如何在 Pandas 的组内使用 cumsum？
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How can I use cumsum within a group in Pandas?
提问by Baron Yugovich
I have
我有
df = pd.DataFrame.from_dict({'id': ['A', 'B', 'A', 'C', 'D', 'B', 'C'], 'val': [1,2,-3,1,5,6,-2], 'stuff':['12','23232','13','1234','3235','3236','732323']})
id stuff val
0 A 12 1
1 B 23232 2
2 A 13 -3
3 C 1234 1
4 D 3235 5
5 B 3236 6
6 C 732323 -2
I'd like to get running some of valfor each id, so the desired output looks like this:
我想val为 each运行一些id，因此所需的输出如下所示：
id stuff val cumsum
0 A 12 1 1
1 B 23232 2 2
2 A 13 -3 -2
3 C 1234 1 1
4 D 3235 5 5
5 B 3236 6 8
6 C 732323 -2 -1
This is what I tried:
这是我尝试过的：
df['cumsum'] = df.groupby('id').cumsum(['val'])
and
和
df['cumsum'] = df.groupby('id').cumsum(['val'])
This is the error I got:
这是我得到的错误：
ValueError: Wrong number of items passed 0, placement implies 1
回答by EdChum
You can call transformand pass the cumsumfunction to add that column to your df:
您可以调用transform并传递cumsum函数将该列添加到您的 df：
In [156]:
df['cumsum'] = df.groupby('id')['val'].transform(pd.Series.cumsum)
df
Out[156]:
id stuff val cumsum
0 A 12 1 1
1 B 23232 2 2
2 A 13 -3 -2
3 C 1234 1 1
4 D 3235 5 5
5 B 3236 6 8
6 C 732323 -2 -1
With respect to your error, you can't call cumsumon a Series groupby object, secondly you're passing the name of the column as a list which is meaningless.
关于您的错误，您不能调用cumsumSeries groupby 对象，其次您将列的名称作为毫无意义的列表传递。
So this works:
所以这有效：
In [159]:
df.groupby('id')['val'].cumsum()
Out[159]:
0 1
1 2
2 -2
3 1
4 5
5 8
6 -1
dtype: int64
