python - Pandas 数据框 : how to add column with number of occurrences in other column

Question

我必须遵循 df:

Col1    Col2
test    Something
test2   Something
test3   Something
test    Something
test2   Something
test5   Something

我想得到

Col1    Col2          Occur
test    Something     2
test2   Something     2
test3   Something     1
test    Something     2
test2   Something     2
test5   Something     1

我试过使用:

df["Occur"] = df["Col1"].value_counts()

但这并没有帮助。我有充满“NaN”的 Occur 列

Answer 1

您还可以使用 GroupBy + transform 和 size:

df['Occur'] = df.groupby('Col1')['Col1'].transform('size')

print(df)

    Col1       Col2  Occur
0   test  Something      2
1  test2  Something      2
2  test3  Something      1
3   test  Something      2
4  test2  Something      2
5  test5  Something      1