative value:

    sprintf('%d', -10)  # => "-10"
    sprintf('% d', -10) # => "-10"

### `'#'` Flag

Use an alternate format; varies among types:

    sprintf('%x', 100)  # => "64"
    sprintf('%#x', 100) # => "0x64"

### `'+'` Flag

Add a leading plus sign for a non-negative number:

    sprintf('%x', 100)  # => "64"
    sprintf('%+x', 100) # => "+64"

### `'-'` Flag

Left justify the value in its field:

    sprintf('%6d', 100)  # => "   100"
    sprintf('%-6d', 100) # => "100   "

### `'0'` Flag

Left-pad with zeros instead of spaces:

    sprintf('%6d', 100)  # => "   100"
    sprintf('%06d', 100) # => "000100"

### `'n$'` Flag

Format the (1-based) `n`th argument into this field:

    sprintf("%s %s", 'world', 'hello')     # => "world hello"
    sprintf("%2$s %1$s", 'world', 'hello') # => "hello world"

## Width Specifier

In general, a width specifier determines the minimum width (in characters) of the formatted field:

    sprintf('%10d', 100)  # => "       100"

    # Left-justify if negative.
    sprintf('%-10d', 100) # => "100       "

    # Ignore if too small.
    sprintf('%1d', 100)   # => "100"

If the width specifier is `'*'` instead of an integer, the actual minimum width is taken from the argument list:

    sprintf('%*d', 20, 14) # => "                  14"

## Precision Specifier

A precision specifier is a decimal point followed by zero or more decimal digits.

For integer type specifiers, the precision specifies the minimum number of digits to be written. If the precision is shorter than the integer, the result is padded with leading zeros. There is no modification or truncation of the result if the integer is longer than the precision:

    sprintf('%.3d', 1)    # => "001"
    sprintf('%.3d', 1000) # => "1000"

    # If the precision is 0 and the value is 0, nothing is written
    sprintf('%.d', 0)  # => ""
    sprintf('%.0d', 0) # => ""

For the `a`/`A`, `e`/`E`, `f`/`F` specifiers, the precision specifies the number of digits after the decimal point to be written:

    sprintf('%.2f', 3.14159)  # => "3.14"
    sprintf('%.10f', 3.14159) # => "3.1415900000"

    # With no precision specifier, defaults to 6-digit precision.
    sprintf('%f', 3.14159)    # => "3.141590"

For the `g`/`G` specifiers, the precision specifies the number of significant digits to be written:

    sprintf('%.2g', 123.45)  # => "1.2e+02"
    sprintf('%.3g', 123.45)  # => "123"
    sprintf('%.10g', 123.45) # =>  "123.45"

    # With no precision specifier, defaults to 6 significant digits.
    sprintf('%g', 123.456789) # => "123.457"

For the `s`, `p` specifiers, the precision specifies the number of characters to write:

    sprintf('%s', Time.now)    # => "2022-05-04 11:59:16 -0400"
    sprintf('%.10s', Time.now) # => "2022-05-04"

If the precision specifier is `'*'` instead of a non-negative integer, the actual precision is taken from the argument list:

    sprintf('%.*d', 20, 1)    # => "00000000000000000001"

## Type Specifier Details and Examples

### Specifiers `a` and `A`

Format `argument` as hexadecimal floating-point number:

    sprintf('%a', 3.14159)   # => "0x1.921f9f01b866ep+1"
    sprintf('%a', -3.14159)  # => "-0x1.921f9f01b866ep+1"
    sprintf('%a', 4096)      # => "0x1p+12"
    sprintf('%a', -4096)     # => "-0x1p+12"

    # Capital 'A' means that alphabetical characters are printed in upper case.
    sprintf('%A', 4096)      # => "0X1P+12"
    sprintf('%A', -4096)     # => "-0X1P+12"

### Specifiers `b` and `B`

The two specifiers `b` and `B` behave identically except when flag `'#'`+ is used.

