std::logb
| Defined in header <cmath>
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| 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 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.
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
