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logb

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)
(C99)
Power functions
(C99)
(C99)
Trigonometric and hyperbolic functions
(C99)
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Error and gamma functions
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(C99)
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(C99)
Nearest integer floating point operations
(C99)(C99)(C99)
(C99)
(C99)(C99)(C99)
Floating point manipulation functions
(C99)(C99)
(C99)
logb
(C99)
Classification
(C99)
(C99)
(C99)
Macro constants
 
Defined in header <math.h>
float       logbf( float arg );
(since C99)
double      logb( double arg );
(since C99)
long double logbl( long double arg );
(since C99)

Extracts the value of the exponent from the floating-point argument arg, and returns it as a floating-point value. Formally, the result is the integral part of log
r
|arg|
as a signed floating-point value, for non-zero arg, where r is FLT_RADIX. If arg is subnormal, it is treated as though it was normalized.

Contents

[edit] Parameters

arg - floating point value

[edit] Return value

The floating-point exponent.

Domain or range error may occur if arg is zero.

[edit] Notes

The value of the exponent returned by logb is always 1 less than the exponent retuned by frexp because of the different normalization requirements: for the exponent e returned by logb, |arg*r-e
|
is between 1 and r (typically between 1 and 2), but for the exponent e returned by frexp, |arg*2-e
|
is between 0.5 and 1.

[edit] Example

#include <stdio.h>
#include <math.h>
#include <fenv.h>
#include <errno.h>
#include <float.h>
#include <string.h>
 
#pragma STDC FENV_ACCESS ON
 
void show_fe_exceptions(void)
{
    printf("current exceptions raised: ");
    if(fetestexcept(FE_DIVBYZERO))     printf(" FE_DIVBYZERO");
    if(fetestexcept(FE_INEXACT))       printf(" FE_INEXACT");
    if(fetestexcept(FE_INVALID))       printf(" FE_INVALID");
    if(fetestexcept(FE_OVERFLOW))      printf(" FE_OVERFLOW");
    if(fetestexcept(FE_UNDERFLOW))     printf(" FE_UNDERFLOW");
    if(fetestexcept(FE_ALL_EXCEPT)==0) printf(" none");
    printf("\n");
}
 
int main(void)
{
    printf("logb(DBL_MIN)   = %+.1f\n", logb(DBL_MIN));
    printf("logb(DBL_MAX)   = %+.1f\n", logb(DBL_MAX));
    printf("logb(+INFINITY) = %+f\n",   logb(+INFINITY));
    printf("logb(-INFINITY) = %+f\n",   logb(-INFINITY));
    printf("logb(NAN)       = %f\n",    logb(NAN));
    printf("\n");
 
    /* "divide-by-zero" floating-point exception */
    /* same with logb(-0.0)                      */
    errno = 0; feclearexcept(FE_ALL_EXCEPT);
    printf("logb(0.0)      = %f\n",   logb(0.0));
    printf("%s\n",strerror(errno));
    show_fe_exceptions();
 
    return 0;
}

Possible output:

logb(DBL_MIN)   = -1022.0
logb(DBL_MAX)   = +1023.0
logb(+INFINITY) = +inf
logb(-INFINITY) = +inf
logb(NAN)       = nan
 
logb(0.0)      = -inf
Success
current exceptions raised:  FE_DIVBYZERO

[edit] See also

decomposes a number into significand and a power of 2
(function) [edit]
(C99)
extracts exponent of the number
(function) [edit]
(C99)(C99)
multiplies a number by FLT_RADIX raised to a power
(function) [edit]