对于列中的相似值，添加新列频率

Question

I have a dataframe :

Id  age number 
1    35   7
5    76   9
1    15   0
2    10   4
5    43   8

What i need to get is :

Id  age  number freq
1   35    7     2
5   76    9     2
1   15    0     1
2   10    4     1
5   43    8     1

Add a new colum freq , for each value in a column , we takes all rows with same value in ID and count rows where the value of cat is less.

Answer 1

If need counter in descending order use GroupBy.cumcount :

df['freq'] = df.groupby('Id').cumcount(ascending=False).add(1)

But if need counts values by Id use GroupBy.transform with DataFrameGroupBy.size , output is different:

df['freq'] = df.groupby('Id')['Id'].transform('size')

Alternative with Series.map and Series.value_counts :

df['freq'] = df['Id'].map(df['Id'].value_counts())