Question

我有这个代码打印出一定数量的钱，但有时如果金额达到10美分的倍数，如22.50美元或189.20美元，用户界面会砍掉最后的0，显示22.5美元或189.2美元？

@IBAction func btnCalPress(sender: AnyObject) {
    let tipPer = Double(PercentTip.text!)! / 100
    let billAmt = Double(BillAmount.text!)
    let salesTax = Double(SalesTax.text!)! / 100

    let tipAmt = Double(billAmt! * tipPer)
    let taxAmt = Double(billAmt! * salesTax)
    let totalAmt = tipAmt + taxAmt + billAmt!

    let tipAmountFormat = round(tipAmt * 100) / 100
    let taxAmountFormat = round(taxAmt * 100) / 100
    let totalPayFormat = round(totalAmt * 100) / 100

    TipAmount.text = "Tip: $\(tipAmountFormat)"
    TaxAmount.text = "Tax: $\(taxAmountFormat)"
    TotalAmount.text = "Total: $\(totalPayFormat)"
}

Answer 1

在处理货币时，更容易依赖NSNumberFormatter，这恰好也可以处理某些货币的特定情况。

应用于您的代码：

@IBAction func btnCalPress(sender: AnyObject) {
    let tipPer = Double(PercentTip.text!)! / 100
    let billAmt = Double(BillAmount.text!)!
    let salesTax = Double(SalesTax.text!)! / 100

    let tipAmt = Double(billAmt * tipPer)
    let taxAmt = Double(billAmt * salesTax)
    let totalAmt = tipAmt + taxAmt + billAmt

    let formatter = NSNumberFormatter()
    formatter.numberStyle = .CurrencyStyle

    let tipAmount = formatter.stringFromNumber(tipAmt)!
    let taxAmount = formatter.stringFromNumber(taxAmt)!
    let totalAmount = formatter.stringFromNumber(totalAmt)!

    TipAmount.text = "Tip: \(tipAmount)"
    TaxAmount.text = "Tax: \(taxAmount)"
    TotalAmount.text = "Total: \(totalAmount)"
}

Answer 2

就像@jtbandes所说，在处理货币时，您应该使用NSDecimalNumber，而不是Double或Float，否则您将失去精确度。

虽然浮点类型可以存储大数字：

print(FLT_MAX)    // 3.40282e+38
print(DBL_MAX)    // 1.79769313486232e+308

它们不适合货币，因为如果您的金额具有大量小数位，您将失去精确度：

let amountString = "9999999999.000009"

let amountDouble = Double(amountString)!
print(amountDouble) 
// prints 9999999999.00001 notice how the .000009 was rounded up to .00001

let amountDecimalNumber = NSDecimalNumber(string: amountString)
print(amountDecimalNumber) // prints 9999999999.000009 no precision lost

在处理货币时，没有理由失去精确度，超过数百万的交易，小的舍入错误确实加起来！ 仅限演示文字。

let formatter = NSNumberFormatter()
formatter.numberStyle = .CurrencyStyle
let prettyAmount = formatter.stringFromNumber(amountDecimalNumber)
// prints $9,999,999,999.00

Xcode Swift赚了一大笔钱

2 个答案: