整数除法与地板商相比:为什么这个令人惊讶的结果? [英] Integer division compared to floored quotient: why this surprising result?
问题描述
//
Python 的整数除法"运算符让我惊讶,今天:
文档 读取x 和y".那么,为什么 math.floor(11/1.1) 等于 10,而 11//1.1 等于 9?
因为 1.1 不能完全以二进制形式表示;近似值略高于 1.1 - 因此除法结果有点太小.
尝试以下操作:
在 Python 2 下,在控制台输入:
<预><代码>>>>1.11.1000000000000001在 Python 3.1 中,控制台将显示 1.1
,但在内部,它仍然是相同的数字.
但是:
<预><代码>>>>11/1.110.0正如 gnibbler 指出的那样,这是在浮点数的可用精度限制内内部舍入"的结果.正如 The MYYN 在他的评论中指出的那样,//
使用与 math.floor()
不同的算法来计算楼层划分结果,以保留 a== (a//b)*b + a%b
尽可能好.
如果需要这种精度,请使用 Decimal
类型.
The //
"integer division" operator of Python surprised me, today:
>>> math.floor(11/1.1)
10.0
>>> 11//1.1
9.0
The documentation reads "(floored) quotient of x and y". So, why is math.floor(11/1.1) equal to 10, but 11//1.1 equal to 9?
Because 1.1 can't be represented in binary form exactly; the approximation is a littler higher than 1.1 - therefore the division result is a bit too small.
Try the following:
Under Python 2, type at the console:
>>> 1.1
1.1000000000000001
In Python 3.1, the console will display 1.1
, but internally, it's still the same number.
But:
>>> 11/1.1
10.0
As gnibbler points out, this is the result of "internal rounding" within the available precision limits of floats. And as The MYYN points out in his comment, //
uses a different algorithm to calculate the floor division result than math.floor()
in order to preserve a == (a//b)*b + a%b
as well as possible.
Use the Decimal
type if you need this precision.
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