Pandas：我如何得到一列组计数来填充组中的每一行？

Question

我可以用组计数成功地填充我的新列，但我怀疑有一种更简单的方法：

# How do I simplify this?

def f(gr):

    return pd.Series([gr['class_name'].count()] * gr.shape[0], index=gr.index)

df['class_size'] = df.groupby("class_name").apply(f).reset_index(level=0, drop=True)
column_list = ['class_name', 'class_size']
df[column_list].head(5)

获取：

Answer 1

我想你需要^{}：

df['class_size'] = df.groupby('class_name')['class_name'].transform('size')

或：

df['class_size'] = df.groupby('class_name')['class_name'].transform('count')

What is the difference between size and count in pandas?

Answer 2

根据您的数据帧形状，您也可以只对groupby进行计数：

import pandas as pd
df = pd.DataFrame({'class names':list('abracadabra'),'class count':1})
df.groupby('class names').count().reset_index()