无效的浮点值 - swift 3 ios

问题描述

我有核心的数据存储，我的领域费用安装在Float识别。
expensesAmount的值是6.3。
但是，当我将其检索到变量expensesAmount，如下所示，它变成了6.30000019。
所以我的totalAmount是不正确的。

有人可以帮忙吗？

  let entity：NSManagedObject = data？.object（at：i）as！ NSManagedObject 
如果让expenseAmount = entity.value（forKey：expenseAmount）为？ Float {
 totalAmount + = expensesAmount 
} 
  

解决方案

  int main（int argc，char ** argv）{
 float fval = 6.3f; 
 double dval = 6.3; 
 printf（％。10f：％.17f\\\
，fval，dval）; 
 // 6.3000001907：6.29999999999999980 
} 
  

所以，如果你需要真正的准确性在小数部分，您需要考虑一些其他方式。

编辑：
我检查了NSDecimalNumber和它的工作正常。这里是一个例子：

pre $ let $ b $ = NSDecimalNumber（string：6.3）//（1）6.3
让bval10 = bval.multiplying（by：10.0）// 63.0
let dval = bval.doubleValue
let dval10 = bval10.doubleValue
print（String（format：％.17f， （尾数：63，指数：-1，isNegative）// 6.29999999999999982
print（String（format：％ ：false）
print（bval2）// 6.3
let bval3 = NSDecimalNumber（尾数：123456789，指数：-4，isNegative：true）
print（bval3）// -12345.6789 $ b （6）中可以看到，在（1）中转换6.3时没有四舍五入的结果。注意63.0可以用w / float / double精确表示。

I had core data storage, my field "expensesAmount" in Float identify.The value of expensesAmount is 6.3.But when I retrieve it to variable "expensesAmount" as below, it become 6.30000019.So my totalAmount is not correct.

Can someone help?

let entity:NSManagedObject = data?.object(at: i) as! NSManagedObject
if let expensesAmount = entity.value(forKey: "expensesAmount") as? Float {
   totalAmount += expensesAmount
}

解决方案

I think this is related to how the floating point numbers are expressed with IEEE-754 standard. With the standard, not all kinds of numbers with fraction may necessarily be expressed precisely even with double. This is irrelevant to Swift. The next small code in C will reproduce your issue.

int main(int argc, char **argv) {
  float fval = 6.3f;
  double dval = 6.3;
  printf("%.10f : %.17f\n", fval, dval);
  // 6.3000001907 : 6.29999999999999980
}

So, if you need the real accuracy in fractional part, you need to consider some other way.

EDITED:I checked with NSDecimalNumber and it's working as expected. Here is an example:

    let bval = NSDecimalNumber(string: "6.3")  // (1) 6.3
    let bval10 = bval.multiplying(by: 10.0)  // 63.0
    let dval = bval.doubleValue
    let dval10 = bval10.doubleValue
    print(String(format: "%.17f", dval))  // 6.29999999999999982
    print(String(format: "%.17f", dval10))  // (6) 63.00000000000000000
    let bval2 = NSDecimalNumber(mantissa: 63, exponent: -1, isNegative: false)
    print(bval2)  // 6.3
    let bval3 = NSDecimalNumber(mantissa: 123456789, exponent: -4, isNegative: true)
    print(bval3)  // -12345.6789

As you can see at (6), there's no round off when converting 6.3 at (1). Note 63.0 can be precisely expressed w/ float/double.

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