UIStepper.value becomes scientific notation

Question

This is my code:

@IBAction func stepperChanged(sender: AnyObject) {
    let stepper = sender as! UIStepper
    stepper.stepValue = 0.1
    stepper.minimumValue = 0.0
    stepper.maximumValue = 1.0
    print("stepper value: \(stepper.value)")
}

When I kept pressing the minus button, the output was:

stepper value: 0.2
stepper value: 0.1
stepper value: 1.38777878078145e-16
stepper value: 0.0

Why there was 1.38777878078145e-16 between 0.1 and 0.0?

I want to know the reason and how to fix it.

Thanks!

Why do you reset the stepper's `stepValue`, `minimumValue`, and `maximumValue` every time the stepper's value changes? — rmaddy, May 06 '16 at 04:13
@matt Thanks! but I still have a question: The minus button on stepper should not be clickable when the value reaches minimumValue, but it is still clickable when the value is `1.38777878078145e-16`. The minus button is only disabled when the value really becomes `0.0`. How do i prevent this? And, why it becomes scientific notation rather than just becomes zero? — Brian, May 06 '16 at 04:24
@rmaddy I just wanted to demo my stepper setting here. Yeah it's not a good idea to set them every time, but I think it doesn't matter here. — Brian, May 06 '16 at 04:43

score 4 · Accepted Answer · answered May 06 '16 at 04:46

4

Please try minimum value as 0 and maximum value as 10.

override func viewWillAppear(animated: Bool) {
        sampleUIStepper.minimumValue = 0
        sampleUIStepper.maximumValue = 10
        sampleUIStepper.stepValue = 1
    }

Convert the UIStepper's sender value to required format and use it.

@IBAction func sampleStepperValueChanged(sender: AnyObject) {
        let stepper = sender as! UIStepper

        let convertedValue = (stepper.value/10) as Double
        print(convertedValue)

    }

answered May 06 '16 at 04:46

Alvin George

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Can you explain why this works while my approach doesn't? thanks – Brian May 06 '16 at 04:47
2

1.38777878078145e-16 is an excellent approximation to the desired value of zero, and an excellent approximation is the best you can ever hope for, with finite-precision arithmetic. You're accumulating round-off error each time you add or subtract 0.1. – Alvin George May 06 '16 at 04:55

score 1 · Answer 2 · answered May 06 '16 at 05:06

Doubles are not precise.

The type of UIStepper.value is Double. And Doubles' precision is only 16 - 17 digits. This means that in this code, result is not equal to 0.1:

let a = 0.3
let b = 0.2
let result = a - b
// result might be 0.1000000000000000007890778643

So when you click the minus button, somewhere in the code there is this statement:

value -= stepValue

Since stepValue is also a Double, it is not precise. What is happening is that the step value might be equal to 0.9999999999999999138777878078145 when you click it the third time. Therefore, the stepper's value is not 0.

Another possible explanation would be that value is not accurate, it might be 0.1000000000000000861222121021854

Remember, programming is not math.

How to solve this problem?

So you want to disable the minus button when it is close to 0, right? It's easy!

if stepper.value - 0 < 0.0000001 {
    stepper.value = 0
}

thanks for the explanation. But the inaccuracy can happen at any point, I should not suppose this only happens when the value is close to zero, right? — Brian, May 06 '16 at 06:04

UIStepper.value becomes scientific notation

2 Answers2