std::ldexp,std::ldexpf,std::ldexpl (3) - Linux Manuals

std::ldexp,std::ldexpf,std::ldexpl: std::ldexp,std::ldexpf,std::ldexpl

NAME

std::ldexp,std::ldexpf,std::ldexpl - std::ldexp,std::ldexpf,std::ldexpl

Synopsis

Defined in header <cmath>
float ldexp ( float x, int exp );
float ldexpf( float x, int exp ); (since C++11)
double ldexp ( double x, int exp ); (1) (2)
long double ldexp ( long double x, int exp );
long double ldexpl( long double x, int exp ); (3) (since C++11)
double ldexp ( IntegralType x, int exp ); (4) (since C++11)

1-3) Multiplies a floating point value x by the number 2 raised to the exp power.
4) A set of overloads or a function template accepting an argument of any integral_type. Equivalent to (2) (the argument is cast to double).

Parameters

x - floating point value
exp - integer value

Return value

If no errors occur, x multiplied by 2 to the power of exp (x×2exp
) 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::ldexp is equivalent to std::scalbn.
The function std::ldexp ("load exponent"), together with its dual, std::frexp, can be used to manipulate the representation of a floating-point number without direct bit manipulations.
On many implementations, std::ldexp is less efficient than multiplication or division by a power of two using arithmetic operators.

Example

// Run this code

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

  #pragma STDC FENV_ACCESS ON
  int main()
  {
      std::cout << "ldexp(7, -4) = " << std::ldexp(7, -4) << '\n'
                << "ldexp(1, -1074) = " << std::ldexp(1, -1074)
                << " (minimum positive subnormal double)\n"
                << "ldexp(nextafter(1,0), 1024) = "
                << std::ldexp(std::nextafter(1,0), 1024)
                << " (largest finite double)\n";
      // special values
      std::cout << "ldexp(-0, 10) = " << std::ldexp(-0.0, 10) << '\n'
                << "ldexp(-Inf, -1) = " << std::ldexp(-INFINITY, -1) << '\n';
      // error handling
      errno = 0;
      std::feclearexcept(FE_ALL_EXCEPT);
      std::cout << "ldexp(1, 1024) = " << std::ldexp(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";
  }

Output:

  ldexp(7, -4) = 0.4375
  ldexp(1, -1074) = 4.94066e-324 (minimum positive subnormal double)
  ldexp(nextafter(1,0), 1024) = 1.79769e+308 (largest finite double)
  ldexp(-0, 10) = -0
  ldexp(-Inf, -1) = -inf
  ldexp(1, 1024) = inf
      errno == ERANGE: Numerical result out of range
      FE_OVERFLOW raised