NumberFormatter string(from:NSNumber) 方法有小数位问题

Question

Here is my code

let ns = NumberFormatter.init()
ns.allowsFloats = true
ns.maximumFractionDigits = 18 //This is a variable value
ns.minimumFractionDigits = 18 //This is a variable value
ns.roundingMode = .floor
ns.numberStyle = .decimal
let doubleValueOfDecimal : Double = 12.95699999999998
let numb = NSNumber.init(value: doubleValueOfDecimal)
print(numb)
let string = ns.string(from: numb)
print(string)

The following is the output and input doubleValueOfDecimal = 2.95699999999998 Output 2.95699999999998 Optional("2.956999999999980000") But if I input

doubleValueOfDecimal =  12.95699999999998

The output is

12.95699999999998
Optional("12.957000000000000000")

The string conversion rounds up the value. Can someone explain me how this works?

The string conversion is rounding up the decimal places when I want it to show the exact number.

Answer 1

You are falling down the cracks between the expectations of the behaviour of decimal numbers and the reality that Float and Double are binary floating-point , that is the fractional part of decimal numbers are sums of 1/10's, 1/100's etc. while for binary numbers it is sums of 1/2's, 1/4's etc. and some values exact in one are inexact in the other and vice-versa.

Change your code to include:

let doubleValueOfDecimal : Decimal = Decimal(string:"12.95699999999998")!
let numb = doubleValueOfDecimal as NSDecimalNumber

and the output is probably what you expect:

12.95699999999998
12.956999999999980000

The Decimal type is a decimal floating-point value type, NSDecimalNumber is a subclass of NSNumber which holds a Decimal value.

HTH

(Note: you have to initialise the Decimal from a string as using a numeric literal appears to involve the Swift compiler using binary floating point at some point in the process...)

Answer 2

Use wrapper method of NSNumber class. `

 print(numb.stringValue)

let ns = NumberFormatter.init()
ns.allowsFloats = true
ns.maximumFractionDigits = 18
ns.minimumFractionDigits = 18
ns.roundingMode = .floor
ns.numberStyle = .decimal
let numb = NSNumber.init(value: doubleValueOfDecimal)
print(numb)
let string = ns.string(from: numb)
print(numb.stringValue)

Below Output for 2.95699999999998 , 12.95699999999998 values.

Output

2.95699999999998

2.95699999999998

12.95699999999998

12.95699999999998