conditional replace based off prior value in same column of pandas dataframe python

Question

Feel like I\'ve looked just about everywhere and I know its probably something very simple. I\'m working with a pandas dataframe and looking to fill/replace data in one of

Answer 1

Here's a kind of brute-force method. There is probably something more elegant, but you could explicitly loop over the rows like this:

df = pd.DataFrame([0, -1, -1, -1, 0 , 0, 0, 1, 0])
df.columns = ['A']
df['B'] = df['A']

# loop here
for i in range(1,len(df)):
     if df.A[i] == 0 and df.B[i-1] == -1:
             df.B[i] = -1
     else:
             df.B[i] = df.A[i]

This gives you the result you seek:

>>> df['B']
0    0
1   -1
2   -1
3   -1
4   -1
5   -1
6   -1
7    1
8    0

Answer 2

using Where

df['B'] = df.A[0:len(df.A)-1].where((df.A==0 ) & (df.B.shift(-1)==-1),-1)
df['B'] = df['B'].fillna(df.A)