Output the full pricision of a double value.

Question

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My question is to give the exact value of R^n. where R is a real number ( 0.0 < R < 99.999 ) and n is an integer such that 0 < n <= 25.

Example:

C#

95.123^12

The answer is

C#

548815620517731830194541.899025343415715973535967221869852721

Another example is:

C#

0.4321^20

The output should be:

C#

.00000005148554641076956121994511276767154838481760200726351203835429763013462401

My code is:

C#

var result = Math.Pow(R, n);
string s = result.ToString("R");
Console.WriteLine(s);

What I get is:

C#

5.4881562051773216E+23
5.1485546410769524E-08

Posted 17-Jan-16 14:44pm

zhshqzyc

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Comments

PIEBALDconsult 17-Jan-16 20:59pm

Precision 15-16 digits
https://msdn.microsoft.com/en-us/library/678hzkk9.aspx

Maybe try a Custom Numeric Format String? I haven't tried it, but how about ################0.################
https://msdn.microsoft.com/en-us/library/0c899ak8.aspx

[no name] 17-Jan-16 21:02pm

It is incorrect string s = result.ToString("################0.################");.

PIEBALDconsult 17-Jan-16 21:37pm

In what way? You may need to use 308 number signs (#) on each side of the decimal point.

2 solutions

Solution 1

If you want to display, say 10, decimal places, try Console.WriteLine(c.ToString("N10"));.

Posted 17-Jan-16 16:19pm

Abhinav S

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BillWoodruff · Accepted Answer · 2016-01-17T18:33:00

You can convert your 'double result to 'decimal to be able to produce a string result with more digits. A big 'double is going to be rendered with the mantissa/exponent notation because that is what it is, under-the-hood.

C#

private string PowerDoubleToDecimal(double doubleValue, double toPower, string formatString)
{
    decimal result = Convert.ToDecimal(Math.Pow(doubleValue, toPower));
    return string.Format(result.ToString(), formatString);
}

// test in some method
string test1 = PowerDoubleToDecimal(95.123, 12.0, "C");
Console.WriteLine(test1);

string test1a = PowerDoubleToDecimal(0.4321, 20.0, "C");
Console.WriteLine(test1a);

// output
548815620517732000000000

0.0000000514855464107695

Jon Skeet has public domain code, 'DoubleConverter:' [^] to get a much better 'transcription' from a Double to String. Using Skeet's code, I get these results from your two trials:

C#

private string DblToExactString(double doubleValue, double toPower)
{
    double result = Math.Pow(doubleValue, toPower);
    return SkeetDoubleConverter.ToExactString(result);
}

// test in some method
string test2 = DblToExactString(95.123, 12.0);
Console.WriteLine(test2);

string test2a = DblToExactString(0.4321, 20.0);
Console.WriteLine(test2a);

548815620517732158537728

0.0000000514855464107695238993420865959704801895213677198626101016998291015625

And, I note the results are not the same as yours. That leads to the question: if you need this high-precision for some mission-critical application (as in aviation, finance, health-care, etc.), would you bet your company's future on 3rd. party code (even, as is the case with Jon Skeet, a guru's guru in .NET) if it comes from a source you believe reliable ?

I wouldn't :) I'd want to spend the money for a library from a very well-known company with years of experience in whatever specialized high-precision performance I needed.

There are many special libraries available for handling high-precision floating point numbers (some open source, some offering both free and commercial versions, some purely commercial). For links to other libraries: [^].