This section discusses the conversion specifications for floating-point numbers: the ‘%f’, ‘%F’, ‘%e’, ‘%E’, ‘%g’, and ‘%G’ conversions.
The ‘%f’ and ‘%F’ conversions print their argument in fixed-point
notation, producing output of the form
[-]ddd.ddd,
where the number of digits following the decimal point is controlled
by the precision you specify.
The ‘%e’ conversion prints its argument in exponential notation,
producing output of the form
[-]d.ddde[+|-]dd.
Again, the number of digits following the decimal point is controlled by
the precision. The exponent always contains at least two digits. The
‘%E’ conversion is similar but the exponent is marked with the letter
‘E’ instead of ‘e’.
The ‘%g’ and ‘%G’ conversions print the argument in the style
of ‘%e’ or ‘%E’ (respectively) if the exponent would be less
than -4 or greater than or equal to the precision; otherwise they use
the ‘%f’ or ‘%F’ style. A precision of 0, is taken as 1.
Trailing zeros are removed from the fractional portion of the result and
a decimal-point character appears only if it is followed by a digit.
The ‘%a’ and ‘%A’ conversions are meant for representing
floating-point numbers exactly in textual form so that they can be
exchanged as texts between different programs and/or machines. The
numbers are represented in the form
[-]0xh.hhhp[+|-]dd.
At the left of the decimal-point character exactly one digit is print.
This character is only 0 if the number is denormalized.
Otherwise the value is unspecified; it is implementation dependent how many
bits are used. The number of hexadecimal digits on the right side of
the decimal-point character is equal to the precision. If the precision
is zero it is determined to be large enough to provide an exact
representation of the number (or it is large enough to distinguish two
adjacent values if the FLT_RADIX is not a power of 2,
see Floating Point Parameters). For the ‘%a’ conversion
lower-case characters are used to represent the hexadecimal number and
the prefix and exponent sign are printed as 0x and p
respectively. Otherwise upper-case characters are used and 0X
and P are used for the representation of prefix and exponent
string. The exponent to the base of two is printed as a decimal number
using at least one digit but at most as many digits as necessary to
represent the value exactly.
If the value to be printed represents infinity or a NaN, the output is
[-]inf or nan respectively if the conversion
specifier is ‘%a’, ‘%e’, ‘%f’, or ‘%g’ and it is
[-]INF or NAN respectively if the conversion is
‘%A’, ‘%E’, ‘%F’ or ‘%G’. On some implementations, a NaN
may result in longer output with information about the payload of the
NaN; ISO C23 defines a macro _PRINTF_NAN_LEN_MAX giving the
maximum length of such output.
The following flags can be used to modify the behavior:
Left-justify the result in the field. Normally the result is right-justified.
Always include a plus or minus sign in the result.
If the result doesn’t start with a plus or minus sign, prefix it with a space instead. Since the ‘+’ flag ensures that the result includes a sign, this flag is ignored if you supply both of them.
Specifies that the result should always include a decimal point, even if no digits follow it. For the ‘%g’ and ‘%G’ conversions, this also forces trailing zeros after the decimal point to be left in place where they would otherwise be removed.
Separate the digits of the integer part of the result into groups as
specified by the locale specified for the LC_NUMERIC category;
see Generic Numeric Formatting Parameters. This flag is a GNU extension.
Pad the field with zeros instead of spaces; the zeros are placed after any sign. This flag is ignored if the ‘-’ flag is also specified.
The precision specifies how many digits follow the decimal-point
character for the ‘%f’, ‘%F’, ‘%e’, and ‘%E’ conversions.
For these conversions, the default precision is 6. If the precision
is explicitly 0, this suppresses the decimal point character
entirely. For the ‘%g’ and ‘%G’ conversions, the precision
specifies how many significant digits to print. Significant digits are
the first digit before the decimal point, and all the digits after it.
If the precision is 0 or not specified for ‘%g’ or ‘%G’,
it is treated like a value of 1. If the value being printed
cannot be expressed accurately in the specified number of digits, the
value is rounded to the nearest number that fits.
Without a type modifier, the floating-point conversions use an argument
of type double. (By the default argument promotions, any
float arguments are automatically converted to double.)
The following type modifier is supported:
An uppercase ‘L’ specifies that the argument is a long
double.
Here are some examples showing how numbers print using the various floating-point conversions. All of the numbers were printed using this template string:
"|%13.4a|%13.4f|%13.4e|%13.4g|\n"
Here is the output:
| 0x0.0000p+0| 0.0000| 0.0000e+00| 0| | 0x1.0000p-1| 0.5000| 5.0000e-01| 0.5| | 0x1.0000p+0| 1.0000| 1.0000e+00| 1| | -0x1.0000p+0| -1.0000| -1.0000e+00| -1| | 0x1.9000p+6| 100.0000| 1.0000e+02| 100| | 0x1.f400p+9| 1000.0000| 1.0000e+03| 1000| | 0x1.3880p+13| 10000.0000| 1.0000e+04| 1e+04| | 0x1.81c8p+13| 12345.0000| 1.2345e+04| 1.234e+04| | 0x1.86a0p+16| 100000.0000| 1.0000e+05| 1e+05| | 0x1.e240p+16| 123456.0000| 1.2346e+05| 1.235e+05|
Notice how the ‘%g’ conversion drops trailing zeros.