©
本文檔使用 php中文網手册 發布
Defined in header <math.h> | ||
---|---|---|
float log2f( float arg ); | (1) | (since C99) |
double log2( double arg ); | (2) | (since C99) |
long double log2l( long double arg ); | (3) | (since C99) |
Defined in header <tgmath.h> | ||
#define log2( arg ) | (4) | (since C99) |
1-3) Computes the base 2
logarithm of arg
.
4) Type-generic macro: If arg
has type long double
, log2l
is called. Otherwise, if arg
has integer type or the type double
, log2
is called. Otherwise, log2f
is called.
arg | - | floating point value |
---|
If no errors occur, the base-2 logarithm of arg
(log
2(arg) or lb(arg)) is returned.
If a domain error occurs, an implementation-defined value is returned (NaN where supported).
If a pole error occurs, -HUGE_VAL
, -HUGE_VALF
, or -HUGE_VALL
is returned.
Errors are reported as specified in math_errhandling.
Domain error occurs if arg
is less than zero.
Pole error may occur if arg
is zero.
If the implementation supports IEEE floating-point arithmetic (IEC 60559),
If the argument is ±0, -∞ is returned and FE_DIVBYZERO
is raised.
If the argument is 1, +0 is returned
If the argument is negative, NaN is returned and FE_INVALID
is raised.
If the argument is +∞, +∞ is returned
If the argument is NaN, NaN is returned
For integer arg
, the binary logarithm can be interpreted as the zero-based index of the most significant 1 bit in the input.
#include <stdio.h>#include <math.h>#include <float.h>#include <errno.h>#include <fenv.h>#pragma STDC FENV_ACCESS ON int main(void){ printf("log2(65536) = %f\n", log2(65536)); printf("log2(0.125) = %f\n", log2(0.125)); printf("log2(0x020f) = %f (highest set bit is in position 9)\n", log2(0x020f)); printf("base-5 logarithm of 125 = %f\n", log2(125)/log2(5)); // special values printf("log2(1) = %f\n", log2(1)); printf("log2(+Inf) = %f\n", log2(INFINITY)); //error handling errno = 0; feclearexcept(FE_ALL_EXCEPT); printf("log2(0) = %f\n", log2(0)); if(errno == ERANGE) perror(" errno == ERANGE"); if(fetestexcept(FE_DIVBYZERO)) puts(" FE_DIVBYZERO raised");}
Possible output:
log2(65536) = 16.000000log2(0.125) = -3.000000log2(0x020f) = 9.041659 (highest set bit is in position 9)base-5 logarithm of 125 = 3.000000log2(1) = 0.000000log2(+Inf) = inflog2(0) = -inf errno == ERANGE: Numerical result out of range FE_DIVBYZERO raised
C11 standard (ISO/IEC 9899:2011):
7.12.6.10 The log2 functions (p: 246)
7.25 Type-generic math <tgmath.h> (p: 373-375)
F.10.3.10 The log2 functions (p: 522)
C99 standard (ISO/IEC 9899:1999):
7.12.6.10 The log2 functions (p: 226)
7.22 Type-generic math <tgmath.h> (p: 335-337)
F.9.3.10 The log2 functions (p: 459)
loglogflogl (C99)(C99) | computes natural (base-e) logarithm (ln(x)) (function) |
---|---|
log10log10flog10l (C99)(C99) | computes common (base-10) logarithm (log10(x)) (function) |
log1plog1pflog1pl (C99)(C99)(C99) | computes natural (base-e) logarithm of 1 plus the given number (ln(1+x)) (function) |
exp2exp2fexp2l (C99)(C99)(C99) | computes 2 raised to the given power (2x) (function) |
| C++ documentation for log2 |
© cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.