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ctanl div double_t erf erfc erfcf erfcl erff erfl exp exp2 exp2f exp2l expf expl expm1 expm1f expm1l fabs fabsf fabsl fdim feclearexcept fegetenv fegetexceptflag fegetround feholdexcept feraiseexcept fesetenv fesetexceptflag fesetround fetestexcept feupdateenv FE_ALL_EXCEPT FE_DFL_ENV FE_DIVBYZERO FE_DOWNWARD FE_INEXACT FE_INVALID FE_OVERFLOW FE_TONEAREST FE_TOWARDZERO FE_UNDERFLOW FE_UPWARD Floating-point environment float_t floor floorf floorl fma fmaf fmal fmax fmaxf fmaxl fmin fminf fminl fmod fmodf fmodl fpclassify FP_INFINITE FP_NAN FP_NORMAL FP_SUBNORMAL FP_ZERO frexp frexpf frexpl HUGE_VAL HUGE_VALF HUGE_VALL hypot hypotf hypotl I ilogb ilogbf ilogbl imaginary imaxabs imaxdiv INFINITY isfinite isgreater isgreaterequal isinf isless islessequal islessgreater isnan isnormal isunordered labs ldexp ldexpf ldexpl ldiv lgamma lgammaf lgammal llabs lldiv llrint llrintf llrintl llround llroundf llroundl log log10 log10f log10l log1p log1pf log1pl log2 log2f log2l logb logbf logbl logf logl lrint lrintf lrintl lround lroundf lroundl MATH_ERREXCEPT math_errhandling MATH_ERRNO modf modff modfl nan NAN nanf nanl nearbyint nearbyintf nearbyintl nextafter nextafterf nextafterl nexttoward nexttowardf nexttowardl Numerics pow powf powl Pseudo-random number generation rand RAND_MAX remainder remainderf remainderl remquo remquof remquol rint rintf rintl round roundf roundl scalbln scalblnf scalblnl scalbn scalbnf scalbnl signbit sin sinf sinh sinhf sinhl sinl sqrt sqrtf sqrtl srand tan tanf tanh tanhf tanhl tanl tgamma tgammaf tgammal trunc truncf truncl Type-generic math _Complex_I _Imaginary_I Program support abort atexit at_quick_exit exit EXIT_FAILURE EXIT_SUCCESS getenv getenv_s jmp_buf longjmp Program support utilities quick_exit raise setjmp SIGABRT SIGFPE SIGILL SIGINT signal SIGSEGV SIGTERM sig_atomic_t SIG_DFL SIG_ERR SIG_IGN system _Exit Strings atof atoi atol atoll btowc c16rtomb c32rtomb char16_t char32_t isalnum isalpha isblank iscntrl isdigit isgraph islower isprint ispunct isspace isupper iswalnum iswalpha iswblank iswcntrl iswctype iswdigit iswgraph iswlower iswprint iswpunct iswspace iswupper iswxdigit isxdigit mblen mbrlen mbrtoc16 mbrtoc32 mbrtowc mbsinit mbsrtowcs mbsrtowcs_s mbstate_t mbstowcs mbstowcs_s mbtowc memchr memcmp memcpy memcpy_s memmove memmove_s memset memset_s Null-terminated byte strings Null-terminated multibyte strings Null-terminated wide strings strcat strcat_s strchr strcmp strcoll strcpy strcpy_s strcspn strerror strerrorlen_s strerror_s Strings library strlen strncat Thread support call_once cnd_broadcast cnd_destroy cnd_init cnd_signal cnd_timedwait cnd_wait mtx_destroy mtx_init mtx_lock mtx_plain mtx_recursive mtx_timed mtx_timedlock mtx_trylock mtx_unlock once_flag ONCE_FLAG_INIT thrd_busy thrd_create thrd_current thrd_detach thrd_equal thrd_error thrd_exit thrd_join thrd_nomem thrd_sleep thrd_success thrd_timedout thrd_yield Thread support library thread_local tss_create tss_delete TSS_DTOR_ITERATIONS tss_get tss_set Type support Boolean type support library Fixed width integer types FLT_EVAL_METHOD FLT_ROUNDS max_align_t NULL Numeric limits offsetof ptrdiff_t size_t Type support Variadic functions Variadic functions va_arg va_copy va_end va_list va_start
characters

