Substitute previous row value if value equal to 0 in Python(Can't use drop duplicates)

Question

I want to check if the value of a column in a dataframe is equal to 0 and if it is 0, I want to substitute the previous row's value of that column.

Suppose my data is as follows,

a=[0,0,1,2,5,6,0,0,8,9,0,11,0,13]

Output should be

a=[0,0,1,2,5,6,6,6,8,9,9,11,11,13]

I tried the following command,

a = a.replace(0, np.NaN).ffill()

but I am getting,

a=[NaN,NaN,1,2,5,6,6,6,8,9,9,11,11,13]

I can't use drop_duplicates because the values would occur again in my array. So I need to check the previous value alone and do the calculation.

Can anybody help me with this?

Thanks

Answer 1

This should do the trick.

previousValue = 0
for i in range(len(a)):
    if a[i] == 0:
        a[i] = previousValue
    else:
        previousValue = a[i]

Or the more concise way would be

for i in range(1, len(a)):
    if a[i] == 0:
        a[i] = a[i-1]