Question

Is there a way to format a double number that always have n digits sepecified by user?

For example if user want to see always 4 digits, take the following numbers as example:

Original             Formatted
-------             ---------
3.42421             3.424
265.6250            265.6
812.50              812.5
12.68798            12.68
0.68787             0.687

I made up this but it just allows for number of floating points! it is not what I wanted!

    public string ToEngV(double d, int percision = 0)
    {
        string zeros = string.Empty;

        if (percision <= 0)
        {
                zeros += "0";
        }
        else if (percision > 0)
        {
            for (int i = 0; i < percision; i++)
            {
                zeros += "0";
            }
        }

       return String.Format("{0:0." + zeros + "}", d) 
     }

Imagine I call the above method for a number like 812.50 and I set the precision to (this is now used for all numbers I am going to format). Obviously the output will be 812.5

But if I give the another number like 1.61826 I will get 1.6 and this ruins the formatting in the page I show these number to users. I need that to be 1.618

Thus I want my method to always show N digit!

Answer 1

I’m not sure if your asking to round or truncate numbers, so I wrote this method:

public static string ToEngV(this double d, int digits, bool round)
{
    var lenght = Math.Truncate(d).ToString().Length;

    if (lenght > digits)
    {
        throw new ArgumentException("...");
    }

    int decimals = digits - lenght;

    if (round)
    {
        return Math.Round(d, decimals).ToString();
    }
    else
    {
        int pow = (int)Math.Pow(10, decimals);
        return (Math.Truncate(d * pow) / pow).ToString();
    }
}

Example:

var numbers = new double[] { 3.42421, 265.6250, 812.50, 12.68798, 0.68787 };
foreach (var number in numbers)
{
    Console.WriteLine(number.ToEngV(4, false));
}
Console.WriteLine()
foreach (var number in numbers)
{
    Console.WriteLine(number.ToEngV(4, true));
}

Output:

3.424
265.6
812.5
12.68
0.687

3.424
265.6
812.5
12.69
0.688

Note that if your number has more integer digits than digits you will get an ArgumentException.