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(libc.info)Floating-Point Conversions

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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`.'DDD`e'[`+'|`-']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.  A precision of `0', is taken as 1. is 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 is the form [`-']`0x'H`.'HHH`p'[`+'|`-']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, Note: 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', or `%G'.

   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;
     Note: General Numeric.  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', `%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.