Efficient pandas/numpy function for time since change

Question

Given a Series , I would like to efficiently compute how many observations have passed since there was a change. Here is a simple example:

ser = pd.         


        

Answer 1

You can use groupby.cumcount:

ser.groupby(ser).cumcount()

#0    0
#1    1
#2    2
#3    3
#4    0
#5    1
#6    2
#7    0
#8    0
#dtype: int64