NSNumberFormatter numberFromString is adding a small fraction when converting certain numbers

问题

I'm seeing some strange bugs in my iPhone app that I have narrowed down to my use of NSNumberFormatter.

A stripped down example...

In Xcode playground I have:

let numberFormatter = NSNumberFormatter()
//numberFormatter.numberStyle = .DecimalStyle - does not change behavior

let numberString = "546000.06"

let number: NSNumber = numberFormatter.numberFromString(numberString)!

print("number: \(number)")

let number1: NSNumber = NSDecimalNumber(string: numberString)

print("number1: \(number1)")

This is the output:

number: 546000.0600000001
number1: 546000.06

Note that setting the numberStyle to .DecimalStyle doesn't change anything.

This issue only happens for certain numeric values (for example, 8.03 is another one). I thought NSNumberFormatter was safe for this type of conversion and I haven't seen much noise about this issue on the internet so I want to assume it is something I am doing wrong.

Can anyone explain what I am seeing? Any help is very much appreciated!

回答1:

It looks like there is an issue with NSNumberFormatter. There are certain values where this rounding error creeps up. In the XCode 7.2.1 playground, it shows up around 8.03.

One way that I've solved this is to round the decimal number. Since the difference is +/- a tiny amount, rounding to 4 fraction places should work. You can use various rounding modes. In this example I used RoundPlain.

var initialValue = NSDecimalNumber(string: "7")
let handler = NSDecimalNumberHandler(roundingMode: NSRoundingMode.RoundPlain, scale: 4, raiseOnExactness: false, raiseOnOverflow: false, raiseOnUnderflow: false, raiseOnDivideByZero: false) 
let numberFormatter = NSNumberFormatter()

for index in 1...300 {    
    initialValue = initialValue.decimalNumberByAdding(0.01)
    let stringValue = "\(initialValue)"

    var number = numberFormatter.numberFromString(stringValue)
    var decimalNumber: NSDecimalNumber = NSDecimalNumber(decimal: number!.decimalValue)
    decimalNumber = decimalNumber.decimalNumberByRoundingAccordingToBehavior(handler)

    print("stringValue = \(stringValue), decimalNumber = \(decimalNumber), number = \(number!)")
}

回答2:

You are seeing a rounding error.

If you would like to display a number to the user with two fraction digits, set maximumFractionDigits:

let numberFormatter = NSNumberFormatter()
numberFormatter.maximumFractionDigits = 2

let firstNumber = NSNumber(float:546000.06)
let firstNumberString = numberFormatter.stringFromNumber(firstNumber)!

let secondNumber = NSNumber(float:5.1337)
let secondNumberString = numberFormatter.stringFromNumber(secondNumber)!

print("firstNumber: \(firstNumberString)")
print("secondNumber: \(secondNumberString)")

The output will be:

"firstNumber: 546000.06\n"
"secondNumber: 5.13\n"

If you try to parse a number from a string, then you are all set.

let thirdNumber = numberFormatter.numberFromString("546000.06")!
print("thirdNumber \(thirdNumber.className): \(thirdNumber)")

The last line prints the description of the object itself (w/ rounding error):

"thirdNumber __NSCFNumber: 546000.0600000001\n"

Update (2016-03-18)

You want to parse the following string:

let currencyString = "$546,000.06"

First, we create a formatter with a locale and .CurrencyStyle. Be aware that the locale depends on the string you try to parse and not on the system you are running on.

let currencyFormatter = NSNumberFormatter.init()
currencyFormatter.locale = NSLocale.init(localeIdentifier: "en_US")
currencyFormatter.numberStyle = .CurrencyStyle

We can now use the formatter to prase the string.

let currencyNumber = currencyFormatter.numberFromString(currencyString)!

You are now free to store this object, for example in CoreData. However, if you wan't to present the value to the user (or print it on the console), you have to use a NSNumberFormatter. If you print the object directly, the description (or debugDescription) of the object is used.

So, let's create another formatter to print the value of your NSNumber object:

let outputFormatter = NSNumberFormatter.init()
outputFormatter.maximumFractionDigits = 10

Using many fraction digits should print any possible rounding errors. But

print("Number: \(outputFormatter.stringFromNumber(currencyNumber)!)")

outputs the desired result:

"Number: 546000.06\n"

What you have seen is the result of the internal representation of an NSNumber object (with description/debugDescription).

来源：https://stackoverflow.com/questions/36044784/nsnumberformatter-numberfromstring-is-adding-a-small-fraction-when-converting-ce