NAME¶

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

Synopsis¶

Defined in header <cmath>
float log2 ( float num );

double log2 ( double num ); (until C++23)

long double log2 ( long double num );
/* floating-point-type */ (since C++23)
log2 ( /* floating-point-type */ num ); (constexpr since C++26)
float log2f( float num ); (1) (2) (since C++11)
(constexpr since C++26)
long double log2l( long double num ); (3) (since C++11)
(constexpr since C++26)
Additional overloads (since C++11)
Defined in header <cmath>
template< class Integer > (A) (constexpr since C++26)
double log2 ( Integer num );

1-3) Computes the binary (base-2) logarithm of num.
The library provides overloads of std::log2 for all cv-unqualified floating-point
types as the type of the parameter.
(since C++23)

A) Additional overloads are provided for all integer types, which are (since C++11)
treated as double.

Parameters¶

num - floating-point or integer value

Return value¶

If no errors occur, the base-2 logarithm of num (log
2(num) or lb(num)) 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 num is less than zero.

Pole error may occur if num 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 num, the binary logarithm can be interpreted as the zero-based index of
the most significant 1 bit in the input.

The additional overloads are not required to be provided exactly as (A). They only
need to be sufficient to ensure that for their argument num of integer type,
std::log2(num) has the same effect as std::log2(static_cast<double>(num)).

Example¶

// Run this code

#include <cerrno>
#include <cfenv>
#include <cmath>
#include <cstring>
#include <iostream>
// #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
log1pl (\({\small\ln{(1+x)}}\)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)
C documentation for
log2