Namespaces
Variants
Views
Actions

log2, log2f, log2l

From cppreference.com
< c‎ | numeric‎ | math
 
 
 
Common mathematical functions
Functions
Basic operations
(C99)
(C99)
(C99)
(C99)
(C99)
(C99)(C99)(C99)
Exponential functions
(C99)
(C99)
(C99)
log2
(C99)
Power functions
(C99)
(C99)
Trigonometric and hyperbolic functions
(C99)
(C99)
(C99)
Error and gamma functions
(C99)
(C99)
(C99)
(C99)
Nearest integer floating point operations
(C99)(C99)(C99)
(C99)
(C99)(C99)(C99)
Floating point manipulation functions
(C99)(C99)
(C99)
(C99)
Classification
(C99)
(C99)
(C99)
Macro constants
 
Defined in header <math.h>
float       log2f( float arg );
(1) (since C99)
double      log2( double arg );
(2) (since C99)
long double log2l( long double arg );
(3) (since C99)
Defined in header <tgmath.h>
#define log2( arg )
(4) (since C99)
1-3) Computes the base 2 logarithm of arg.
4) Type-generic macro: If arg has type long double, log2l is called. Otherwise, if arg has integer type or the type double, log2 is called. Otherwise, log2f is called.

Contents

[edit] Parameters

arg - floating point value

[edit] 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.

[edit] 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

[edit] Notes

For integer arg, the binary logarithm can be interpreted as the zero-based index of the most significant 1 bit in the input.

[edit] Example

#include <stdio.h>
#include <math.h>
#include <float.h>
#include <errno.h>
#include <fenv.h>
#pragma STDC FENV_ACCESS ON
int main(void)
{
    printf("log2(65536) = %f\n", log2(65536));
    printf("log2(0.125) = %f\n", log2(0.125));
    printf("log2(0x020f) = %f (highest set bit is in position 9)\n", log2(0x020f));
    printf("base-5 logarithm of 125 = %f\n", log2(125)/log2(5));
    // special values
    printf("log2(1) = %f\n", log2(1));
    printf("log2(+Inf) = %f\n", log2(INFINITY));
    //error handling
    errno = 0; feclearexcept(FE_ALL_EXCEPT);
    printf("log2(0) = %f\n", log2(0));
    if(errno == ERANGE) perror("    errno == ERANGE");
    if(fetestexcept(FE_DIVBYZERO)) puts("    FE_DIVBYZERO raised");
}

Possible output:

log2(65536) = 16.000000
log2(0.125) = -3.000000
log2(0x020f) = 9.041659 (highest set bit is in position 9)
base-5 logarithm of 125 = 3.000000
log2(1) = 0.000000
log2(+Inf) = inf
log2(0) = -inf
    errno == ERANGE: Numerical result out of range
    FE_DIVBYZERO raised

[edit] See also

(C99)(C99)
computes natural (base-e) logarithm (ln(x))
(function) [edit]
computes common (base-10) logarithm (log10(x))
(function) [edit]
(C99)(C99)(C99)
computes natural (base-e) logarithm of 1 plus the given number (ln(1+x))
(function) [edit]
(C99)(C99)(C99)
computes 2 raised to the given power (2x)
(function) [edit]