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>
int exponent
float scalbn ( float num, int exp );

double scalbn ( double num, int exp (since C++11)
); (until C++23)

long double scalbn ( long double num, int
exp );
constexpr /* floating-point-type */
scalbn ( /* (since C++23)
floating-point-type */ num, int exp );
(since C++11)
float scalbnf( float num, int exp ); (2) (constexpr since
C++23)
long double scalbnl( long double num, int (since C++11)
exp ); (3) (constexpr since
C++23)
long exponent
float scalbln ( float num, long exp
);

double scalbln ( double num, long exp (1) (since C++11)
); (until C++23)

long double scalbln ( long double num,
long exp );
constexpr /* floating-point-type */
scalbln ( /* (since C++23)
floating-point-type */ num, long exp );
float scalblnf( float num, long exp (since C++11)
); (4) (5) (constexpr since
C++23)
long double scalblnl( long double num, (since C++11)
long exp ); (6) (constexpr since
C++23)
Additional overloads
Defined in header <cmath>
template< class Integer > (since C++11)
double scalbn( Integer num, int exp ); (A) (constexpr since
C++23)
template< class Integer > (since C++11)
double scalbln( Integer num, long exp ); (B) (constexpr since
C++23)

1-6) 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.
(since C++23)
A,B) Additional overloads are provided for all integer types, which are treated as
double.

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 (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).

Example¶

// Run this code

#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

See also¶

frexp
frexpf decomposes a number into significand and base-2 exponent
frexpl (function)
(C++11)
(C++11)
ldexp
ldexpf multiplies a number by 2 raised to an integral power
ldexpl (function)
(C++11)
(C++11)
C documentation for
scalbn