NumberFormatter 的 generateDecimalNumbers 不起作用

问题描述

我的函数是将字符串转换为十进制

My function is converting a string to Decimal

func getDecimalFromString(_ strValue: String) -> NSDecimalNumber {
    let formatter = NumberFormatter()
    formatter.maximumFractionDigits = 1
    formatter.generatesDecimalNumbers = true
    return formatter.number(from: strValue) as? NSDecimalNumber ?? 0
}

但它没有按预期工作.有时它会像这样返回

But it is not working as per expectation. Sometimes it's returning like

Optional(8.300000000000001)
Optional(8.199999999999999)

而不是 8.3 或 8.2.在字符串中，我有类似8.3"或8.2"的值，但转换后的小数不符合我的要求.有什么建议我犯错了吗?

instead of 8.3 or 8.2. In the string, I have value like "8.3" or "8.2" but the converted decimal is not as per my requirements. Any suggestion where I made mistake?

推荐答案

将 generatesDecimalNumbers 设置为 true 并不像预期的那样工作.返回的值是 NSDecimalNumber 的一个实例(可以准确地表示值 8.3)，但显然格式化程序首先将字符串转换为二进制浮点数(并且可以不准确地表示 8.3).因此返回的十进制值只是大致正确.

Setting generatesDecimalNumbers to true does not work as one might expect. The returned value is an instance of NSDecimalNumber (which can represent the value 8.3 exactly), but apparently the formatter converts the string to a binary floating number first (and that can not represent 8.3 exactly). Therefore the returned decimal value is only approximately correct.

这也被报告为一个错误:

That has also been reported as a bug:

• NSDecimalNumbers from NSNumberFormatter 受二进制逼近影响错误

• NSDecimalNumbers from NSNumberFormatter are affected by binary approximation error

还要注意(与文档相反)，maximumFractionDigits 属性在 解析 字符串时不起作用变成一个数字.

Note also that (contrary to the documentation), the maximumFractionDigits property has no effect when parsing a string into a number.

有一个简单的解决方案:使用

There is a simple solution: Use

NSDecimalNumber(string: strValue) // or
NSDecimalNumber(string: strValue, locale: Locale.current)

相反，取决于字符串是否本地化.

instead, depending on whether the string is localized or not.

或者使用 Swift 3 Decimal 类型:

Or with the Swift 3 Decimal type:

Decimal(string: strValue) // or
Decimal(string: strValue, locale: .current)

例子:

if let d = Decimal(string: "8.2") {
    print(d) // 8.2
}

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