Double to String Precision Issue (Swift & Obj-C)

Double_to_String.swift

let value : Double = 1234567890.1234567

// 1. String format

"\(value)" == "1234567890.1234567"  // false
"\(value)" // "1234567890.12346"
value.description
String(format: "%f", value) == "1234567890.1234567"     // false
String(format: "%f", value) // "1234567890.123457"
String(format: "%.7f", value) == "1234567890.1234567"   // true!
String(format: "%.7f", value) // "1234567890.1234567"
String(format: "%.8f", value) == "1234567890.1234567"   // false
String(format: "%.8f", value) // "1234567890.12345672" wtf
String(format: "%.9f", value) == "1234567890.1234567"   // false
String(format: "%.9f", value) // "1234567890.123456717" wtf


// 2. NSNumber & NumberFormatter

let number = NSNumber(value: value)
number.doubleValue == value         // true
number.stringValue == "1234567890.1234567"  // false
number.stringValue // "1234567890.123457"

let nf = NumberFormatter()
nf.numberStyle = .decimal
nf.usesGroupingSeparator = false


// 2.1 Significant Digits = 17

nf.usesSignificantDigits = true

nf.maximumSignificantDigits = 14
nf.string(from: number) == "1234567890.1234567" // false
nf.string(from: number) // "1234567890.1235"
nf.maximumSignificantDigits = 15
nf.string(from: number) == "1234567890.1234567" // false
nf.string(from: number) // "1234567890.12346"
nf.maximumSignificantDigits = 16
nf.string(from: number) == "1234567890.1234567" // false
nf.string(from: number) // "1234567890.12346" wtf
nf.maximumSignificantDigits = 17
nf.string(from: number) == "1234567890.1234567" // false
nf.string(from: number) // "1234567890.12346" wtf


// 2.2 minimum and maximum number of (integer, fraction) digits = (10, 7)

nf.usesSignificantDigits = false

nf.maximumIntegerDigits = 10
nf.maximumFractionDigits = 4
nf.string(from: number) == "1234567890.1234567" // false
nf.string(from: number) // "1234567890.1235"
nf.maximumFractionDigits = 5
nf.string(from: number) == "1234567890.1234567" // false
nf.string(from: number) // "1234567890.12346"
nf.maximumFractionDigits = 6
nf.string(from: number) == "1234567890.1234567" // false
nf.string(from: number) // "1234567890.12346" wtf
nf.maximumFractionDigits = 7
nf.string(from: number) == "1234567890.1234567" // false
nf.string(from: number) // "1234567890.12346" wtf


// 3. NSDecimalNumber & Decimal

let num = NSDecimalNumber(value: value)
num.doubleValue == value                // false!
num.stringValue == "1234567890.1234567" // false
num.stringValue // "1234567890.1234569216" wtf
num.description == "1234567890.1234567" // false

var decimal = Decimal(value)    // 1234567890.1234569216
let decimalString = NSDecimalString(&decimal, nil)  // "1234567890.1234569216"
decimalString == "1234567890.1234567"   // false


// 4. Break into fractional and integral parts

let sep = modf(value)
sep.0   // integral：1234567890
sep.1   // fractional：0.1234567165374756

sep.1 == value - sep.0  // true
sep.1 == 0.1234567      // false!


// 5. JSONEncoder (weird workaround)

let array = [1234567890.1234567]    // [1234567891.123457] wtf
array[0] == 1234567890.1234567      // true

let jsonEncoder = JSONEncoder()
if let data = try? jsonEncoder.encode(array) {
    if var str = String(data: data, encoding: .utf8) {
        // "[1234567891.1234567]"
        str.removeFirst()
        str.removeLast()
        str == "1234567890.1234567" // true!
    }
}