Allows values of floating type to be used directly in expressions.

A floating constant is a non-lvalue expression having the form:

significand exponent(optional) suffix(optional) |

Where the significand has the form.

whole-number(optional) `.` (optional) fraction(optional) |

The exponent has the form.

`e` | `E` exponent-sign(optional) digit-sequence | (1) | |

`p` | `P` exponent-sign(optional) digit-sequence | (2) | (since C99) |

1) The exponent syntax for a decimal floating-point constant

2) The exponent syntax for hexadecimal floating-point constant

If the significand begins with the character sequence For a double d = 0x1.2p3; // hex fraction 1.2 (decimal 1.125) scaled by 2^3, that is 9.0 | (since C99) |

For a *decimal floating constant*, the significand is interpreted as a decimal rational number, and the digit-sequence of the exponent is interpreted as the integer power of 10 to which the significand has to be scaled.

double d = 1.2e3; // decimal fraction 1.2 scaled by 10^3, that is 1200.0

An unsuffixed floating constant has type `double`

. If suffix is the letter `f`

or `F`

, the floating constant has type `float`

. If suffix is the letter `l`

or `L`

, the floating constant has type `long double`

.

The result of evaluating a floating constant is either the nearest representable value or the larger or smaller representable value immediately adjacent to the nearest representable value, chosen in an implementation-defined manner (in other words, default rounding direction during translation is implementation-defined).

Floating-point constants may convert to more range and precision than is indicated by their type, if indicated by | (since C99) |

The result of evaluating a hexadecimal floating constant, if FLT_RADIX is 2, is the exact value represented by the floating constant, correctly rounded to the target type. | (since C99) |

If the exponent is present and fractional part is not used, the decimal separator may be omitted:

double x = 1e0; // floating-point 1.0 (period not used)

For decimal floating constants, the exponent part is optional. If it is omitted, the period is not optional, and either the whole-number or the fraction must be present.

double x = 1.; // floating-point 1.0 (fractional part optional) double y = .1; // floating-point 0.1 (whole-number part optional)

For hexadecimal floating constants, the exponent is not optional to avoid ambiguity resulting from an f suffix being mistaken as a hexadecimal digit. | (since C99) |

Default rounding direction and precision are in effect when the floating constants are converted into internal representations, and floating-point exceptions are not raised even if ` #pragma STDC FENV_ACCESS`

is in effect (for execution-time conversion of character strings, `strtod`

can be used). Note that this differs from arithmetic constant expressions of floating type.

Letters in the floating constants are case-insensitive: `0x1.ep+3`

and `0X1.EP+3`

represent the same floating-point value 15.0.

The decimal point specified by `setlocale`

has no effect on the syntax of floating constants: the decimal point character is always the period.

Unlike integers, not every floating value can be represented directly by decimal or even hexadecimal constant syntax: macros NAN and INFINITY as well as functions such as `nan`

offer ways to generate those special values. Note that `0x1.FFFFFEp128f`

, which might appear to be an IEEE float NaN, in fact overflows to an infinity in that format.

There are no negative floating constants; an expression such as `-1.2`

is the arithmetic operator unary minus applied to the floating constant `1.2`

. Note that the special value negative zero may be constructed with `-0.0`

.

#include <stdio.h> int main(void) { printf("15.0 = %a\n", 15.0); printf("0x1.ep+3 = %f\n", 0x1.ep+3); // Constants outside the range of type double. printf("+2.0e+308 --> %g\n", 2.0e+308); printf("+1.0e-324 --> %g\n", 1.0e-324); printf("-1.0e-324 --> %g\n", -1.0e-324); printf("-2.0e+308 --> %g\n", -2.0e+308); }

Output:

15.0 = 0x1.ep+3 0x1.ep+3 = 15.000000 +2.0e+308 --> inf +1.0e-324 --> 0 -1.0e-324 --> -0 -2.0e+308 --> -inf

- C11 standard (ISO/IEC 9899:2011):
- 6.4.4.2 Floating constants (p: 65-66)
- C99 standard (ISO/IEC 9899:1999):
- 6.4.4.2 Floating constants (p: 57-58)
- C89/C90 standard (ISO/IEC 9899:1990):
- 3.1.3.1 Floating constants

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