How to round a number?

[WilliamGS]

Hi all. Is there a way or function to round a number?. I am using VC++ 6.0

Thanks in advance.
William GS

 

[uolj]

Add 0.5 and cast to an int (and then back to double if you want).

double d1 = 1.3, d2 = 1.7;
double roundedDown = static_cast<int>(d1 + 0.5);
double roundedUp = static_cast<int>(d2 + 0.5);
// roundedDown = 1.0
// roundedUp = 2.0

 

[Salem]

Mmm, doesn't round say -1.7 very well.

--

 

[uolj]

double roundIt(double d)
{
  return static_cast<int>((d < 0.0) ? (d - 0.5) : (d + 0.5));
}

 

[globos]

//! Rounding of \p v to the number of digit \p digit_count.
double round (double v, int digit_count = 0) 
{
  assert (digit_count >= 0);//positive_digit_count

  double dec_place = 1;
  for (int i = 1; i <= digit_count; i++)
    dec_place = dec_place * 10;

  int sign = v > 0 ? 1 : -1;
  double Result = (abs (v) * dec_place) + 0.5;
  int new_integer = (int) Result;
  Result = sign * (((double) new_integer) / dec_place);

  return Result;
}

--
Globos

 

[cpjust]

Since a double only has 15 digits of precision, you don't need a whole int, a char is more than enough. You should be asserting if the abs(digit_count) is > 15.
This way a positive number might mean the number of digits to keep after the decimal point, or a negative number might mean the number of digits to round before the decimal place.

ex.
round( 3.14, 1 ) would return 3.1
round( 3.14, 0 ) would return 3.0
round( 3.50, -1 ) would return 4.0
round( 314, -2 ) would return 310

Also, instead of using abs() and casting to an int, you should use fabs() so that decimal numbers won't get lost.