iphone / Objective C - Comparing doubles not working

Question

I think I\'m going insane. \"counter\" and \"interval\" are both doubles. This is happening on accelerometer:didAccelerate at an interval of (.01) . \"counter\" should even

Answer 1

Don't ever compare doubles or floats with equality - they might look the same at the number of significant figures your are examining but the computer sees more.

For this purpose, the Foundation Framework provides "epsilon" values for different types such as "float" and "double". If the distance between two numbers is smaller than epsilon, you can assume these two numbers are equal.

In your case, you would use it as follow:

- (BOOL)firstDouble:(double)first isEqualTo:(double)second {
    if(fabs(first - second) < DBL_EPSILON)
        return YES;
    else
        return NO;
}

Or in Swift 4:

func doublesAreEqual(first: Double, second: Double) -> Bool {
    if fabs(first - second) < .ulpOfOne {
        return true
    }
    return false
}

Two very useful links:

What Every Computer Scientist Should Know About Floating-Point Arithmetic

Interesting discussion of Unit in Last Place (ULP) and usage in Swift

Friday Q&A 2011-01-04: Practical Floating Point