NAME¶

std::log2,std::log2f,std::log2l - std::log2,std::log2f,std::log2l

Synopsis¶

Defined in header <cmath>
float log2 ( float arg ); (1) (since C++11)
float log2f( float arg );
double log2 ( double arg ); (2) (since C++11)
long double log2 ( long double arg ); (3) (since C++11)
long double log2l( long double arg );
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_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 <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¶

log
logf computes natural (base e) logarithm (\({\small \ln{x} }\)ln(x))
logl (function)
(C++11)
(C++11)
log10
log10f computes common (base 10) logarithm (\({\small \log_{10}{x} }\)log[10](x))
log10l (function)
(C++11)
(C++11)
log1p
log1pf natural logarithm (to base e) of 1 plus the given number (\({\small
log1pl \ln{(1+x)} }\)ln(1+x))
(C++11) (function)
(C++11)
(C++11)
exp2
exp2f
exp2l returns 2 raised to the given power (\({\small 2^x}\)2^x)
(C++11) (function)
(C++11)
(C++11)