Scalbn
Defined in header <cmath>
.
Description
Multiplies a floating point value num
by FLT_RADIX
raised to power exp
.
The library provides overloads of std::scalbn and std::scalbln for all cv-unqualified floating-point types as the type of the parameter num
(od C++23).
Additional Overloads are provided for all integer types, which are treated as double.
Declarations
- C++23
- C++11
// 1)
constexpr /* floating-point-type */
scalbn ( /* floating-point-type */ num, int exp );
// 2)
constexpr float scalbnf( float num, int exp );
// 3)
constexpr long double scalbnl( long double num, int exp );
// 4)
constexpr /* floating-point-type */
scalbln ( /* floating-point-type */ num, long exp );
// 5)
constexpr float scalblnf( float num, long exp );
// 6)
constexpr long double scalblnl( long double num, long exp );
// 7)
template< class Integer >
constexpr double scalbn( Integer num, int exp );
// 8)
template< class Integer >
constexpr double scalbln( Integer num, long exp );
// 1)
float scalbn ( float num, int exp );
// 2)
double scalbn ( double num, int exp );
// 3)
long double scalbn ( long double num, int exp );
// 4)
float scalbnf( float num, int exp );
// 5)
long double scalbnl( long double num, int exp );
// 6)
float scalbln ( float num, long exp );
// 7)
double scalbln ( double num, long exp );
// 8)
long double scalbln ( long double num, long exp );
// 9)
float scalblnf( float num, long exp );
// 10)
long double scalblnl( long double num, long exp );
// 11)
template< class Integer >
constexpr double scalbn( Integer num, int exp );
// 12)
template< class Integer >
constexpr double scalbln( Integer num, long exp );
Parameters
num
- floating-point or integer value
exp
- integer value
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.
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
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 Additional Overloads. 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)
Examples
#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";
}
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