std::log2,std::log2f,std::log2l (3) - Linux Manuals

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

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