PS. I wouldn’t've been able to code that function without your teachings. ;D

I’ve rewrote your function like this:

bool IsEqual(double dX, double dY)
{
const double dEpsilon = 1; // dX and dY have to be within 1% of eachother
return fabs(dX - dY) <= dEpsilon * ((dX + dY) / 100);
}

I’ve tested this and it works fine. (99 will equal 100, but 98 will not equal 100.)

; )

For example, .0095 has two placeholder zeros, and so is only 2 significant figures. 34.90 has 4 significant figures.

There are two types of errors we need to watch out for with floating point values: rounding errors, and precision errors.

Rounding errors can happen with numbers of any length, because some numbers have infinite representations in binary (0.1 for example), and those representations will be truncated. Rounding errors typically make your answer wrong by 0.000001, or some small number like that.

The second and more potentially serious error are precision errors, where your number can’t be stored because the floating point representation doesn’t have enough memory. Precision errors are more serious because they can affect your answer by a much larger degree of magnitude than rounding errors.

0.0095 * 36.75 = 0.349125, which is 6 significant figures, so in this case, you’ll probably be fine in terms of precision errors. Consequently, your answer will only be affected by small rounding errors.

But consider a case like: 0.0095 * 36.7513. Even though 36.7513 is 4 decimal places, it’s 6 significant digits. When multiplied by 0.0095, the answer is 0.34913735. A float will truncate this to 0.349137.

Once you get into larger dollar amounts, floats are even less suitable. Consider $100264.75. This is a number with 8 significant digits, even though it only uses 2 decimal places. Already a float is not going to be able to hold this number. As you do mathematical operations on it, it’s going to drift farther and farther from your intended answer.

In short, if accuracy is important, use double. Only use floats when accuracy is not that important (eg. in games, where it doesn’t really matter if your character has 137.24 or 137.25 strength).