12.12.5 Floating-Point Conversions

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 C2X 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.

‘0’

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:

‘L’

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.