NAME¶

std::scalbn,std::scalbnf,std::scalbnl,std::scalbln,std::scalblnf,std::scalblnl - std::scalbn,std::scalbnf,std::scalbnl,std::scalbln,std::scalblnf,std::scalblnl

Synopsis¶

Defined in header <cmath>
float scalbn ( float x, int exp ); (1) (since C++11)
(constexpr since C++23)
float scalbnf( float x, int exp ); (2) (since C++11)
(constexpr since C++23)
double scalbn ( double x, int exp ); (3) (since C++11)
(constexpr since C++23)
long double scalbn ( long double x, int exp ); (4) (since C++11)
(constexpr since C++23)
long double scalbnl( long double x, int exp ); (5) (since C++11)
(constexpr since C++23)
double scalbn ( IntegralType x, int exp ); (6) (since C++11)
(constexpr since C++23)
float scalbln ( float x, long exp ); (7) (since C++11)
(constexpr since C++23)
float scalblnf( float x, long exp ); (8) (since C++11)
(constexpr since C++23)
double scalbln ( double x, long exp ); (9) (since C++11)
(constexpr since C++23)
long double scalbln ( long double x, long exp ); (10) (since C++11)
(constexpr since C++23)
long double scalblnl( long double x, long exp ); (11) (since C++11)
(constexpr since C++23)
double scalbln ( IntegralType x, long exp ); (12) (since C++11)
(constexpr since C++23)

1-5,7-11) Multiplies a floating point value x by FLT_RADIX raised to power exp.
6,12) A set of overloads or a function template accepting an argument of any
integral type. Equivalent to (3) or (9) (the argument is cast to double).

Parameters¶

x - floating point value
exp - integer value

Return value¶

If no errors occur, x multiplied by FLT_RADIX to the power of arg (x×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 x is ±0, it is returned, unmodified
* If x is ±∞, it is returned, unmodified
* If exp is 0, then x is returned, unmodified
* If x 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 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

Example¶

// Run this code

#include <iostream>
#include <cmath>
#include <cerrno>
#include <cstring>
#include <cfenv>

// #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

See also¶

frexp
frexpf decomposes a number into significand and a power of 2
frexpl (function)
(C++11)
(C++11)
ldexp
ldexpf multiplies a number by 2 raised to a power
ldexpl (function)
(C++11)
(C++11)