Python pandas: add index column based on existing columns, with duplicates sharing the same index

Question

I would like to add an index column based on existing columns. Duplicates would share the same index. For example,

If the values for the two columns ['old_index','year'] are the same, then the new index would be same. The value in the column 'num' does not matter.

I'm wondering if anyone can help. Thank you very much!

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– Diesan Romero, Commented Jul 9, 2021 at 16:04

jonny_two_knives · Accepted Answer · 2021-07-09 14:56:36Z

1


df['new_id'] = df.groupby(df.columns.tolist(), sort=False).ngroup() + 1
df


index   year    id  new_id
0   1   2000    5   1
1   2   1996    3   2
2   2   1996    3   2
3   4   1994    2   3
4   4   1999    4   4
5   4   1999    4   4
6   12  1989    1   5
7   12  1989    1   5
8   12  1985    0   6
9   12  2011    6   7

Give this a try, but let me know if it isn't fully what you are looking for.

edited Jul 9, 2021 at 14:56

answered Jul 9, 2021 at 14:04

jonny_two_knives

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5 Comments

Arist Over a year ago

Thank you for your help. It did work. I just edited the question, could you please check it? I'm looking for a way that works on selected columns.

jonny_two_knives Over a year ago

@Arist. I just edited my answer. Try that new line of code

Arist Over a year ago

yes it works. I really appreciate your help :)

jonny_two_knives Over a year ago

Be sure to click the check arrow to indicate your question has been answered next to my submission. @Arist

Arist Over a year ago

Just did. I'm new to Stackoverflow. Thanks for reminding me.

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