What should I set in maximumFractionDigits to minimize data loss? - ios

Question

Introduction
I noticed the NumberFormatter#maximumFractionDigits default is 3.
I have confirmed:
import Foundation
let nf = NumberFormatter()
nf.numberStyle = .decimal
print(nf.maximumFractionDigits) //=> 3
nf.string(for: Decimal(string: "100.1111111")) //=> "100.111"
I have tried to set Int.max
I set Int.max to maximumFractionDigits:
import Foundation
let nf = NumberFormatter()
nf.numberStyle = .decimal
nf.maximumFractionDigits = Int.max
nf.string(for: Decimal(string: "100.1111111")) // => "100"
Why!? become "100"!?
In my research
I read Foundation > NSNumberFormatter > NumberFormatter source code:
open var maximumFractionDigits: Int
I have confirmed maximumFractionDigits data type is Int.
Question
How to set max into maximumFractionDigits?
I want to show a server response without loss, as much as possible.
of course, a server response is String in json. But all most calculation in ios app with Decimal from the String. So this goal is to convert Decimal to String for UILabel.
Q1. nf.maximumFractionDigits = Int.max. Why loss data? this is bug on NumberFormatter?
Q2. How to set max into maximumFractionDigits correct?
Goal
I want to minimize data loss.

Answer 1

Q1. nf.maximumFractionDigits = Int.max. Why loss data? this is bug on NumberFormatter?
When not clearly documented, every Int parameter may have a limitation depending on the implementation details. If you passed a value exceeding this limitation, a runtime error might cause crash, or might be silently ignored, all such things depend on the implementation detail.
As far as I tested, the maximum number you can set to maximumFractionDigits is the same value with Int32.max.
let nf = NumberFormatter()
nf.numberStyle = .decimal
nf.maximumFractionDigits = Int(Int32.max)+1
print(nf.string(for: Decimal(string: "123.45678901234567890123456789012345678"))!)
//->123
nf.maximumFractionDigits = Int(Int32.max)
print(nf.string(for: Decimal(string: "123.45678901234567890123456789012345678"))!)
//->123.45678901234567890123456789012345678
You can call it a bug, but, the maximum significant digits which NumberFormatter can handle is 38-digit, of Decimal. Who want to make a precise definition for values more than millions of times bigger than expected practical values?
Q2. How to set max into maximumFractionDigits correct?
As noted above, the significant digits held in Decimal is 38. You can write something like this:
let nf = NumberFormatter()
nf.numberStyle = .decimal
nf.usesSignificantDigits = true
nf.maximumSignificantDigits = 38
print(nf.string(for: Decimal(string: "123.45678901234567890123456789012345678"))!)
//->123.45678901234567890123456789012345678