C++

Reference

<cmath> (math.h)

function
<cmath> <ctgmath>

log2

     double log2  (double x);      float log2f (float x);long double log2l (long double x);
     double log2 (double x);      float log2 (float x);long double log2 (long double x);     double log2 (T x);           // additional overloads for integral types
Compute binary logarithm
Returns the binary (base-2) logarithm of x.

Header <tgmath.h> provides a type-generic macro version of this function.
Additional overloads are provided in this header (<cmath>) for the integral types: These overloads effectively cast x to a double before calculations (defined for T being any integral type).

Parameters

x
Value whose logarithm is calculated.
If the argument is negative, a domain error occurs.

Return Value

The binary logarithm of x: log2x.
If x is negative, it causes a domain error:
If x is zero, it may cause a pole error (depending on the library implementation).

If a domain error occurs:
- And math_errhandling has MATH_ERRNO set: the global variable errno is set to EDOM.
- And math_errhandling has MATH_ERREXCEPT set: FE_INVALID is raised.

If a pole error occurs:
- And math_errhandling has MATH_ERRNO set: the global variable errno is set to ERANGE.
- And math_errhandling has MATH_ERREXCEPT set: FE_DIVBYZERO is raised.

Example

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/* log2 example */
#include <stdio.h>      /* printf */
#include <math.h>       /* log2 */

int main ()
{
  double param, result;
  param = 1024.0;
  result = log2 (param);
  printf ("log2 (%f) = %f.\n", param, result );
  return 0;
}

Output:

log2 (1024.000000) = 10.000000


See also

exp2
Compute binary exponential function (function)
log
Compute natural logarithm (function)
pow
Raise to power (function)