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Python 3.x舍入行为

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问题：Python 3.x舍入行为
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问题：Python 3.x舍入行为

我只是在重新阅读Python 3.0的新增功能，它指出：

round（）函数的舍入策略和返回类型已更改。现在，精确的中途案例将舍入到最接近的偶数结果，而不是从零舍入。（例如，round（2.5）现在返回2而不是3。）

以及关于round的文档：

对于支持round（）的内置类型，将值四舍五入为乘幂n的最接近10的倍数；如果两个倍数相等接近，则四舍五入取整为偶数选择

因此，在v2.7.3下：

In [85]: round(2.5)
Out[85]: 3.0

In [86]: round(3.5)
Out[86]: 4.0

如我所料 但是，现在在v3.2.3下：

In [32]: round(2.5)
Out[32]: 2

In [33]: round(3.5)
Out[33]: 4

这似乎是违反直觉的，与我对四舍五入（并可能绊倒人）的理解相反。英语不是我的母语，但是直到我读了这篇文章，我才认为我知道四舍五入的含义：-/我确定在引入v3时一定对此进行了一些讨论，但我找不到很好的理由。我的搜索。

  1. 有人知道为什么将其更改为此吗？
  2. 是否有其他主流编程语言（例如C，C ++，Java，Perl等）进行这种（对我来说是不一致的）舍入？

我在这里想念什么？

更新：@ Li-aungYip的评论有关“银行取整”为我提供了正确的搜索词/关键字进行搜索，我发现了这样一个问题：为什么.NET默认使用银行取整？，所以我会仔细阅读。

点击查看英文原文

I was just re-reading What’s New In Python 3.0 and it states:

The round() function rounding strategy and return type have changed. Exact halfway cases are now rounded to the nearest even result instead of away from zero. (For example, round(2.5) now returns 2 rather than 3.)

and the documentation for round:

For the built-in types supporting round(), values are rounded to the closest multiple of 10 to the power minus n; if two multiples are equally close, rounding is done toward the even choice

So, under v2.7.3:

In [85]: round(2.5)
Out[85]: 3.0

In [86]: round(3.5)
Out[86]: 4.0

as I’d have expected. However, now under v3.2.3:

In [32]: round(2.5)
Out[32]: 2

In [33]: round(3.5)
Out[33]: 4

This seems counter-intuitive and contrary to what I understand about rounding (and bound to trip up people). English isn’t my native language but until I read this I thought I knew what rounding meant :-/ I am sure at the time v3 was introduced there must have been some discussion of this, but I was unable to find a good reason in my search.

  1. Does anyone have insight into why this was changed to this?
  2. Are there any other mainstream programming languages (e.g., C, C++, Java, Perl, ..) that do this sort of (to me inconsistent) rounding?

What am I missing here?

UPDATE: @Li-aungYip’s comment re “Banker’s rounding” gave me the right search term/keywords to search for and I found this SO question: Why does .NET use banker’s rounding as default?, so I will be reading that carefully.

回答 0

如今，Python 3.0的方法被认为是标准的舍入方法，尽管某些语言实现尚未上市。

简单的“总是向上取整0.5”技术会导致对较高数字的轻微偏向。通过大量计算，这可能很重要。Python 3.0方法消除了这个问题。

常用的舍入方法不止一种。浮点数学的国际标准IEEE 754定义了五种不同的舍入方法（Python 3.0使用的是默认的一种）。还有其他。

这种行为并未得到应有的广泛了解。如果我没记错的话，AppleScript是这种舍入方法的早期采用者。roundAppleScript中的命令实际上确实提供了几个选项，但是IEEE 754中的默认设置是roundtoward-even round。他实现的“学校”：round 2.5 rounding as taught in school是有效的AppleScript命令。:-)

点击查看英文原文

Python 3’s way (called “round half to even” or “banker’s rounding”) is considered the standard rounding method these days, though some language implementations aren’t on the bus yet.

The simple “always round 0.5 up” technique results in a slight bias toward the higher number. With large numbers of calculations, this can be significant. The Python 3.0 approach eliminates this issue.

There is more than one method of rounding in common use. IEEE 754, the international standard for floating-point math, defines five different rounding methods (the one used by Python 3.0 is the default). And there are others.

This behavior is not as widely known as it ought to be. AppleScript was, if I remember correctly, an early adopter of this rounding method. The round command in AppleScript offers several options, but round-toward-even is the default as it is in IEEE 754. Apparently the engineer who implemented the round command got so fed up with all the requests to “make it work like I learned in school” that he implemented just that: round 2.5 rounding as taught in school is a valid AppleScript command. :-)