Confusion with unwrapping an optional value when rounding numbers - swift

Question

Very new to Swift concepts and I'm having trouble conceptualizing how to convert a long decimal value type var distance: String? into a shorter one. This code is crashing due to a:
Fatal error: Unexpectedly found nil while unwrapping an Optional value
let distance = Int(item.distance!) // long decimal value
let x = Float((distance)!)
let y = Double(round(1000*x)/1000)
print(y)

Answer 1

A couple of observations:
You should not only excise the Int from your code snippet, but the Float, too. If your numbers can be large, Float can impose undesirable limitations in the precision of your calculation. So, you would likely want to remove both Int and Float like so:
guard let string = item.distance, let value = Double(string) else {
return
}
let result: Double = (value * 1000).rounded() / 1000
If you’re doing this rounding just so you can show it to 3 decimal places in your UI, you probably wouldn’t round the value at all, but rather just round the output using a NumberFormatter, e.g.:
let formatter = NumberFormatter()
formatter.numberStyle = .decimal
formatter.minimumFractionDigits = 3
formatter.maximumFractionDigits = 3
guard let string = item.distance, let value = Double(string) else {
return
}
let result: String = formatter.string(for: value)
We do this when showing fractional values in our UI because:
The resulting string will be “localized”. This is important because not all locales use . for the decimal point (e.g., in many countries, the value ½ is displayed as 0,5, not 0.5). We always want to show numbers in our UI in the manner preferred by the user. The NumberFormatter does all of this localization of string representations of numbers for us.
If the value had trailing zeros, the above formatter will generate all of those decimal places (e.g. 0.5 will be shown as 0.500), which is often preferable when dealing with decimal places in your UI. (And even if if you don’t want those trailing zeros, you’d still use the NumberFormatter and just set minimumFractionDigits to whatever is appropriate for your app.)
If you really need to round the number to three decimal places (which is unlikely in this case, but we encounter this in financial apps), you shouldn’t use Double at all, but rather Decimal. Again, I’m guessing this is unlikely in your scenario, but I mention it for the sake of completeness.

Answer 2

First of all, This will help you find the issue:
if let distance = item.distance {
if let distanceInt = Int(distance) {
let x = Float(distanceInt)
let y = Double(round(1000*x)/1000)
print(y)
} else {
print("Distance (\(distance)) is not convertible to Int. It has a value, but this value is not representing an integer number.")
}
} else {
print("distance is nil. It should be some number but it is not set yet")
}
Here you can see this string: "0.45991480288961" can not be converted to an Int. So you need to convert it directly to a Double:
if let distance = item.distance {
if let distanceDouble = Double(distance) {
let x = Float(distanceDouble)
let y = Double(round(1000*x)/1000)
print(y)
} else {
print("Distance (\(distance)) is not convertible to Double. It has a value, but this value is not representing a double number.")
}
} else {
print("distance is nil. It should be some number but it is not set yet")
}