Dataframe fill column with previous value until condition

Question

I have a dataframe that looks like this:

Step	Text	Parameter
15	print	1
16	control	2
17	printout	3
18	print2	1
19	Nan	2
20	Nan	3
21	Nan	4
22	Nan	1
23	Nan	2
24	Nan	1

And I want my dataframe to look like this:

Step	Text	Parameter
15	print	1
15	print	2
15	print	3
16	control	1
16	control	2
17	control	3
17	control	4
18	printout	1
18	printout	2
19	print2	1

So basically when I have "1" in Parameter column, I need the next value from Step and Text. Any ideas?:)

Answer 1

You can use repeat on a custom group:

# ensure NaN
df['Text'] = df['Text'].replace('Nan', pd.NA)

# get the number of rows per group starting with 1
n = df.groupby(df['Parameter'].eq(1).cumsum()).size()

# repeat the index of the non NaN values as many times
idx = df['Text'].dropna().index.repeat(n)

# replace the values ignoring the index
# (using the underlying numpy array)
df[['Step', 'Text']] = df.loc[idx, ['Step', 'Text']].to_numpy()

output:

  Step      Text  Parameter
0   15     print          1
1   15     print          2
2   15     print          3
3   16   control          1
4   16   control          2
5   16   control          3
6   16   control          4
7   17  printout          1
8   17  printout          2
9   18    print2          1