Round Function 在 v2.6.6 中不起作用，但在 v3.4.2 中有效 [英] Round Function does not work in v2.6.6, but works in v3.4.2

问题描述

我的 round 函数在 linux python 2.6.6 中不起作用，而在 Windows 3.4.2 中运行良好使用以下类型的代码后:

Array[i] = round(math.e ** AnotherArray[i], 4)v.3.4.2:0.0025999999999999999 =>0.0026v.2.6.6:0.0025999999999999999 =>0.0025999999999999999

解决方案

它们的工作原理相同，但 Python 2.7 及更高版本在打印它们的 representations 时会舍入浮点数，为了不让用户被(独立于语言和机器的)浮点运算的限制.

十进制数0.0026不能完全表示为二进制float，所以总会有一些舍入误差.

如果你想减少混淆，只需打印数字:

<预><代码>>>>a = 0.0025999999999999999>>>b = 轮 (a,5)>>>b # 这会调用 repr(b)0.0025999999999999999>>>print b # 这会调用 str(b)0.0026

实际上，这些舍入误差很少有影响，尽管您需要注意它们，尤其是在比较相等性时.

以下循环不会在 0 处停止:

x = 1.0而 x != 0:打印 xx -= 0.1

为什么?一起来看看:

<预><代码>>>>x = 1.0>>>而 x != 0:... 打印 repr(x)... x -= 0.1...如果 x<0:中断...1.00.900000000000000020.800000000000000040.700000000000000070.600000000000000090.500000000000000110.400000000000000130.300000000000000160.200000000000000150.100000000000000141.3877787807814457e-16

因此，请始终将sys.float_info.epsilon 考虑在内:

<预><代码>>>>x = 1.0>>>而 abs(x) >sys.float_info.epsilon:... 打印 x... x -= 0.1...1.00.90.80.70.60.50.40.30.20.1

My round function does not work in linux python 2.6.6, whereas it works fine in Windows 3.4.2 after using the following type of code:

Array[i] = round(math.e ** AnotherArray[i], 4)

v.3.4.2:  0.0025999999999999999 => 0.0026
v.2.6.6:  0.0025999999999999999 =>  0.0025999999999999999

解决方案

They both work the same, but Python 2.7 and up will round floating point numbers when printing their representations, in order to not confuse users by the (language- and machine-independent) limitations of floating point arithmetic.

The decimal number 0.0026 can't be represented exactly as a binary float, so there will always be some rounding error.

If you want less confusion, just print the numbers:

>>> a = 0.0025999999999999999
>>> b = round(a,5)
>>> b                  # this calls repr(b) 
0.0025999999999999999
>>> print b            # this calls str(b)
0.0026

In practice, those rounding errors rarely matter, although you need to be aware of them, especially when comparing for equality.

The following loop doesn't stop at 0:

x = 1.0
while x != 0:
    print x
    x -= 0.1

Why? Let's take a look:

>>> x = 1.0
>>> while x != 0:
...     print repr(x)
...     x -= 0.1
...     if x<0: break
...
1.0
0.90000000000000002
0.80000000000000004
0.70000000000000007
0.60000000000000009
0.50000000000000011
0.40000000000000013
0.30000000000000016
0.20000000000000015
0.10000000000000014
1.3877787807814457e-16

Therefore, always take sys.float_info.epsilon into account:

>>> x = 1.0
>>> while abs(x) > sys.float_info.epsilon:
...     print x
...     x -= 0.1
...
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1

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