Format Specifications ↑

Several Ruby core classes have instance method printf or sprintf:

Each of these methods takes:

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:

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 ↑

Floating-Point Type Specifiers ↑

Other Type Specifiers ↑

Flags ↑

The effect of a flag may vary greatly among type specifiers. These remarks are general in nature. See Specifiers b and B.

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"

'*' Flag ↑

Use the next argument as the field width:

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

'n$' Flag ↑

Format the (1-based) nth 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"

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"

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"