Python Pandas : Conditional rolling count

Question

Here is my input dataframe:

type
a   
a   
a   
a   
a   
b   
b   
a   
a   
a

This is my expected output:

type,   id
a   ,   1
a   ,   2
a   ,   3
a   ,   4
a   ,   5
b   ,   5
b   ,   5
a   ,   6
a   ,   7
a   ,   8

I need to generate ID column based on 'type' column. I have two types 'a' & 'b'.. as long as it is 'a' I want to increment ID. If 'b', keep previous 'a' ID. How can I do this in a Pandas dataframe?

Answer 1

You can count the cumulative sum of a Boolean series indicating when your series equals a value:

df['id'] = df['type'].eq('a').cumsum()