Creating a variable using conditionals python

Question

I have a pandas dataframe as below.

df = [['A',1,3],
        ['A',1,2],
        ['A',0,1],
        ['A',0,1],
        ['A',5,6],
        ['B',0,5],
        ['B',1,9],
        ['B',1,2],
        ['B',1,1]] 

df = pd.DataFrame(df, columns = ['flag', 'A', 'B'])
df

Now I need to create a new variable called 'C' based on the below conditions,

1) For 1st row of each group of flag, 'C' = 'A'

2) ELSE, if A >= previous row of 'C', then 'C' = 'A' else 'C' = previous row 'C'

Below is my expected output:

  flag  A  B  C
0    A  1  3  1
1    A  1  2  1
2    A  0  1  1
3    A  0  1  1
4    A  5  6  5
5    B  0  5  0
6    B  1  9  1
7    B  1  2  1
8    B  1  1  1

I can do it using iterrows, but I need an efficient/vectorized way of doing this, since my dataset is huge

BENY · Accepted Answer · 2019-11-08 02:24:18Z

1

You can do

df['C']=df.groupby('flag').A.cummax()
df
Out[14]: 
  flag  A  B  C
0    A  1  3  1
1    A  1  2  1
2    A  0  1  1
3    A  0  1  1
4    A  5  6  5
5    B  0  5  0
6    B  1  9  1
7    B  1  2  1
8    B  1  1  1

answered Nov 8, 2019 at 2:24

BENY

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