pandas :使用diff和groupby [英] pandas: use diff with groupby

问题描述

我正在使用 pandas (版本0.20. 3)并且我想将diff方法与groupby一起应用，但要使用Databy而不是DataFrame，结果是下划线".

I am using pandas (version 0.20.3) and I want to apply the diff method with groupby but instead of a DataFrame, the result is an "underscore".

这是代码:

import numpy as np
import pandas as pd

# creating the DataFrame
data = np.random.random(18).reshape(6,3)
indexes = ['B']*3 + ['A']*3
columns = ['x', 'y', 'z']
df = pd.DataFrame(data, index=indexes, columns=columns)
df.index.name = 'chain_id'

# Now I want to apply the diff method in function of the chain_id
df.groupby('chain_id').diff()

结果是一个下划线！ 请注意，df.loc ['A'].diff()和df.loc ['B'].diff()返回预期结果，所以我不明白为什么它不适用于groupby.

And the result is an underscore! Note that df.loc['A'].diff() and df.loc['B'].diff() return the expected results so I don't understand why it wouldn't work with groupby.

推荐答案

IIUC，您的错误:无法从重复的轴重新索引

df.reset_index().groupby('chain_id').diff().set_index(df.index)
Out[859]: 
                 x         y         z
chain_id                              
B              NaN       NaN       NaN
B        -0.468771  0.192558 -0.443570
B         0.323697  0.288441  0.441060
A              NaN       NaN       NaN
A        -0.198785  0.056766  0.081513
A         0.138780  0.563841  0.635097

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