I have a pandas DataFrame with a column like:

0
1
1
2
2
3
4
5
5
0
0
0

I would like to leave any leading zeros, but ffill to replace the trailing zeros with one greater than the previous, nonzero value. In this case, I'd like the output to be:

0
1
1
2
2
3
4
5
5
6
6
6

How can I go about doing this?

I have a pandas DataFrame with a column like:

0
1
1
2
2
3
4
5
5
0
0
0

I would like to leave any leading zeros, but ffill to replace the trailing zeros with one greater than the previous, nonzero value. In this case, I'd like the output to be:

0
1
1
2
2
3
4
5
5
6
6
6

How can I go about doing this?

• python
• pandas
• ffill

• Do you have any other edge cases? Or is the column always zero from the last non zero value? – Tom McLean Commented Nov 19, 2024 at 16:02
• So far this is my edge case. I've been able to handle any others without much issue. Mozway's answer works for my needs for now, but I'll update if I run into any edge cases that it can't handle. – Aaron Horowitz Commented Nov 19, 2024 at 16:09

1 Answer 1

You could mask, increment and ffill:

m = df['col'].eq(0)
s = df['col'].mask(m)
df['out'] = s.fillna(s.add(1).ffill().fillna(0)).convert_dtypes()

Or, if you really want to only target the trailing zeros:

df['out'] = df['col'].mask(df['col'].eq(0)[::-1].cummin(), df['col'].max()+1)

Output:

    col  out
0     0    0
1     1    1
2     1    1
3     2    2
4     2    2
5     3    3
6     4    4
7     5    5
8     5    5
9     0    6
10    0    6
11    0    6

Intermediates (first approach):

    col  out      m    s  s.add(1)  .ffill()  .fillna(0)
0     0    0   True  NaN       NaN       NaN         0.0
1     1    1  False  1.0       2.0       2.0         2.0
2     1    1  False  1.0       2.0       2.0         2.0
3     2    2  False  2.0       3.0       3.0         3.0
4     2    2  False  2.0       3.0       3.0         3.0
5     3    3  False  3.0       4.0       4.0         4.0
6     4    4  False  4.0       5.0       5.0         5.0
7     5    5  False  5.0       6.0       6.0         6.0
8     5    5  False  5.0       6.0       6.0         6.0
9     0    6   True  NaN       NaN       6.0         6.0
10    0    6   True  NaN       NaN       6.0         6.0
11    0    6   True  NaN       NaN       6.0         6.0

Intermediates (second approach):

    col  out      m    s  df['col'].eq(0)  [::-1].cummin()
0     0    0   True  NaN             True            False
1     1    1  False  1.0            False            False
2     1    1  False  1.0            False            False
3     2    2  False  2.0            False            False
4     2    2  False  2.0            False            False
5     3    3  False  3.0            False            False
6     4    4  False  4.0            False            False
7     5    5  False  5.0            False            False
8     5    5  False  5.0            False            False
9     0    6   True  NaN             True             True
10    0    6   True  NaN             True             True
11    0    6   True  NaN             True             True

applying per group:

Assuming a group LOT_ID and the target column STEP_NUMBER:

df['out'] = (df.groupby('LOT_ID')['STEP_NUMBER']
             .transform(lambda x: x.mask(x.eq(0)[::-1].cummin(), x.max()+1))
            )