Question

In C# I want a function that rounds a given double to a given amount of decimals. I always want my function to return a value (which can be a string) with the given amount of decimals. If necessary, trailing zeros need to be added.

Example:

string result = MyRoundingFunction(1.01234567, 3);
// this must return "1.012"

That’s easy, it’s just rounding and converting to string. But here comes the problem:

string result2 = MyRoundingFuntion(1.01, 3);
// this must return "1.010"

Is there a convenient/standard way to do this, or do I manually need to add trailing zeros?

Any help is appreciated. Note that in the real life application I can’t hard code the number of decimals.

Answer 1

You can create a formatter like this example:

int numDigitsAfterPoint = 5;
double num = 1.25d;
string result = num.ToString("0." + new string('0', numDigitsAfterPoint));

or (more easily)

string result = num.ToString("F" + numDigitsAfterPoint);

As a sidenote, ToString uses the MidpointRounding.AwayFromZero instead of the MidpointRounding.ToEven (also called Banker’s Rounding). As an example:

var x1 = 1.25.ToString("F1");
var x2 = 1.35.ToString("F1");
var x3 = Math.Round(1.25, 1).ToString();
var x4 = Math.Round(1.35, 1).ToString();

These will produce different result (because Math.Round normally uses MidpointRounding.ToEven)

And note that internally ToString() seems to do some “magic” before rounding digits. For doubles, if you ask him less than 15 digits, I think it rounds to 15 digits first and then rounds to the right number of digits. See here https://ideone.com/ZBEis9