Pandas: How to access the value of the index

Question

I have a dataframe and would like to use the values in the index to create another column.
For instance:

df=pd.DataFrame({'idx1':range(0,5), 'idx2':range(10000,10005), 'value':np.random.randn(5)})
df.set_index(keys=['idx1','idx2'], inplace=True)
print df

               value
idx1 idx2           
0    10000 -1.470367
1    10001  0.260693
2    10002 -0.732319
3    10003 -0.116977
4    10004  1.106644

I'd like to do something like this:

df['idx1_mod']= df['idx1'] + 100

(Actually, I want to do more complicated things, but basically I need the value of the index.)

Right now I'm resorting to reseting the index (to get the index fields as columns), doing my calcs with access to the columns, and then re-creating the index. I'm sure I'm missing something obvious, but I've looked a ton and keep missing it!

Note - I also tried df.iterrows(), but it seems that gives a copy of the row and doesn't let me update the original dataframe.

HYRY · Accepted Answer · 2013-11-15 03:15:17Z

10

df["idx1_mod"] = df.index.get_level_values(0).values + 100

answered Nov 15, 2013 at 3:15

HYRY

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1 Comment

Ross R Over a year ago

Perfect. My actual use case requires more than one field, but this was the clue I needed. For the next questioner, this works for multiple fields: df['idx_mod'] = df.index.map(lambda idx: someFunc(idx[0],idx[1]))

simplemts · Accepted Answer · 2013-11-15 01:27:15Z

3

Try this:

for idx in range(len(df)):
    df['idx1_mod'][idx] = df.index[idx][0] + 100

edited Nov 15, 2013 at 1:27

answered Nov 15, 2013 at 1:21

simplemts

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Comments

Kim · Accepted Answer · 2018-08-11 00:31:09Z

3

You can use drop=False when setting the index to preserve your keys as columns. This should work:

df.set_index(keys=['idx1','idx2'], inplace=True, drop=False)
df['idx1_mod'] = df['idx'] + 100

edited Aug 11, 2018 at 0:31

Kim

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answered Aug 10, 2018 at 23:28

Tim

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