Rounding up c# giving wrong answer

Question

I am programming a stocks/production program for a school project (bike factory) and I have to round some numbers up such as the bikes that have to be produced (due to fallout it are doubles).

If I use the following code:

double test = Math.Ceiling(100 * 1.09);

label75.Text = Convert.ToString(test);

I get as an answer 110, but that's not right, it should be 109 (9% of 100). Although it seems to work with values below 9%.

What am I doing wrong?

Answer 1

doubles and floats are binary floating point types. This means they cannot precisely represent many decimal numbers (like 1.09). This leads to some subtle round off errors. In your case 100 * 1.09 actually results in a number very slightly larger than 109, so the Ceiling function correctly rounds it up to the nearest integer, 110.

Change it to a decimal and you'll get the result you expect:

decimal test = Math.Ceiling(100 * 1.09m); // 109

Note the m in 1.09m identifies it as a decimal literal. This works because the decimal type is specifically designed to represent decimal numbers (rather than just their binary approximations, as double and float do).

Answer 2

Floating point arithmetic is not in base 10, it's in base 2. the double type doesn't have an exact base-2 equivalent of 1.09

To illustrate, if you put a break-point at the end of the following program

public static void Main()
{

    double test = 100 * 1.09;
}

than test would show up as 109.00000000000001