floating constant
Allows values of floating type to be used directly in expressions.
Syntax
A floating constant is a nonlvalue expression having the form:
significand exponent(optional) suffix(optional)  
Where the significand has the form
wholenumber(optional) . (optional) fraction(optional)


The exponent has the form
e  E exponentsign(optional) digitsequence

(1)  
p  P exponentsign(optional) digitsequence

(2)  (since C99)  
Explanation
If the significand begins with the character sequence For a hexadecimal floating constant, the significand is interpreted as a hexadecimal rational number, and the digitsequence of the exponent is interpreted as the integer power of 2 to which the significand has to be scaled. 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 digitsequence 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 implementationdefined manner (in other words, default rounding direction during translation is implementationdefined).
Floatingpoint constants may convert to more range and precision than is indicated by their type, if indicated by FLT_EVAL_METHOD. For example, the constant 0.1f may act as if it were 0.1L in an expression. 
(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; // floatingpoint 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 wholenumber or the fraction must be present.
double x = 1.; // floatingpoint 1.0 (fractional part optional) double y = .1; // floatingpoint 0.1 (wholenumber 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) 
Notes
Default rounding direction and precision are in effect when the floating constants are converted into internal representations, and floatingpoint exceptions are not raised even if #pragma STDC FENV_ACCESS is in effect (for executiontime 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 caseinsensitive: 0x1.ep+3
and 0X1.EP+3
represent the same floatingpoint 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.
Example
#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("+1.0e324 > %g\n", 1.0e324); printf("1.0e324 > %g\n", 1.0e324); printf("2.0e+308 > %g\n", 2.0e+308); }
Output:
15.0 = 0x1.ep+3 0x1.ep+3 = 15.000000 +2.0e+308 > inf +1.0e324 > 0 1.0e324 > 0 2.0e+308 > inf
References
 C11 standard (ISO/IEC 9899:2011):
 6.4.4.2 Floating constants (p: 6566)
 C99 standard (ISO/IEC 9899:1999):
 6.4.4.2 Floating constants (p: 5758)
 C89/C90 standard (ISO/IEC 9899:1990):
 3.1.3.1 Floating constants