识别 pandas 组中的第一个非零元素 [英] Identify first non-zero element within a group in pandas

问题描述

我有一个如下所示的数据框.最右边的列是我想要的列:

I have a dataframe that looks like the following. The rightmost column is my desired column:

Group   Value   Target_Column   
1        0         0     
1        0         0  
1        1         1
1        2         0
2        0         0
2        1         1
2        0         0
2        1         0

如何识别组(Group)中的第一个非零值，然后创建一个保留第一个非零值并显示所有其他零的列?

How do I identify the first non-zero value in a group(Group) and then create a column that retains the first non-zero value and show all else as zeroes?

如解决方案中所述，我一直在尝试利用idxmax进行此操作: 在的每个列中查找第一个非零值熊猫DataFrame

I have been trying to leverage idxmax for this as stated in this solution: Find first non-zero value in each column of pandas DataFrame

import pandas as pd
df = pd.DataFrame({'Group': [1,1,1,1,2,2,2,2], 'Value': [0,0,1,1,0,1,0,1]})
df.ne(0).idxmax()
g = df.groupby('Group').Value
g.ne(0).idxmax()

推荐答案

使用idxmax

df['Newcol']=0
df.loc[df.Value.ne(0).groupby(df['Group']).idxmax(),'Newcol']=1
df
Out[41]: 
   Group  Value  Target_Column  Newcol
0      1      0              0       0
1      1      0              0       0
2      1      1              1       1
3      1      2              0       0
4      2      0              0       0
5      2      1              1       1
6      2      0              0       0
7      2      1              0       0

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