C＃银行家的舍入误差 [英] C# banker's rounding error
问题描述
double a = 18.565
return Math.Round(a,2)
..返回18.57。结果
。对于我试图银行家舍和预期一样每隔数，例如Math.Round（2.565,2）返回2.56。
..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.
任何线索为何以及何时发生这种情况？
它是错误还是我失去了一些关于银行家的舍入？
Any clue why and when that happens? Is it error or am I missing something about banker's rounding?
谢谢..
推荐答案
由于马修说，18.565无法准确表示。使用的实际值是18.565000000000001278976924368180334568023681640625（使用 Math.Round 将考虑一个值，该值的真正的超越半路点，但作为可以精确地表示，因为恰好是在哪些是尽量靠近中途点的这一点。但是，我还没有看到，描述在其中的应用的情况下的任何文件，并明确它不是在这种情况下发生的。我不想依赖它。
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.Round will 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.
即使舍入值是不完全的，当然18.57。它实际上18.57000000000000028421709430404007434844970703125。
Even the rounded value isn't exactly 18.57 of course. It's actually 18.57000000000000028421709430404007434844970703125.
从根本上说，如果你真的，真的很在乎准确的代表，你应该使用十进制值的十进制。这不只是在 Math.Round 术语 - 它去处理浮点值的每一个环节。
Fundamentally, if you really, really care about representing decimal values accurately, you should be using decimal. That's not just in terms of Math.Round - it goes to every aspect of handling floating point values.
这确实的给予 Math.Round 当然，正确的价值，：
That does give the right value for Math.Round, of course:
decimal m = 18.565m;
Console.WriteLine(Math.Round(m, 2)); // Prints 18.56
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