Python 3 奇怪的除法 [英] Python 3 strange division

问题描述

大约半小时思考我做错了什么！?"在 5 行代码上.. 因为 Python3 以某种方式四舍五入大整数.任何人都知道为什么会出现这样的问题:

Python2:

int(6366805760909027985741435139224001 # 这是 7**40./7) == 909543680129861140820205019889143 # 7**39

Python3:

int(6366805760909027985741435139224001/7) == 909543680129861204865300750663680 # 我不知道这是什么.

解决方案

Python 3 不是舍入大整数".它的作用是在除法后返回一个浮点数.因此，在 Python 2 中:

<预><代码>>>>4/22

在 Python 3 中:

<预><代码>>>>4/22.0

原因很简单.在 Python 2 中，/ 在使用整数时进行整数除法会产生一些令人惊讶的结果:

<预><代码>>>>5/22

哎呀.在 Python 3 中这是固定的:

<预><代码>>>>5/22.5

这意味着在 Python 3 中，你的除法返回一个浮点数:

<预><代码>>>>6366805760909027985741435139224001/79.095436801298612e+32

此浮点数的准确度低于您需要的数字.然后，您使用 int() 将其转换为整数，您会得到一个意想不到的数字.

您应该改为使用整数除法(在 Python 2 和 Python 3 中):

<预><代码>>>>6366805760909027985741435139224001//7909543680129861140820205019889143L

(尾随 L 表示它是一个长整数，在 Python 3 中长整数和普通整数合并了，所以没有尾随 L).

About half an hour thinking "what am i doing wrong!?" on the 5-lines code.. because Python3 is somehow rounding big integers. Anyone know why there is a problem such:

Python2:

int(6366805760909027985741435139224001        # This is 7**40.
    / 7) == 909543680129861140820205019889143 # 7**39

Python3:

int(6366805760909027985741435139224001 
    / 7) == 909543680129861204865300750663680 # I have no idea what this is.

解决方案

Python 3 is not "rounding big integers". What it does is that it will return a float after division. Hence, in Python 2:

>>> 4/2
2

while in Python 3:

>>> 4/2
2.0

The reason for this is simple. In Python 2, / being integer division when you use integers have some surprising results:

>>> 5/2
2

Ooops. In Python 3 this is fixed:

>>> 5/2
2.5

This means that in Python 3, your division returns a float:

>>> 6366805760909027985741435139224001/7
9.095436801298612e+32

This float has less accuracy than the digits you need. You then convert this to an integer with int(), and you get a number you don't expect.

You should instead use integer division (in both Python 2 and Python 3):

>>> 6366805760909027985741435139224001//7
909543680129861140820205019889143L

(The trailing L means it's a long integer, in Python 3 the long and the normal integer is merged, so there is no trailing L).

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