python - 按日期查找第一次出现的值

Question

我的数据集包含两列:名称和日期。这些名字在不同的日期重复。我正在尝试获取唯一名称的列表以及该名称第一次出现的日期。

这是数据示例:

    Name    Date
0   Smith   2021-01-02
1   Johnson 2021-01-03
2   Williams    2021-01-04
3   Brown   2021-01-05
4   Jones   2021-01-06
5   Garcia  2021-01-07
6   Miller  2021-01-08
7   Davis   2021-01-09
8   Rodriguez   2020-01-10
9   Martinez    2018-01-11
10  Smith   2021-01-12
11  Johnson 2017-01-13
12  Williams    2016-01-24
13  Brown   2011-12-15
14  Jones   2009-01-16
15  Garcia  2008-11-17
16  Miller  2021-02-18
17  Davis   2004-03-09
18  Rodriguez   2001-01-20
19  Martinez    2002-01-21
20  Smith   2010-05-22
21  Johnson 2011-12-23
22  Williams    2006-01-04
23  Brown   2006-01-25
24  Jones   2014-01-04
25  Garcia  2013-01-13
26  Miller  2021-01-28
27  Davis   2021-01-29
28  Rodriguez   2021-01-30
29  Martinez    2021-01-31

我尝试使用 pandas groupby 函数，但没有得到正确的结果:

    #Sample Data
import pandas as pd

df = pd.read_excel("SampleData2.xlsx")
df["first_test"] = df.groupby("Name")["Date"].transform("first")
df

我得到这样的导出:

    Name    Date    first_test
0   Smith   2021-01-02  2021-01-02
1   Johnson 2021-01-03  2021-01-03
2   Williams    2021-01-04  2021-01-04
3   Brown   2021-01-05  2021-01-05
4   Jones   2021-01-06  2021-01-06
5   Garcia  2021-01-07  2021-01-07
6   Miller  2021-01-08  2021-01-08
7   Davis   2021-01-09  2021-01-09
8   Rodriguez   2020-01-10  2020-01-10
9   Martinez    2018-01-11  2018-01-11
10  Smith   2021-01-12  2021-01-02
11  Johnson 2017-01-13  2021-01-03
12  Williams    2016-01-24  2021-01-04
13  Brown   2011-12-15  2021-01-05
14  Jones   2009-01-16  2021-01-06
15  Garcia  2008-11-17  2021-01-07
16  Miller  2021-02-18  2021-01-08
17  Davis   2004-03-09  2021-01-09
18  Rodriguez   2001-01-20  2020-01-10
19  Martinez    2002-01-21  2018-01-11
20  Smith   2010-05-22  2021-01-02
21  Johnson 2011-12-23  2021-01-03
22  Williams    2006-01-04  2021-01-04
23  Brown   2006-01-25  2021-01-05
24  Jones   2014-01-04  2021-01-06
25  Garcia  2013-01-13  2021-01-07
26  Miller  2021-01-28  2021-01-08
27  Davis   2021-01-29  2021-01-09
28  Rodriguez   2021-01-30  2020-01-10
29  Martinez    2021-01-31  2018-01-11

但是，我想要得到的是:

Name    FirstDate
Smith   5/22/2010
Johnson 12/23/2011
Williams    1/4/2006
Brown   1/25/2006
Jones   1/16/2009
Garcia  11/17/2008
Miller  1/8/2021
Davis   3/9/2004
Rodriguez   1/20/2001
Martinez    1/21/2002

知道如何做到这一点吗？

Answer 1

按名称分组，然后取最小日期。像这样的事情:

df.groupby('Name').agg({'Date': 'min'})

根据您提供的df，我发现:

                 Date
Name                 
Brown      2006-01-25
Davis      2004-03-09
Garcia     2008-11-17
Johnson    2011-12-23
Jones      2009-01-16
Martinez   2002-01-21
Miller     2021-01-08
Rodriguez  2001-01-20
Smith      2010-05-22
Williams   2006-01-04