How do I only fill down when a column is 1 in Pandas DataFrame

user_439497 0
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I have a signal column and I want to create a NewSignal column that only takes the value of Signal when Allowed column is 1, otherwise it should keep its own previous value:

df:
        Signal  Allowed  NewSignal
    0       -1        0          0
    1       -1        0          0
    2       -1        0          0
    3       -1        1         -1
    4       -1        1         -1
    5       -1        1         -1
    6       -1        1         -1
    7       -1        0         -1
    8       -1        0         -1
    9        1        0         -1
    10       1        1          1
    11       1        1          1
    12       1        1          1

When I try to create NewSignal like this:

df['NewSignal'] = 0
df['NewSignal'] = np.where(df['Allowed'] == 1,df['Signal'],df['NewSignal'].shift(1))

NewSignal is NOT correct on row 7:

    Signal  Allowed  NewSignal
0       -1        0        NaN
1       -1        0        0.0
2       -1        0        0.0
3       -1        1       -1.0
4       -1        1       -1.0
5       -1        1       -1.0
6       -1        1       -1.0
7       -1        0        0.0
8       -1        0        0.0
9        1        0        0.0
10       1        1        1.0
11       1        1        1.0
12       1        1        1.0
pandas numpy python
1 Answers

Try with ffill with where

df['Nsg'] = df.Signal.where(df.Allowed==1).ffill()
Out[253]: 
0     NaN
1     NaN
2     NaN
3    -1.0
4    -1.0
5    -1.0
6    -1.0
7    -1.0
8    -1.0
9    -1.0
10    1.0
11    1.0
12    1.0
Name: Signal, dtype: float64
user_79645270
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