std::logb
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<tbody> </tbody> float logb( float arg ); |
( C++11) | |
double logb( double arg ); |
( C++11) | |
long double logb( long double arg ); |
( C++11) | |
double logb( Integral arg ); |
( C++11) | |
arg , . , logr|arg| , ,
r std::numeic_limits<T>::radix T arg. arg , .:
Extracts the value of the exponent from the floating-point argument
r|arg| as a signed floating-point value, for non-zero arg, where
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.| arg |
.
,
arg . std::logb 1 std::frexp - :
| 1
|
e std::logb, |arg*r-e| 1
r ( 1 2), e std::frexp, |arg*2-e|
0.5 1.:
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
| is between 1 and
| is between
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.#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';
}
:
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
.
(C++11)(C++11) |
2 () |
(C++11)(C++11)(C++11) |
() |
(C++11)(C++11)(C++11)(C++11)(C++11)(C++11) |
FLT_RADIX () |