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

Parameters

Return value

If no errors occur, num multiplied by FLT_RADIX to the power of exp ( num×FLT_RADIX exp ) 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

#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

See also