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ilogb

From cppreference.com
< c‎ | numeric‎ | math
 
 
 
Common mathematical functions
Functions
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Exponential functions
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Power functions
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Trigonometric and hyperbolic functions
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Error and gamma functions
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Nearest integer floating point operations
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Floating point manipulation functions
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ilogb
(C99)
(C99)
Classification
(C99)
(C99)
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Macro constants
 
Defined in header <math.h>
int ilogbf( float arg );
(since C99)
int ilogb( double arg );
(since C99)
int ilogbl( long double arg );
(since C99)
#define FP_ILOGB0 /*implementation-defined*/
(since C99)
#define FP_ILOGBNAN /*implementation-defined*/
(since C99)

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

Contents

[edit] Parameters

arg - floating point value

[edit] Return value

The floating-point exponent, cast to integer.

Domain or range error may occur if arg is zero, FP_ILOGB0 is returned in that case.

Domain or range error may occur if arg is infinite, INT_MAX is returned in that case.

Domain or range error may occur if arg is NaN, FP_ILOGBNAN is returned in that case.

If the result cannot be represented as int, the result is undefined.

[edit] Notes

The value of the exponent returned by ilogb is always 1 less than the exponent retuned by frexp because of the different normalization requirements: for the exponent e returned by ilogb, |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 <limits.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("FP_ILOGB0   = %+d\n", FP_ILOGB0);
    printf("FP_ILOGBNAN = %+d\n", FP_ILOGBNAN);
    printf("INT_MAX     = %+d\n", INT_MAX);
    printf("\n");
 
    printf("ilogb(DBL_MIN) = %+d\n", ilogb(DBL_MIN));
    printf("ilogb(DBL_MAX) = %+d\n", ilogb(DBL_MAX));
    printf("\n");
 
    errno = 0; feclearexcept(FE_ALL_EXCEPT);
    printf("ilogb(0.0)      = %d\n",   ilogb(0.0));
    printf("%s\n",strerror(errno));
    show_fe_exceptions();
    printf("\n");
 
    errno = 0; feclearexcept(FE_ALL_EXCEPT);
    printf("ilogb(INFINITY) = %d\n",   ilogb(INFINITY));
    printf("%s\n",strerror(errno));
    show_fe_exceptions();
    printf("\n");
 
    errno = 0; feclearexcept(FE_ALL_EXCEPT);
    printf("ilogb(NAN)      = %d\n",   ilogb(NAN));
    printf("%s\n",strerror(errno));
    show_fe_exceptions();
 
    return 0;
}

Possible output:

FP_ILOGB0   = -2147483648
FP_ILOGBNAN = -2147483648
INT_MAX     = +2147483647
 
ilogb(DBL_MIN) = -1022
ilogb(DBL_MAX) = +1023
 
ilogb(0.0)      = -2147483648
Success
current exceptions raised:  none
 
ilogb(INFINITY) = 2147483647
Success
current exceptions raised:  none
 
ilogb(NAN)      = -2147483648
Success
current exceptions raised:  none

[edit] See also

breaks a number into significand and a power of 2
(function) [edit]
(C99)(C99)(C99)
extracts exponent of the given number
(function) [edit]
(C99)(C99)(C99)(C99)(C99)(C99)
computes efficiently a number times FLT_RADIX raised to a power
(function) [edit]