log2, log2f, log2l
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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.[edit] Parameters
arg
-
floating point value
[edit] Return value
If no errors occur, the base-2 logarithm of arg (log2(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.
[edit] 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
[edit] Notes
For integer arg, the binary logarithm can be interpreted as the zero-based index of the most significant 1 bit in the input.
[edit] Example
Run this code
#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
[edit] References
- C17 standard (ISO/IEC 9899:2018):
- 7.12.6.10 The log2 functions (p: 179)
- 7.25 Type-generic math <tgmath.h> (p: 272-273)
- F.10.3.10 The log2 functions (p: 381)
- 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)
[edit] See also
(C99)(C99)
(function) [edit]
(C99)(C99)(C99)
(function) [edit]
C++ documentation for log2