Question

Im currently reading Code Complete by Steve McConnell, specifically page 295 on floating-point numbers.

When i ran the following code:

        double nominal = 1.0;
        double sum = 0.0;

        for (int i = 0; i < 10; i++)
        {
            sum += 0.1;
            Console.WriteLine("sum: " + sum.ToString());
        }

        if (equals(nominal,sum))
        {
            Console.WriteLine("Numbers are the same");
        }
        else
        {
            Console.WriteLine("Numbers are different");
        }

I got a print out of
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Numbers are different

How come I didn’t get the output that is suppose to happen?
ie:
0.1
0.2
0.30000000000000004
0.4
0.5
0.6
0.7
0.79999999999999999
0.89999999999999999
0.99999999999999999
Numbers are different

Is C# rounding numbers when I do an implicit conversion from double to string? I think so because when i debug the application and step through the for loop, i can see the non-terminating repeating decimal numbers. What do you think? Am i right or wrong?

Answer 1

double.ToString() uses the general format which defaults to 15 decimal places if the precision is not specified. So it does do a little rounding which is why you’re seeing what you’re seeing. For example 0.89999999999999999 which you specified in your question is 17 decimal places. You could actually see the entire number by doing sum.ToString("g17").

You can find .net’s Standard Numeric Format Strings and their default precision here: http://msdn.microsoft.com/en-us/library/dwhawy9k(VS.80).aspx