左对齐 pandas 滚动对象 [英] Left-align a pandas rolling object

问题描述

使用大熊猫0.18.1，我想获取一栏数据框的滚动平均值.从0.18.0版开始，这是通过rolling()对象完成的.这些滚动对象的默认设置是右对齐的.您可以传递一个布尔参数，center = True，以将滚动对象与中心值对齐，但是似乎没有办法将其左对齐.这是一个示例:

Using pandas 0.18.1, I'd like to take the rolling average of a one-column dataframe. Since version 0.18.0, this is done with rolling() objects. The default for these rolling objects is to be right-justified. There is a boolean argument you can pass, center=True, to align the rolling object to the center value, but there doesn't seem to be a way to left-align it. Here's an example:

df = pandas.DataFrame({'A': [2,3,6,8,20, 27]})
df
    A
0   2
1   3
2   6
3   8
4  20
5  27

标准方法会自动向右对齐，因此前两个变量的窗口大小为三时没有任何值:

The standard method automatically aligns to the right, so there's no value at the first two indecies with a window of size three:

 df.rolling(window=3).mean()
           A
0        NaN
1        NaN
2   3.666667
3   5.666667
4  11.333333
5  18.333333

我们可以像这样对操作进行居中对齐:

We can center-align the operation like this:

df.rolling(window=3).mean(center=True)
           A
0        NaN
1   3.666667
2   5.666667
3  11.333333
4  18.333333
5        NaN

但是我要找的是这个

df.rolling(3).mean()
            A
 0   3.666667
 1   5.666667
 2  11.333333
 3  18.333333
 4        NaN
 5        NaN

我可以通过以下方法完成此任务:使用默认的右对齐方式，然后对其重新编制索引，或者通过反转行的顺序，然后将其右对齐"，但是这些都是应解决的方法简单直接的操作.

I can accomplish this by doing it with the default right alignment, and then re-indexing it, or by reversing the order of the rows and then doing it "right-aligned" but these are work-arounds for what should be a straight-forward operation.

推荐答案

我认为您可以使用

I think you can use shift:

a = df.rolling(window=3).mean().shift(-2)
print (a)
           A
0   3.666667
1   5.666667
2  11.333333
3  18.333333
4        NaN
5        NaN

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