how to add incremental number to specific column in pandas - pandas

Question

I have following dataframe in pandas
code tank length dia diff
123 3 625 210 -0.38
123 5 635 210 1.2
I want to add 1 only in length for 5 times if the diff is positive and subtract 1 if the dip is negative. My desired dataframe looks like
code tank length diameter
123 3 625 210
123 3 624 210
123 3 623 210
123 3 622 210
123 3 621 210
123 3 620 210
123 5 635 210
123 5 636 210
123 5 637 210
123 5 638 210
123 5 639 210
123 5 640 210
I am doing following in pandas.
df.add(1)
But, its adding 1 to all the columns.

Answer 1

Use Index.repeat 6 times, then add counter values by GroupBy.cumcount and last create default RangeIndex by DataFrame.set_index:
df1 = df.loc[df.index.repeat(6)].copy()
df1['length'] += df1.groupby(level=0).cumcount()
df1 = df1.reset_index(drop=True)
Or:
df1 = (df.loc[df.index.repeat(6)]
.assign(length = lambda x: x.groupby(level=0).cumcount() + x['length'])
.reset_index(drop=True))
print (df1)
code tank length dia
0 123 3 625 210
1 123 3 626 210
2 123 3 627 210
3 123 3 628 210
4 123 3 629 210
5 123 3 630 210
6 123 5 635 210
7 123 5 636 210
8 123 5 637 210
9 123 5 638 210
10 123 5 639 210
11 123 5 640 210
EDIT:
df1 = df.loc[df.index.repeat(6)].copy()
add = df1.groupby(level=0).cumcount()
mask = df1['diff'] < 0
df1['length'] = np.where(mask, df1['length'] - add, df1['length'] + add)
df1 = df1.reset_index(drop=True)
print (df1)
code tank length dia diff
0 123 3 625 210 -0.38
1 123 3 624 210 -0.38
2 123 3 623 210 -0.38
3 123 3 622 210 -0.38
4 123 3 621 210 -0.38
5 123 3 620 210 -0.38
6 123 5 635 210 1.20
7 123 5 636 210 1.20
8 123 5 637 210 1.20
9 123 5 638 210 1.20
10 123 5 639 210 1.20
11 123 5 640 210 1.20

Answer 2

We can use pd.concat, np.cumsum and groupby + .add.
If you want to substract, simply multiply addition * -1 so for example: (np.cumsum(np.ones(n))-1) * -1
n = 6
new = pd.concat([df]*n).sort_values(['code', 'length']).reset_index(drop=True)
addition = np.cumsum(np.ones(n))-1
new['length'] = new.groupby(['code', 'tank'])['length'].apply(lambda x: x.add(addition))
Output
code tank length dia
0 123 3 625.0 210
1 123 3 626.0 210
2 123 3 627.0 210
3 123 3 628.0 210
4 123 3 629.0 210
5 123 3 630.0 210
6 123 5 635.0 210
7 123 5 636.0 210
8 123 5 637.0 210
9 123 5 638.0 210
10 123 5 639.0 210
11 123 5 640.0 210