Grouping rows from Pandas Dataframe with column value within 20% of first value in group

Question

Say I have a Pandas Dataframe below:

ID	value
0	1
1	2
2	1.2
3	1.6
4	1.5
5	1.4
6	1.1

I would like to group the Values within 20% margin of the first element of the group.

ID	value	group
0	1	0
2	1.2	0
6	1.1	0
1	2	1
3	1.6	1
4	1.5	2
5	1.4	2

What is the best way to do this?

Answer 1

Try:

df["group"] = np.nan
group = 0
while (m := df["group"].isna()).any():
    val = df.loc[m.idxmax(), "value"]
    df.loc[m & (np.abs(df["value"] - val) <= val * 0.2), "group"] = group
    group += 1

print(df.sort_values(by="group"))

Prints:

   ID  value  group
0   0    1.0    0.0
2   2    1.2    0.0
6   6    1.1    0.0
1   1    2.0    1.0
3   3    1.6    1.0
4   4    1.5    2.0
5   5    1.4    2.0