具有组内值的新列(New column with a value inside a group)

       A           B    C
0    Red  2002-01-13  3.9
1    Red  2002-01-13  4.1
2    Red  2002-01-13  2.3
3    Red  2002-01-14  0.7
4    Red  2002-01-14  5.9
5    Red  2002-01-14  3.0
6    Red  2002-01-15  6.9
7    Red  2002-01-15  6.4
8    Red  2002-01-15  9.9
9   Blue  2006-07-21  7.2
10  Blue  2006-07-21  4.2
11  Blue  2006-07-21  6.1
12  Blue  2006-07-22  0.1
13  Blue  2006-07-22  3.2
14  Blue  2006-07-22  2.7
 
 我想用下一个条件创建一个df['D']列： 
 
 
 
 取每个A组的第一个B日期的最后一个C值。 
 
 
 我的意思是，对于A组Red ，取2.3，因为它是第一个B date 2002-01-13的最后一个C值。 对于A组Blue ，取6.1，因为它是2006-07-21第一个B日期的最后一个C值。 
 
 输出应该是： 
 
       A           B    C    D
0    Red  2002-01-13  3.9  2.3
1    Red  2002-01-13  4.1  2.3
2    Red  2002-01-13  2.3  2.3
3    Red  2002-01-14  0.7  2.3
4    Red  2002-01-14  5.9  2.3
5    Red  2002-01-14  3.0  2.3
6    Red  2002-01-15  6.9  2.3
7    Red  2002-01-15  6.4  2.3
8    Red  2002-01-15  9.9  2.3
9   Blue  2006-07-21  7.2  6.1
10  Blue  2006-07-21  4.2  6.1
11  Blue  2006-07-21  6.1  6.1
12  Blue  2006-07-22  0.1  6.1
13  Blue  2006-07-22  3.2  6.1
14  Blue  2006-07-22  2.7  6.1
 
 我试过了： 
 
df['D'] = df.groupby('A')["C"].transform('last')

       A           B    C
0    Red  2002-01-13  3.9
1    Red  2002-01-13  4.1
2    Red  2002-01-13  2.3
3    Red  2002-01-14  0.7
4    Red  2002-01-14  5.9
5    Red  2002-01-14  3.0
6    Red  2002-01-15  6.9
7    Red  2002-01-15  6.4
8    Red  2002-01-15  9.9
9   Blue  2006-07-21  7.2
10  Blue  2006-07-21  4.2
11  Blue  2006-07-21  6.1
12  Blue  2006-07-22  0.1
13  Blue  2006-07-22  3.2
14  Blue  2006-07-22  2.7
 
I want to create a df['D'] column, with next condition:
 
 
 
Take the last C value of the first B date, by each A group.
 
 
I mean, for A group Red, take 2.3, as it is the last C value of first B date 2002-01-13. For A group Blue, take 6.1, as it is the last C value of first B date 2006-07-21.
 
Output should be:
 
       A           B    C    D
0    Red  2002-01-13  3.9  2.3
1    Red  2002-01-13  4.1  2.3
2    Red  2002-01-13  2.3  2.3
3    Red  2002-01-14  0.7  2.3
4    Red  2002-01-14  5.9  2.3
5    Red  2002-01-14  3.0  2.3
6    Red  2002-01-15  6.9  2.3
7    Red  2002-01-15  6.4  2.3
8    Red  2002-01-15  9.9  2.3
9   Blue  2006-07-21  7.2  6.1
10  Blue  2006-07-21  4.2  6.1
11  Blue  2006-07-21  6.1  6.1
12  Blue  2006-07-22  0.1  6.1
13  Blue  2006-07-22  3.2  6.1
14  Blue  2006-07-22  2.7  6.1
 
I've tried:
 
df['D'] = df.groupby('A')["C"].transform('last')

原文：https://stackoverflow.com/questions/51450485