How do I convert a double to a string of 15 digits?

Question

I have the number 123.1234567890129.

I want the result to be 123.123456789012 without the last digit being rounded.

I've tried:

("123.1234567890129").ToString("G15") //123.123456789013

Answer 1

One way that you could do this is to round to 16 like this

("123.1234567890129").ToString("G16").Substring(0, 16);

Answer 2

1. Since you said double.
2. Since doubles can have ANY number of digits you must round in some way. (you either round down, as inferred or you round up as in practice for this case)
3. Since you imply you only want to see the number of precise digits, you must find out how many digits are on each side of the decimal point (0 to 15 on either side)

An extenstion to round down

public static class DoubleExtensions
{
    public static double RoundDown(this double value, int numDigits)
    {
        double factoral = Math.Pow(10, numDigits);
        return Math.Truncate(value * factoral) / factoral;
    }
}

test case

        const int totalDigits = 15;

        // why start with a string??
        string somestring = "123.1234567890129";
        const int totalDigits = 15;

        // since the title says 'convert a double to a string' lets make it a double eh?
        double d = double.Parse(somestring);
        int value = (int)d;

        double digitsRight = d - value;
        int numLeft = (d - digitsRight).ToString().Count();
        int numRight = totalDigits - numLeft;

        double truncated = d.RoundDown(numRight);

        string s = truncated.ToString("g15");

Answer 3

You can create custom FormatProvider and then create your implementation.

class Program
{
    static void Main(string[] args)
    {
        double number = 123.1234567890129;

        var result = string.Format(new CustomFormatProvider(15), "{0}", number);
    }

}


public class CustomFormatProvider : IFormatProvider, ICustomFormatter
{
    private readonly int _numberOfDigits;

    public CustomFormatProvider(int numberOfDigits)
    {
        _numberOfDigits = numberOfDigits;
    }

    public object GetFormat(Type formatType) => formatType == typeof(ICustomFormatter) ? this : null;

    public string Format(string format, object arg, IFormatProvider formatProvider)
    {
        if (!Equals(formatProvider))
            return null;

        if (!(arg is double))
        {
            return null;
        }

        var input = ((double)arg).ToString("R");

        return input.Length > _numberOfDigits + 1 ? input.Substring(0, _numberOfDigits + 1) : input; // +1 because of dot
    }

Unfortunately you cannot do in this way:

var result = number.ToString(new CustomFormatProvider(15));

because of value types limitation.. Double supports only CultureInfo and NumberFormatInfo formatters. If you pass different formatter it will return default instance: NumberFormatInfo.CurrentInfo'. You can make small workaround by usingstring.Format` method.

Answer 4

New to the community. First answer here. :)

I think you are looking for something like this. Works with or without decimal. This will cut the digits after the 15th digit only irrespective of length of the number. You can get the user to decide the accuracy by getting the precision value as a user input and performing that condition check accordingly. I used 15 because you mentioned it. Let me know if it works for you. Cheers!

        string newstr;
        int strlength,substrval;
        double number;
        string strnum = "123.1234567890129";
        strlength = strnum.Length;
        if(strlength>15)
        {
            strlength = 15;
        }
        substrval = strlength;
        foreach(char x in strnum)
        {
            if(x=='.')
            {
                substrval++;
            }
        }
        newstr = strnum.Substring(0, substrval);
        number=Convert.ToDouble(newstr);

Answer 5

Alife Goodacre, code is printing "123.12345678901" insted "123.123456789012"

there should be Substring(0, 16) insted of Substring(0, 15)

Convert.ToDouble("123.1234567890129").ToString("G16").Substring(0, 16)

OutPut Screen with Code.