Python Pandas groupby：获取一个组的大小，知道它的 id（来自 .grouper.group_info[0]）
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Pandas groupby: get size of a group knowing its id (from .grouper.group_info[0])
提问by piokuc
In the following snippet datais a pandas.DataFrameand indicesis a set of columns of the data. After grouping the data with groupbyI am interested in the ids of the groups, but only those with a size greater than a threshold (say: 3).
在下面的代码段中data是一个pandas.DataFrame和indices是data. 将数据分组后，groupby我对组的 id 感兴趣，但只对大小大于阈值的那些感兴趣（比如：3）。
group_ids=data.groupby(list(data.columns[list(indices)])).grouper.group_info[0]
Now, how can I find which group has a size greater than or equal 3 knowing the id of the group? I only want ids of groups with a certain size.
现在，我怎样才能知道哪个组的大小大于或等于 3 知道组的 id？我只想要具有特定大小的组的 ID。
#TODO: filter out ids from group_ids which correspond to groups with sizes < 3
采纳答案by Andy Hayden
One way is to use the sizemethod of the groupby:
g = data.groupby(...)
size = g.size()
size[size > 3]
For example, here there is only one group of size > 1:
例如，这里只有一组大小 > 1：
In [11]: df = pd.DataFrame([[1, 2], [3, 4], [1,6]], columns=['A', 'B'])
In [12]: df
Out[12]:
A B
0 1 2
1 3 4
2 1 6
In [13]: g = df.groupby('A')
In [14]: size = g.size()
In [15]: size[size > 1]
Out[15]:
A
1 2
dtype: int64
If you were interested in just restricting the DataFrame to those in large groups you could use the filtermethod:
如果您只想将 DataFrame 限制为大型组，则可以使用filter方法：
In [21]: g.filter(lambda x: len(x) > 1)
Out[21]:
A B
0 1 2
2 1 6
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