std::logb

 

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Template:ddcl list begin <tr class="t-dsc-header">

<td>

Defined in header <cmath>

</td>

<td></td> <td></td> </tr> <tr class="t-dcl ">

<td >

float       logb( float arg );

</td>

<td class="t-dcl-nopad"> </td> <td > (since C++11) </td> </tr> <tr class="t-dcl ">

<td >

double      logb( double arg );

</td>

<td class="t-dcl-nopad"> </td> <td > (since C++11) </td> </tr> <tr class="t-dcl ">

<td >

long double logb( long double arg );

</td>

<td class="t-dcl-nopad"> </td> <td > (since C++11) </td> </tr> <tr class="t-dcl ">

<td >

double      logb( Integral arg );

</td>

<td class="t-dcl-nopad"> </td> <td > (since C++11) </td> </tr> Template:ddcl list end

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 logr|arg| as a signed floating-point value, for non-zero arg, where r is std::numeic_limits<T>::radix and T is the floating-point type of arg. If arg is subnormal, it is treated as though it was normalized.

Parameters

arg

-

floating point value

Return value

The floating-point exponent.

Domain or range error may occur if arg is zero.

Notes

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

Example

Compares different floating-point decomposition functions

Run this code

#include <iostream>
#include <cmath>
#include <limits>
int main()
{
    double f = 123.45;
    std::cout << "Given the number " << f << " or " << std::hexfloat
              << f << std::defaultfloat << " in hex,\n";
 
    double f3;
    double f2 = std::modf(f, &f3);
    std::cout << "modf() makes " << f3 << " + " << f2 << '\n';
 
    int i;
    f2 = std::frexp(f, &i);
    std::cout << "frexp() makes " << f2 << " * 2^" << i << '\n';
 
    i = std::ilogb(f);
    std::cout << "logb()/ilogb() make " << f/std::scalbn(1.0, i) << " * "
              << std::numeric_limits<double>::radix
              << "^" << std::ilogb(f) << '\n';
}

Output:

Given the number 123.45 or 0x1.edccccccccccdp+6 in hex,
modf() makes 123 + 0.45
frexp() makes 0.964453 * 2^7
logb()/ilogb() make 1.92891 * 2^6

See also

Template:cpp/numeric/math/dcl list frexpTemplate:cpp/numeric/math/dcl list ilogbTemplate:cpp/numeric/math/dcl list scalbn