how to select index in dataframe python code example

Example 1: pandas df by row index

indices = [133, 22, 19, 203, 14, 1]
df_by_indices = df.iloc[indices, :]

Example 2: add an index column in range dataframe

df = df.loc[df.index.repeat(df['a'])]   
df['c'] = df.groupby(level=0).cumcount() + 1
df = df.reset_index(drop=True)
print (df)
   a  b  c
0  1  x  1
1  2  y  1
2  2  y  2
3  3  z  1
4  3  z  2
5  3  z  3