std::scalbn, std::scalbnf, std::scalbnl, std::scalbln, std::scalblnf, std::scalblnl
<cmath>
double scalbn ( double num, int exp );
(until C++23)
scalbn ( /* floating-point-type */ num, int exp );
(constexpr since C++23)
(constexpr since C++23)
double scalbln ( double num, long exp );
(until C++23)
scalbln ( /* floating-point-type */ num, long exp );
(constexpr since C++23)
(constexpr since C++23)
<cmath>
double scalbn( Integer num, int exp );
(constexpr since C++23)
double scalbln( Integer num, long exp );
(constexpr since C++23)
std::scalbn and std::scalbln for all cv-unqualified floating-point types as the type of the parameter num.(since C++23)[edit] Parameters
[edit] Return value
If no errors occur, num multiplied by FLT_RADIX to the power of exp (num×FLT_RADIXexp
) is returned.
If a range error due to overflow occurs, ±HUGE_VAL , ±HUGE_VALF, or ±HUGE_VALL is returned.
If a range error due to underflow occurs, the correct result (after rounding) is returned.
[edit] Error handling
Errors are reported as specified in math_errhandling .
If the implementation supports IEEE floating-point arithmetic (IEC 60559),
- Unless a range error occurs, FE_INEXACT is never raised (the result is exact).
- Unless a range error occurs, the current rounding mode is ignored.
- If num is ±0, it is returned, unmodified.
- If num is ±∞, it is returned, unmodified.
- If exp is 0, then num is returned, unmodified.
- If num is NaN, NaN is returned.
[edit] Notes
On binary systems (where FLT_RADIX is 2), std::scalbn is equivalent to std::ldexp .
Although std::scalbn and std::scalbln are specified to perform the operation efficiently, on many implementations they are less efficient than multiplication or division by a power of two using arithmetic operators.
The function name stands for "new scalb", where scalb was an older non-standard function whose second argument had floating-point type.
The std::scalbln function is provided because the factor required to scale from the smallest positive floating-point value to the largest finite one may be greater than 32767, the standard-guaranteed INT_MAX . In particular, for the 80-bit long double, the factor is 32828.
The GNU implementation does not set errno regardless of math_errhandling.
The additional overloads are not required to be provided exactly as (A,B). They only need to be sufficient to ensure that for their argument num of integer type:
- std::scalbn(num, exp) has the same effect as std::scalbn(static_cast<double>(num), exp).
- std::scalbln(num, exp) has the same effect as std::scalbln(static_cast<double>(num), exp).
[edit] Example
#include <cerrno> #include <cfenv> #include <cmath> #include <cstring> #include <iostream> // #pragma STDC FENV_ACCESS ON int main() { std::cout << "scalbn(7, -4) = " << std::scalbn(7, -4) << '\n' << "scalbn(1, -1074) = " << std::scalbn(1, -1074) << " (minimum positive subnormal double)\n" << "scalbn(nextafter(1,0), 1024) = " << std::scalbn(std::nextafter (1,0), 1024) << " (largest finite double)\n"; // special values std::cout << "scalbn(-0, 10) = " << std::scalbn(-0.0, 10) << '\n' << "scalbn(-Inf, -1) = " << std::scalbn(-INFINITY, -1) << '\n'; // error handling errno = 0; std::feclearexcept (FE_ALL_EXCEPT ); std::cout << "scalbn(1, 1024) = " << std::scalbn(1, 1024) << '\n'; if (errno == ERANGE ) std::cout << " errno == ERANGE: " << std::strerror (errno) << '\n'; if (std::fetestexcept (FE_OVERFLOW )) std::cout << " FE_OVERFLOW raised\n"; }
Possible output:
scalbn(7, -4) = 0.4375 scalbn(1, -1074) = 4.94066e-324 (minimum positive subnormal double) scalbn(nextafter(1,0), 1024) = 1.79769e+308 (largest finite double) scalbn(-0, 10) = -0 scalbn(-Inf, -1) = -inf scalbn(1, 1024) = inf errno == ERANGE: Numerical result out of range FE_OVERFLOW raised