[FIXED] How to substitute with 0, the first of 2 consecutive values in a pandas column with groupby

Issue

I have the following pandas dataframe

import pandas as pd
foo = pd.DataFrame({'id': [1,1,1,1,1,2,2,2,2,2],
                    'col_a': [0,1,1,0,1,1,1,0,1,1]})

I would like to create a new column (col_a_new) which will be the same as col_a but substitute with 0 the 1st out of the 2 consecutive 1s in col_a, by id

The resulting dataframe looks like this:

foo = pd.DataFrame({'id': [1,1,1,1,1,2,2,2,2,2],
                        'col_a': [0,1,1,0,1,1,1,0,1,1],
             'col_a_new': [0,0,1,0,1,0,1,0,0,1]})

Any ideas ?

Solution

Other approach: Just group by id and define new values using appropriate conditions.

(foo.groupby("id").col_a
    .transform(lambda series: [0 if i < len(series) - 1 
                               and series.tolist()[i+1] == 1 
                               else x for i, x in enumerate(series.tolist())]))

Answered By - mcsoini