C# 银行家的舍入误差
double a = 18.565
return Math.Round(a,2)
..返回 18.57。
对于其他每个数字，我尝试了银行家的四舍五入，结果按预期工作，例如 Math.Round(2.565,2) 返回 2.56。
知道为什么以及何时发生这种情况吗？ 这是错误还是我遗漏了有关银行家四舍五入的信息？
谢谢..
double a = 18.565
return Math.Round(a,2)
..returns 18.57.
For every other number I tried banker's rounding worked as expected, for example Math.Round(2.565,2) returned 2.56.
Any clue why and when that happens?
Is it error or am I missing something about banker's rounding?
Thanks..
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正如Matthew所说，18.565无法准确表示。实际使用的值是18.565000000000001278976924368180334568023681640625（使用DoubleConverter找到），显然超过了一半-方式。现在我有一种隐隐的感觉，有时
Math.Round会考虑一个实际上超出中点的值，但它是尽可能接近可以准确表示的中点，就像恰好在该点一样。但是，我还没有看到任何描述其应用情况的文档，并且显然在本例中没有发生这种情况。我不想依赖它。当然，即使四舍五入的值也不完全是 18.57。实际上是18.57000000000000028421709430404007434844970703125。
从根本上讲，如果您真的非常关心准确表示十进制值，那么您应该使用
decimal。这不仅仅是在Math.Round方面 - 它涉及处理浮点值的各个方面。当然，这确实为
Math.Round提供了正确的值：As Matthew said, 18.565 can't be accurately represented. The actual value used is 18.565000000000001278976924368180334568023681640625 (found using DoubleConverter), which is clearly beyond half-way. Now I've a sneaking feeling that sometimes
Math.Roundwill consider a value which is actually beyond the half-way point, but which is as close to the half-way point as can be accurately represented, as being exactly at that point. However, I haven't seen any documentation describing the situations in which that's applied, and clearly it's not happening in this case. I wouldn't want to rely on it.Even the rounded value isn't exactly 18.57 of course. It's actually 18.57000000000000028421709430404007434844970703125.
Fundamentally, if you really, really care about representing decimal values accurately, you should be using
decimal. That's not just in terms ofMath.Round- it goes to every aspect of handling floating point values.That does give the right value for
Math.Round, of course:18.565 不能精确地表示为双精度数。因此，二进制表示稍高，因此向上舍入。如果使用decimal：
可以精确表示，就不会有这个问题。
18.565 can not be exactly represented as a double. Thus, the binary representation is slightly higher, so it rounds up. If you use decimal:
it can be exactly represented, and you won't have this issue.
我的猜测是 FP 表示意味着它实际上不是尾随 5；而是尾随 5。 FP的危险！
不过，这工作得很好：
My guess is that the FP representation means it isn't actually a trailing 5; the dangers of FP!
This works fine, though:
Double 是一个浮点值，所以如果你将其写为 18.565，它实际上在内存中的值类似于 18.56500000000000000000000000000000001，因此它大于中点。
Double is a floating point value, so maybe if you write it as 18.565, it is actually in memory something like 18.56500000000000000000000000000000001, and hence it is more than the midpoint.