Finding the number of places after the decimal point of a Double

Question

I have a Double value:

double a = 4.5565;

What is the easiest way to calculate the number of digits after the decimal point (4 in this case).

I know that I can convert to string and do a split and take the length. But is there an easier way?

Answer 1

There's no easy way, especially since the number of digits mathematically speaking might be far more than displayed. For example, 4.5565 is actually stored as 4.556499999999999772626324556767940521240234375 (thanks to harold for calculating that). You're very unlikely to find a useful solution to this problem.

EDIT

You could come up with some algorithm that works like this: if, as you calculate the decimal representation, you find a certain number of 9s (or zeros) in succession, you round up (or down) to the last place before the series of 9s (or zeros) began. I suspect that you would find more trouble down that road than you would anticipate.

Answer 2

var precision = 0;
var x = 1.345678901m;

while (x*(decimal)Math.Pow(10,precision) != 
         Math.Round(x*(decimal)Math.Pow(10,precision))) 
   precision++;

precision will be equal to the number of significant digits of the decimal value (setting x to 1.23456000 will result in a precision of 5 even though 8 digits were originally specified in the literal). This executes in time proportional to the number of decimal places. It counts the number of fractional digits ONLY; you can count the number of places to the left of the decimal point by taking the integer part of Math.Log10(x). It works best with decimals as they have better value precision so there is less rounding error.

Answer 3

Write a function

int CountDigitsAfterDecimal(double value)
        {
            bool start = false;
            int count = 0;
            foreach (var s in value.ToString())
            {
                if (s == '.')
                {
                    start = true;
                }
                else if (start)
                {
                    count++;
                }
            }

            return count;
        }

Answer 4

I think this might be a solution:

 private static int getDecimalCount(double val)
 {
     int i=0;
     while (Math.Round(val, i) != val)
         i++;
     return i;
 }

double val9 = 4.5565d; int count9 = getDecimalCount(val9);//result: 4

Sorry for the duplication -> https://stackoverflow.com/a/35238462/1266873

Edit: Workaround for @chri3g91 's mentioned error:

public static class DecimalExtensions
{
    public static int GetDecimalCount(this double val)
    {
        int i = 0;
        //Doubles can be rounded to 15 digits max. ref: https://stackoverflow.com/a/33714700/1266873
        while (i < 16 && Math.Round(val, i) != val)
            i++;
        return i;
    }
    public static int GetDecimalCount(this decimal val)
    {
        int i = 0;
        while (Math.Round(val, i) != val)
            i++;
        return i;
    }
}
 

Answer 5

base on james answer bat much clearer:

int num = dValue.ToString().Length - (((int)dValue).ToString().Length + 1);

num is the exact number of digits after the decimal point. without including 0 like this(25.520000) in this case, you will get num= 2

Answer 6

I Think String solution is best : ((a-(int)a)+"").length-2

Answer 7

I'll perhaps use this code if I needed,

myDoubleNumber.ToString("R").Split('.')[1].Length

"R" here is Round Trip Format Specifier

We need to check for the index bounds first of course.

Answer 8

Another solution would be to use some string functions:

private int GetSignificantDecimalPlaces(decimal number, bool trimTrailingZeros = true)
{
  var stemp = Convert.ToString(number);
  
  if (stemp.IndexOf(Application.CurrentCulture.NumberFormat.NumberDecimalSeparator) < 0)
    return 0;

  if (trimTrailingZeros)
    stemp = stemp.TrimEnd('0');

  return stemp.Length - 1 - stemp.IndexOf(Application.CurrentCulture.NumberFormat.NumberDecimalSeparator);
}

Remember to use System.Windows.Forms to get access to Application.CurrentCulture