NumberFormatter Fraction Digits confusion (swift) - ios

Question

I am trying to format numbers so that there are always 4 digits after the decimal place. For example:
1 // 1.0000
0 // 0.0000
1.23 // 1.2300
1.234 // 1.2340
1.2345 // 1.2345
1.23456 // 1.2346 **[edited]**
I have tried all kinds of combinations of the following:
let formatter = NumberFormatter()
formatter.usesSignificantDigits = true // I believe this the default so not required
formatter.numberStyle = .decimal
formatter.maximumSignificantDigits = 4
formatter.minimumSignificantDigits = 4
formatter.maximumFractionDigits = 4
formatter.minimumFractionDigits = 4
let p = formatter.string(from: NSNumber(value: percentage))
debugPrint("p = \(p)")
But in two of the cases, this is what I get:
0 // 0.000
0.0123456 // 0.01234
Here is an example:
and the debug output:
"p = 0.9375"
"p = 0.000"
"p = 0.03125"
"p = 0.000"
"p = 0.03125"
What am I missing?
[I thought I had seen really good explanation in here some time ago, but can no longer find it - if anyone could drop a link to it, that would be great too!]

Answer 1

If you are trying to dictate the number of decimal places, then simply remove this significant digits stuff:
let formatter = NumberFormatter()
formatter.numberStyle = .decimal
formatter.maximumFractionDigits = 4
formatter.minimumFractionDigits = 4
let values: [Double] = [
1, // 1.0000
0, // 0.0000
1.23, // 1.2300
1.234, // 1.2340
1.2345, // 1.2345
1.23456 // 1.2346 ... if you really want 1.2345, then change formatter’s `roundingMode` to `.down`.
]
let strings = values.map { formatter.string(for: $0) }
That yields the four digits after the decimal point, as desired.