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I have a DataFrame. To do a statistical conditional test, I split it into two based on a boolean column ('mar'). I want to use the ratio of counts between the two tables to add a column expressing the proportion of true values in the 'mar' column for each combination of the other columns, as seen below.

>>> df_nomar
   alc  cig  mar  cnt
1    1    1    0  538
3    1    0    0  456
5    0    1    0   43
7    0    0    0  279

>>> df_mar
   alc  cig  mar  cnt
0    1    1    1  911
2    1    0    1   44
4    0    1    1    3
6    0    0    1    2
>>> df_mar.loc[:, 'prop'] = np.array(df_mar['cnt'])/(np.array(df_mar['cnt']) + np.array(df_nomar['cnt']))
/usr/local/lib/python3.5/dist-packages/pandas/core/indexing.py:296: SettingWithCopyWarning: 
A value is trying to be set on a copy of a slice from a DataFrame.
Try using .loc[row_indexer,col_indexer] = value instead

See the caveats in the documentation: http://pandas.pydata.org/pandas-docs/stable/indexing.html#indexing-view-versus-copy
  self.obj[key] = _infer_fill_value(value)
/usr/local/lib/python3.5/dist-packages/pandas/core/indexing.py:476: SettingWithCopyWarning: 
A value is trying to be set on a copy of a slice from a DataFrame.
Try using .loc[row_indexer,col_indexer] = value instead

See the caveats in the documentation: http://pandas.pydata.org/pandas-docs/stable/indexing.html#indexing-view-versus-copy
  self.obj[item] = s

>>> df_mar
   alc  cig  mar  cnt      prop
0    1    1    1  911  0.628709
2    1    0    1   44  0.088000
4    0    1    1    3  0.065217
6    0    0    1    2  0.007117

I've gone to the suggested page to investigate the warning. When I assign the new column, I am using the form df_mar.loc[:, 'prop'] = ..., just as suggested.

So why am I still getting this warning?

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1 回答 1

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如果对齐数据的两个DataFrames的大小相同，您似乎需要：reset_index

a = df_mar['cnt'].reset_index(drop=True)
b = df_nomar['cnt'].reset_index(drop=True)
df_mar['prop'] = (a/(a + b)).values

另一种解决方案是转换为numpy arrayby values：

a = df_mar['cnt'].values
b = df_nomar['cnt'].values
df_mar['prop'] = a / (a + b)

print (df_mar)
   alc  cig  mar  cnt      prop
0    1    1    1  911  0.628709
2    1    0    1   44  0.088000
4    0    1    1    3  0.065217
6    0    0    1    2  0.007117

这个熊猫警告来自哪里

它显然来自上面的代码。如果 filter DataFrames 则需要copy：

df_nomar = df[df['mar'] == 0].copy()
df_mar = df[df['mar'] == 1].copy()

如果您df稍后修改值，您会发现修改不会传播回原始数据 ( df_nomarand df_mar)，并且 Pandas 会发出警告。

于 2017-12-12T15:51:24.437 回答