c++ rounding of numbers away from zero

问题:

Hi i want to round double numbers like this (away from zero) in C++:

  4.2 ---->   5
  5.7 ---->   6
 -7.8 ---->  -8
-34.2 ----> -35

What is the efficient way to do this?

回答1:

inline double myround(double x)
{
  return x < 0 ? floor(x) : ceil(x);
}

As mentioned in the article Huppie cites, this is best expressed as a template that works across all float types

See http://en.cppreference.com/w/cpp/numeric/math/floor and http://en.cppreference.com/w/cpp/numeric/math/floor

or, thanks to Pax, a non-function version:

x = (x < 0) ? floor(x) : ceil(x);

回答2:

There is a nice article about a similar problem on CPlusPlus.com. The easy solution to your problem should be something like this:

double customRound( double value ) const {
   return value < 0 ? floor( value ) : ceil( value );
}

A better solution is the one mentioned in the article, which uses a template:

//--------------------------------------------------------------------------
// symmetric round up
// Bias: away from zero
template <typename FloatType>
FloatType ceil0( const FloatType& value )
{
   FloatType result = std::ceil( std::fabs( value ) );
   return (value < 0.0) ? -result : result;
}

回答3:

The x < 0 ? floor(x) : ceil(x); approach of Ruben Bartelink is good. Yet consider what happens with special cases of x = -0.0, x = NaN.

Rather than have myround(-0.0) potentially return +0.0¹ and have myround(NaN) return with a changed payload of the NaN, consider the below.

myround_alt(-0.0) returns -0.0.

myround_alt(NaN) more likely returns an unchanged payload NaN. Not-a-number stuff is tricky and not well defined. IAC, it is the myround_alt(-0.0) --> -0.0 I am seeking.

inline double myround_alt(double x) {
  if (x > 0) return ceil(x);
  if (x < 0) return floor(x);
  return x;
}

---

¹IEC 60559 floating-point arithmetic specifies ceil(±0) returns ±0 so this approach not needed with implementations that strictly follow that spec. Yet many C floating point implementation do not follow that (C does not require it) or fail in such comer cases like this.

回答4:

try

 double rounded = _copysign(ceil(abs(x)), x);