允許直接在表達式中使用浮點類型的值。

句法

浮點常量是一個非左值表達式,其形式為:

significand exponent(optional) suffix(optional)



有效數(shù)字的形式。

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



指數(shù)的形式。

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

(1)


p | P exponent-sign(optional) digit-sequence

(2)

(since C99)

1)十進制浮點常量的指數(shù)語法

2)十六進制浮點常量的指數(shù)語法

說明

如果有效數(shù)字以字符序列0x或0X開頭,則浮點常量是一個十六進制浮點常量。否則,它是一個十進制浮點常量。對于十六進制浮點常量,有效數(shù)被解釋為十六進制有理數(shù),并且指數(shù)的數(shù)字序列被解釋為必須縮放有效數(shù)的2的整數(shù)次冪。雙d = 0x1.2p3; //十六進制小數(shù)1.2(十進制1.125)按2 ^ 3縮放,即9.0

(自C99以來)

對于十進制浮點常量,有效數(shù)被解釋為十進制有理數(shù),并且指數(shù)的數(shù)字序列被解釋為10的整數(shù)冪,有效數(shù)必須被縮放到該冪。

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

一個沒有任何固定的浮點常量具有類型double。如果后綴是字母f或者F,浮動常數(shù)是類型的float。如果后綴是字母l或者L,浮動常數(shù)是類型的long double。

評估浮點常量的結(jié)果或者是最接近的可表示值,或者是以實現(xiàn)定義的方式選擇的緊鄰最接近的可表示值的較大或較小的可表示值(換句話說,翻譯期間的默認舍入方向是實現(xiàn)定義的) 。

如果由 FLT_EVAL_METHOD 指示,則浮點常量可能會轉(zhuǎn)換為更多的范圍和精度。例如,在表達式中,常數(shù)0.1f可能表現(xiàn)為0.1L。

(自C99以來)

如果 FLT_RADIX 為2,則評估十六進制浮點常量的結(jié)果是由浮點常量表示的精確值,正確地舍入到目標類型。

(自C99以來)

如果指數(shù)存在且不使用小數(shù)部分,則可以省略小數(shù)點分隔符:

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

對于十進制浮點常量,指數(shù)部分是可選的。如果省略,則該期間不是可選的,并且必須存在整數(shù)或分數(shù)。

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)

對于十六進制浮點常量,指數(shù)不是可選的,以避免f后綴被誤認為十六進制數(shù)字而引起的歧義。

(自C99以來)

當(dāng)浮點常量轉(zhuǎn)換為內(nèi)部表示時,默認的舍入方向和精度有效,即使#pragma STDC FENV_ACCESS有效(對于字符串的執(zhí)行時間轉(zhuǎn)換,strtod也可以使用),浮點異常不會   產(chǎn)生。請注意,這與浮點類型的算術(shù)常量表達式不同。

在浮動常量字母是不區(qū)分大小寫的:0x1.ep+30X1.EP+3表示相同的浮點值15.0。

指定的小數(shù)點setlocale對浮點常量的語法沒有影響:小數(shù)點字符始終為句點。

與整數(shù)不同,并不是每個浮點值都可以用十進制或甚至十六進制常量語法直接表示:宏 NAN 和 INFINITY 以及諸如nan提供生成這些特殊值的方式的函數(shù)。請注意0x1.FFFFFEp128f,它可能看起來像一個 IEEE 浮點型NaN,實際上以這種格式溢出到無窮大。

沒有負向浮動常量; 一個表達式,如-1.2算術(shù)運算符一元減號應(yīng)用于浮點常量1.2。請注意,特殊值負零可以用-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);}

輸出:

15.0     = 0x1.ep+30x1.ep+3 = 15.000000+2.0e+308 --> inf+1.0e-324 --> 0-1.0e-324 --> -0-2.0e+308 --> -inf

參考

  • 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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