The GNU C Library - Floating-Point Conversions

Floating-Point Conversions

This section discusses the conversion specifications for floating-point numbers: the `%f', `%e', `%E', `%g', and `%G' conversions.

The `%f' conversion prints its 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' style. 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 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.

`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', `%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 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:

	"|%12.4f|%12.4e|%12.4g|\n"

Here is the output:

	|      0.0000|  0.0000e+00|           0|
	|      1.0000|  1.0000e+00|           1|
	|     -1.0000| -1.0000e+00|          -1|
	|    100.0000|  1.0000e+02|         100|
	|   1000.0000|  1.0000e+03|        1000|
	|  10000.0000|  1.0000e+04|       1e+04|
	|  12345.0000|  1.2345e+04|   1.234e+04|
	| 100000.0000|  1.0000e+05|       1e+05|
	| 123456.0000|  1.2346e+05|   1.234e+05|

Notice how the `%g' conversion drops trailing zeros.