How to select rows in the last key of first level in a Multiindex dataframe?

Question

I have a Pandas DataFrame that looks like the following:

                       data
date       signal                     
2012-11-01 a           0.04
           b           0.03
2012-12-01 a          -0.01
           b           0.00
2013-01-01 a          -0.00
           b          -0.01

I am trying to get only the last row based on the first level of the multiindex, which is date in this case.

2013-01-01 a          -0.00
           b          -0.01

The first level index is datetime. What would be the most elegant way to select the last row?

Answer 1

One way is to access the MultiIndex's levels directly (and use the last one):

In [11]: df.index.levels
Out[11]: [Index([bar, baz, foo, qux], dtype=object), Index([one, two], dtype=object)]

In [12]: df.index.levels[0][-1]
Out[12]: 'qux'

And select these rows with ix :

In [13]: df.ix[df.index.levels[0][-1]]
Out[13]:
            0         1         2         3
one  1.225973 -0.703952  0.265889  1.069345
two -1.521503  0.024696  0.109501 -1.584634

In [14]: df.ix[df.index.levels[0][-1]:]
Out[14]:
                0         1         2         3
qux one  1.225973 -0.703952  0.265889  1.069345
    two -1.521503  0.024696  0.109501 -1.584634

(Using @Jeff's example DataFrame .)

Perhaps a more elegant way is to use tail (if you knew there would always be two):

In [15]: df.tail(2)
Out[15]:
                0         1         2         3
qux one  1.225973 -0.703952  0.265889  1.069345
    two -1.521503  0.024696  0.109501 -1.584634

Answer 2

In 0.11 (coming this week), this is a reasonable way to do this

In [50]: arrays = [np.array(['bar', 'bar', 'baz', 'baz', 'foo', 'foo', 'qux', 'qux']),
   .....:           np.array(['one', 'two', 'one', 'two', 'one', 'two', 'one', 'two'])]

In [51]: df = pd.DataFrame(np.random.randn(8, 4), index=arrays)

In [52]: df
Out[52]: 
                0         1         2         3
bar one -1.798562  0.852583 -0.148094 -2.107990
    two -1.091486 -0.748130  0.519758  2.621751
baz one -1.257548  0.210936 -0.338363 -0.141486
    two -0.810674  0.323798 -0.030920 -0.510224
foo one -0.427309  0.933469 -1.259559 -0.771702
    two -2.060524  0.795388 -1.458060 -1.762406
qux one -0.574841  0.023691 -1.567137  0.462715
    two  0.936323  0.346049 -0.709112  0.045066

In [53]: df.loc['qux'].iloc[[-1]]
Out[53]: 
            0         1         2         3
two  0.936323  0.346049 -0.709112  0.045066

This will work in 0.10.1

In [63]: df.ix['qux'].ix[-1]
Out[63]: 
0    0.936323
1    0.346049
2   -0.709112
3    0.045066
Name: two, dtype: float64

And another way (this works in 0.10.1) as well

In [59]: df.xs(('qux','two'))
Out[59]: 
0    0.936323
1    0.346049
2   -0.709112
3    0.045066
Name: (qux, two), dtype: float64

Answer 3

If you have a dataframe df with a MultiIndex already defined, then:

df2 = df.ix[df.index[len(df.index)-1][0]]

would also work.

Answer 4

您可以使用iloc获取最后一行：

   df.iloc[-1]