Pandas 數據框獲取每組的第一行並復制到其他行

Question

我有一個如下所示的 Pandas DataFrame。

df = pd.DataFrame({'id' : [1,1,1,2,2,3,3,3,3,4,4,5,6,6,6,7,7],
                'value'  : ["first","second","second","first",
                            "second","first","third","fourth",
                            "fifth","second","fifth","first",
                            "first","second","third","fourth","fifth"]})

我想通過 ["id","value"] 對其進行分組，獲取每個組的第一行，並使用其值覆蓋該組剩余行中的值（因此結果表的維度相同作為源表）

        id   value
0        1   first
1        1  second
2        1  second
3        2  second
4        2   first
5        3   first
6        3   third
7        3  fourth
8        3   fifth
9        4  second
10       4   fifth
11       5   first
12       6   first
13       6  second
14       6   third
15       7  fourth
16       7   fifth

預期結果

        id   value
0        1   first
1        1   first
2        1   first
3        2  second
4        2  second
5        3   first
6        3   first
7        3   first
8        3   first
9        4  second
10       4  second
11       5   first
12       6   first
13       6   first
14       6   first
15       7  fourth
16       7  fourth

我嘗試了很多方法，但無濟於事......有什么想法嗎？

Answer 1

將GroupBy.transform與GroupBy.first GroupBy.transform使用：

df['value'] = df.groupby('id')['value'].transform('first')