log2, log2f, log2l
From cppreference.com
| Defined in header <math.h>
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||
| 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
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 raisedReferences
- 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)
See also
| (C99)(C99) |
computes natural (base-e) logarithm (ln(x)) (function) |
| (C99)(C99) |
computes common (base-10) logarithm (log10(x)) (function) |
| (C99)(C99)(C99) |
computes natural (base-e) logarithm of 1 plus the given number (ln(1+x)) (function) |
| (C99)(C99)(C99) |
computes 2 raised to the given power (2x) (function) |
| C++ documentation for log2
| |