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.

Answer 1

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