C参考手册
数值 | Numerics
log2l
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.
Parameters
arg |
- |
floating point value |
---|
Return 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.
Error handling
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
Notes
For integer arg
, the binary logarithm can be interpreted as the zero-based index of the most significant 1 bit in the input.
Example
#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) = 16.000000
log2(0.125) = -3.000000
log2(0x020f) = 9.041659 (highest set bit is in position 9)
base-5 logarithm of 125 = 3.000000
log2(1) = 0.000000
log2(+Inf) = inf
log2(0) = -inf
errno == ERANGE: Numerical result out of range
FE_DIVBYZERO raised
References
- 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)
See also
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.
http://en.cppreference.com/w/c/numeric/math/log2