std::log2, std::log2f, std::log2l
Defined in header <cmath> |
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|---|---|---|
float log2 ( float arg ); float log2f( float arg ); |
(1) | (since C++11) |
double log2 ( double arg ); |
(2) | (since C++11) |
long double log2 ( long double arg ); long double log2l( long double arg ); |
(3) | (since C++11) |
double log2 ( IntegralType arg ); |
(4) | (since C++11) |
1-3) Computes the binary (base-
2) logarithm of
arg.
4) A set of overloads or a function template accepting an argument of any
integral type. Equivalent to
(2) (the argument is cast to
double).
Parameters
| arg | - | value of floating-point or Integral type |
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_DIVBYZEROis raised. - If the argument is 1, +0 is returned
- If the argument is negative, NaN is returned and
FE_INVALIDis 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 <iostream>
#include <cmath>
#include <cerrno>
#include <cstring>
#include <cfenv>
// #pragma STDC FENV_ACCESS ON
int main()
{
std::cout << "log2(65536) = " << std::log2(65536) << '\n'
<< "log2(0.125) = " << std::log2(0.125) << '\n'
<< "log2(0x020f) = " << std::log2(0x020f)
<< " (highest set bit is in position 9)\n"
<< "base-5 logarithm of 125 = " << std::log2(125)/std::log2(5) << '\n';
// special values
std::cout << "log2(1) = " << std::log2(1) << '\n'
<< "log2(+Inf) = " << std::log2(INFINITY) << '\n';
// error handling
errno = 0;
std::feclearexcept(FE_ALL_EXCEPT);
std::cout << "log2(0) = " << std::log2(0) << '\n';
if (errno == ERANGE)
std::cout << " errno == ERANGE: " << std::strerror(errno) << '\n';
if (std::fetestexcept(FE_DIVBYZERO))
std::cout << " FE_DIVBYZERO raised\n";
}
Possible output:
log2(65536) = 16
log2(0.125) = -3
log2(0x020f) = 9.04166 (highest set bit is in position 9)
base-5 logarithm of 125 = 3
log2(1) = 0
log2(+Inf) = inf
log2(0) = -inf
errno == ERANGE: Numerical result out of range
FE_DIVBYZERO raised
See also
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(C++11)(C++11)
|
computes natural (base e) logarithm (\({\small \ln{x} }\)ln(x)) (function) |
|
(C++11)(C++11)
|
computes common (base 10) logarithm (\({\small \log_{10}{x} }\)log10(x)) (function) |
|
(C++11)(C++11)(C++11)
|
natural logarithm (to base e) of 1 plus the given number (\({\small \ln{(1+x)} }\)ln(1+x)) (function) |
|
(C++11)(C++11)(C++11)
|
returns 2 raised to the given power (\({\small 2^x}\)2x) (function) |
C documentation for log2 |
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