NAME¶

std::logb,std::logbf,std::logbl - std::logb,std::logbf,std::logbl

Synopsis¶

Defined in header <cmath>
float logb ( float arg ); (1) (since C++11)
(constexpr since C++23)
float logbf( float arg ); (2) (since C++11)
(constexpr since C++23)
double logb ( double arg ); (3) (since C++11)
(constexpr since C++23)
long double logb ( long double arg ); (4) (since C++11)
(constexpr since C++23)
long double logbl( long double arg ); (5) (since C++11)
(constexpr since C++23)
double logb ( IntegralType arg ); (6) (since C++11)
(constexpr since C++23)

1-5) Extracts the value of the unbiased radix-independent exponent from the
floating-point argument arg, and returns it as a floating-point value.
6) A set of overloads or a function template accepting an argument of any integral
type. Equivalent to (3) (the argument is cast to double).

Formally, the unbiased exponent is the signed integral part of log
r|arg| (returned by this function as a floating-point value), for non-zero arg,
where r is std::numeric_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¶

If no errors occur, the unbiased exponent of arg is returned as a signed
floating-point value.

If a domain error occurs, an implementation-defined value is returned

If a pole error occurs, -HUGE_VAL, -HUGE_VALF, or -HUGE_VALL is returned.

Error handling¶

Errors are reported as specified in math_errhandling.

Domain or range error may occur if arg is zero.

If the implementation supports IEEE floating-point arithmetic (IEC 60559),

* If arg is ±0, -∞ is returned and FE_DIVBYZERO is raised.
* If arg is ±∞, +∞ is returned
* If arg is NaN, NaN is returned.
* In all other cases, the result is exact (FE_INEXACT is never raised) and the
current rounding mode is ignored

Notes¶

POSIX requires that a pole error occurs if arg is ±0.

The value of the exponent returned by std::logb is always 1 less than the exponent
returned 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>
#include <cfenv>
#pragma STDC FENV_ACCESS ON
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';

// error handling
std::feclearexcept(FE_ALL_EXCEPT);
std::cout << "logb(0) = " << std::logb(0) << '\n';
if (std::fetestexcept(FE_DIVBYZERO))
std::cout << " FE_DIVBYZERO raised\n";
}

Possible 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
logb(0) = -Inf
FE_DIVBYZERO raised

See also¶

frexp
frexpf decomposes a number into significand and a power of 2
frexpl (function)
(C++11)
(C++11)
ilogb
ilogbf
ilogbl extracts exponent of the number
(C++11) (function)
(C++11)
(C++11)
scalbn
scalbnf
scalbnl
scalbln
scalblnf
scalblnl multiplies a number by FLT_RADIX raised to a power
(C++11) (function)
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)