使用groupby转换 pandas [英] Inplace transformation pandas with groupby
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问题描述
是否可以使用groupby
语句对DataFrame进行就地突变?
Would it be possible to mutate DataFrame inplace with groupby
statement?
import pandas as pd
dt = pd.DataFrame({
"LETTER": ["a", "b", "c", "a", "b"],
"VALUE" : [10 , 12 , 13, 0, 15]
})
def __add_new_col(dt_):
dt_['NEW_COL'] = dt_['VALUE'] - dt_['VALUE'].mean()
return dt_
pass
dt.groupby("LETTER").apply(__add_new_col)
LETTER VALUE NEW_COL
0 a 10 5.0
1 b 12 -1.5
2 c 13 0.0
3 a 0 -5.0
4 b 15 1.5
dt
LETTER VALUE
0 a 10
1 b 12
2 c 13
3 a 0
4 b 15
在R data.table中,可以使用:=
运算符,例如dt[, col := ... , by ='LETTER']
In R data.table it is possible by using :=
operator e.g. dt[, col := ... , by ='LETTER']
推荐答案
I think you can use transform
which return Series
same length and same index as df
with substracting:
print (dt.groupby("LETTER")['VALUE'].transform('mean'))
0 5.0
1 13.5
2 13.0
3 5.0
4 13.5
Name: VALUE, dtype: float64
dt['NEW_COL'] = dt['VALUE'] - dt.groupby("LETTER")['VALUE'].transform('mean')
print (dt)
LETTER VALUE NEW_COL
0 a 10 5.0
1 b 12 -1.5
2 c 13 0.0
3 a 0 -5.0
4 b 15 1.5
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