Format `argument` as a binary integer:

    sprintf('%b', 1)  # => "1"
    sprintf('%b', 4)  # => "100"

    # Prefix '..' for negative value.
    sprintf('%b', -4) # => "..100"

    # Alternate format.
    sprintf('%#b', 4)  # => "0b100"
    sprintf('%#B', 4)  # => "0B100"

### Specifier `c`

Format `argument` as a single character:

    sprintf('%c', 'A') # => "A"
    sprintf('%c', 65)  # => "A"

This behaves like `String#<<`, except for raising `ArgumentError` instead of `RangeError`.

### Specifier `d`

Format `argument` as a decimal integer:

    sprintf('%d', 100)  # => "100"
    sprintf('%d', -100) # => "-100"

Flag `'#'` does not apply.

### Specifiers `e` and `E`

Format `argument` in scientific notation:

    sprintf('%e', 3.14159)  # => "3.141590e+00"
    sprintf('%E', -3.14159) # => "-3.141590E+00"

### Specifier `f`

Format `argument` as a floating-point number:

    sprintf('%f', 3.14159)  # => "3.141590"
    sprintf('%f', -3.14159) # => "-3.141590"

Flag `'#'` does not apply.

### Specifiers `g` and `G`

Format `argument` using exponential form (`e`/`E` specifier) if the exponent is less than -4 or greater than or equal to the precision. Otherwise format `argument` using floating-point form (`f` specifier):

    sprintf('%g', 100)  # => "100"
    sprintf('%g', 100.0)  # => "100"
    sprintf('%g', 3.14159)  # => "3.14159"
    sprintf('%g', 100000000000)  # => "1e+11"
    sprintf('%g', 0.000000000001)  # => "1e-12"

    # Capital 'G' means use capital 'E'.
    sprintf('%G', 100000000000)  # => "1E+11"
    sprintf('%G', 0.000000000001)  # => "1E-12"

    # Alternate format.
    sprintf('%#g', 100000000000)  # => "1.00000e+11"
    sprintf('%#g', 0.000000000001)  # => "1.00000e-12"
    sprintf('%#G', 100000000000)  # => "1.00000E+11"
    sprintf('%#G', 0.000000000001)  # => "1.00000E-12"

### Specifier `o`

Format `argument` as an octal integer. If `argument` is negative, it will be formatted as a two’s complement prefixed with `..7`:

    sprintf('%o', 16)   # => "20"

    # Prefix '..7' for negative value.
    sprintf('%o', -16)  # => "..760"

    # Prefix zero for alternate format if positive.
    sprintf('%#o', 16)  # => "020"
    sprintf('%#o', -16) # => "..760"

### Specifier `p`

Format `argument` as a string via `argument.inspect`:

    t = Time.now
    sprintf('%p', t)   # => "2022-05-01 13:42:07.1645683 -0500"

### Specifier `s`

Format `argument` as a string via `argument.to_s`:

    t = Time.now
    sprintf('%s', t) # => "2022-05-01 13:42:07 -0500"

Flag `'#'` does not apply.

### Specifiers `x` and `X`

Format `argument` as a hexadecimal integer. If `argument` is negative, it will be formatted as a two’s complement prefixed with `..f`:

    sprintf('%x', 100)   # => "64"

    # Prefix '..f' for negative value.
    sprintf('%x', -100)  # => "..f9c"

    # Use alternate format.
    sprintf('%#x', 100)  # => "0x64"

    # Alternate format for negative value.
    sprintf('%#x', -100) # => "0x..f9c"

### Specifier `%`

Format `argument` (`'%'`) as a single percent character:

    sprintf('%d %%', 100) # => "100 %"

Flags do not apply.

## Reference by Name

For more complex formatting, Ruby supports a reference by name. %<name>s style uses format style, but %{name} style doesn’t.

Examples:

    sprintf("%<foo>d : %<bar>f", { :foo => 1, :bar => 2 }) # => 1 : 2.000000
    sprintf("%{foo}f", { :foo => 1 })                      # => "1f"