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Difference between revisions of "cpp/numeric/math/log2"

From cppreference.com
< cpp‎ | numeric‎ | math
m (Undo revision 106555 by 98.24.33.186 (talk) that's not a c++20 concept)
m (Text replace - "Integral arg" to "IntegralType arg")
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{{dcl | since=c++11 |num=4|
 
{{dcl | since=c++11 |num=4|
−
double      log2( Integral arg );
+
double      log2( IntegralType arg );
 
}}
 
}}
 
{{dcl end}}
 
{{dcl end}}

Revision as of 20:03, 17 October 2018

 
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Defined in header <cmath>
float       log2( float arg );
 (1) (since C++11)
double      log2( double arg );
 (2) (since C++11)
long double log2( 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).

Contents

Parameters

arg - value of floating-point or Integral type

Return value

If no errors occur, the base-2 logarithm of arg (log2(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

(C++11)(C++11)
 computes natural (base e) logarithm (ln(x))
(function) [edit]
(C++11)(C++11)
 computes common (base 10) logarithm (log10(x))
(function) [edit]
(C++11)(C++11)(C++11)
 natural logarithm (to base e) of 1 plus the given number (ln(1+x))
(function) [edit]
(C++11)(C++11)(C++11)
 returns 2 raised to the given power (2x)
(function) [edit]
C documentation for log2
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