Python 如何为 a size() 列指定名称？
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How to assign a name to the a size() column?
提问by d1337
I am using .size() on a groupby result in order to count how many items are in each group.
我在 groupby 结果上使用 .size() 以计算每个组中有多少项目。
I would like the result to be saved to a new column name without manually editing the column names array, how can it be done?
我想在不手动编辑列名数组的情况下将结果保存到新的列名，怎么办？
Thanks
谢谢
This is what I have tried:
这是我尝试过的：
grpd = df.groupby(['A','B'])
grpd['size'] = grpd.size()
grpd
and the error I got:
和我得到的错误：
TypeError: 'DataFrameGroupBy' object does not support item assignment (on the second line)
TypeError: 'DataFrameGroupBy' 对象不支持项目分配（在第二行）
采纳答案by Dan Allan
The result of df.groupby(...)is not a DataFrame. To get a DataFrame back, you have to apply a function to each group, transform each element of a group, or filter the groups.
的结果df.groupby(...)不是 DataFrame。要取回 DataFrame，您必须对每个组应用一个函数、转换组的每个元素或过滤组。
It seems like you want a DataFrame that contains (1) all your original data in dfand (2) the count of how much data is in each group. These things have different lengths, so if they need to go into the same DataFrame, you'll need to list the size redundantly, i.e., for each row in each group.
似乎您想要一个包含 (1) 所有原始数据df和 (2) 每组中有多少数据的数据帧。这些东西有不同的长度，所以如果它们需要进入同一个DataFrame，你需要冗余地列出大小，即每组中的每一行。
df['size'] = df.groupby(['A','B']).transform(np.size)
(Aside: It's helpful if you can show succinct sample input and expected results.)
（旁白：如果您能显示简洁的样本输入和预期结果，这会很有帮助。）
回答by jezrael
You need transformsize- lenof dfis same as before:
你需要- of和以前一样：transformsizelendf
Notice:
注意：
Here it is necessary to add one column after groupby, else you get an error. Because GroupBy.sizecount NaNs too, what column is used is not important. All columns working same.
这里有必要在 之后添加一列groupby，否则会出错。因为GroupBy.sizecountNaN也一样，使用什么列并不重要。所有列的工作方式相同。
import pandas as pd
df = pd.DataFrame({'A': ['x', 'x', 'x','y','y']
, 'B': ['a', 'c', 'c','b','b']})
print (df)
A B
0 x a
1 x c
2 x c
3 y b
4 y b
df['size'] = df.groupby(['A', 'B'])['A'].transform('size')
print (df)
A B size
0 x a 1
1 x c 2
2 x c 2
3 y b 2
4 y b 2
If need set column name in aggregating df- lenof dfis obviously NOTsame as before:
如果需要在聚集集列名df-len的df显然不是像以前一样：
import pandas as pd
df = pd.DataFrame({'A': ['x', 'x', 'x','y','y']
, 'B': ['a', 'c', 'c','b','b']})
print (df)
A B
0 x a
1 x c
2 x c
3 y b
4 y b
df = df.groupby(['A', 'B']).size().reset_index(name='Size')
print (df)
A B Size
0 x a 1
1 x c 2
2 y b 2
回答by Sealander
The .size()built-in method of DataFrameGroupBy objects actually returns a Series object with the group sizes and not a DataFrame. If you want a DataFrame whose column is the group sizes, indexed by the groups, with a custom name, you can use the .to_frame()method and use the desired column name as its argument.
.size()DataFrameGroupBy 对象的内置方法实际上返回一个带有组大小的 Series 对象，而不是一个 DataFrame。如果您想要一个 DataFrame，其列是组大小，由组索引，具有自定义名称，您可以使用该.to_frame()方法并使用所需的列名作为其参数。
grpd = df.groupby(['A','B']).size().to_frame('size')
If you wanted the groups to be columns again you could add a .reset_index()at the end.
如果您希望组再次成为列，您可以.reset_index()在末尾添加一个。
回答by Sealander
lets say n is the name of dataframe and cst is the no of items being repeted. Below code gives the count in next column
假设 n 是数据框的名称， cst 是重复的项目数。下面的代码给出了下一列的计数
cstn=Counter(n.cst)
cstlist = pd.DataFrame.from_dict(cstn, orient='index').reset_index()
cstlist.columns=['name','cnt']
n['cnt']=n['cst'].map(cstlist.loc[:, ['name','cnt']].set_index('name').iloc[:,0].to_dict())
Hope this will work
希望这会奏效
