Namespaces
Variants
Views
Actions

std::logb

From cppreference.com
< cpp‎ | numeric‎ | math
Revision as of 15:44, 1 July 2013 by P12bot (Talk | contribs)

 
 
 
Common mathematical functions
Functions
Basic operations
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)(C++11)(C++11)
Exponential functions
(C++11)
(C++11)
(C++11)
(C++11)
Power functions
(C++11)
(C++11)
Trigonometric and hyperbolic functions
(C++11)
(C++11)
(C++11)
Error and gamma functions
(C++11)
(C++11)
(C++11)
(C++11)
Nearest integer floating point operations
(C++11)(C++11)(C++11)
(C++11)
(C++11)
(C++11)(C++11)(C++11)
Floating point manipulation functions
(C++11)(C++11)
(C++11)
logb
(C++11)
(C++11)(C++11)
(C++11)
Classification/Comparison
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
Macro constants
(C++11)(C++11)(C++11)(C++11)(C++11)
 
Defined in header <cmath>
float       logb( float arg );
(since C++11)
double      logb( double arg );
(since C++11)
long double logb( long double arg );
(since C++11)
double      logb( Integral arg );
(since C++11)

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 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.

Contents

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

#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

decomposes a number into significand and a power of 2
(function) [edit]
(C++11)
extracts exponent of the number
(function) [edit]
(C++11)(C++11)
multiplies a number by FLT_RADIX raised to a power
(function) [edit]