swift - NSNumber/NSDecimalNumber 奇怪的行为

Question

对于我刚好选择的测试用例，以下代码会产生奇怪的结果。来自 playground 的代码，然后是结果

import Cocoa

func spellDollars(_ amount: Decimal) -> String {
    let fDollars = NumberFormatter()
    fDollars.numberStyle = NumberFormatter.Style.spellOut
    let fCents = NumberFormatter()
    fCents.format = "00"
    let c = (amount * 100 as NSDecimalNumber).intValue % 100
    let cents = fCents.string(from: NSNumber(value: c))!
    let d  = (amount as NSDecimalNumber).intValue
    print(d, c, amount)
    let dollars = fDollars.string(from: NSNumber(value: d))!.capitalized
    let s = "\(dollars) and \(cents)/100"
    return s
}

print(spellDollars(1019123.08))

结果:

-825551 -32 1019123.0799999997952负八十二万五千五百五十一和 -32/100

Answer 1

归根结底，问题在于您使用 Double 调用了 spellDollars，它转换为 Decimal。你可以这样做:

print(spellDollars(Decimal(string: "1019123.08")!))

或者

print(spellDollars(Decimal(sign: .plus, exponent: -2, significand: 101912308)))

如果不引入此 Double 到 Decimal 转换噪声，这两种方法都可以工作。

---

考虑一个简化的例子:

let decimal1 = Decimal(floatLiteral: 1019123.08)
let decimal2 = Decimal(string: "1019123.08")!
let decimal3 = Decimal(sign: .plus, exponent: -2, significand: 101912308)

for value in [decimal1, decimal2, decimal3] {
    print(value)
}

这将输出:

1019123.0799999997952
1019123.08
1019123.08

要理解为什么第一个示例中的 Double 会转换为 1019123.0799999997952 的 Decimal 值，我建议您引用 What Every Computer Scientist Should Know About Floating-Point Arithmetic .

---

关于您从 intValue 得到 -825551 的原因，我猜这是由于有效数 (10191230799999997952) 太大而无法以整数格式表示而导致的溢出或类似的东西。

我建议快刀斩乱麻，不要使用 NSDecimalNumber 算法来手动舍入值。使用 NSDecimalRound 函数，例如

func spellDollars(_ amount: Decimal) -> String {
    // instantiate formatters

    let dollarsFormatter = NumberFormatter()
    dollarsFormatter.numberStyle = .spellOut
    let centsFormatter = NumberFormatter()
    centsFormatter.minimumIntegerDigits = 2

    // get dollars

    var inputAmount = amount
    var dollars = Decimal.zero
    NSDecimalRound(&dollars, &inputAmount, 0, .down)

    // get dollars and cents

    var dollarsAndCents = Decimal.zero
    NSDecimalRound(&dollarsAndCents, &inputAmount, 2, .plain)

    // get cents

    let cents = (dollarsAndCents - dollars) * 100

    // build strings

    let centsString = centsFormatter.string(for: cents)!
    let dollarsString = dollarsFormatter.string(for: dollars)!.capitalized
    return "\(dollarsString) and \(centsString)/100"
}