问题
What is the best way to reshape the following dataframe in pandas? This DataFrame df has x,y values for each sample (s1 and s2 in this case) and looks like this:
In [23]: df = pandas.DataFrame({"s1_x": scipy.randn(10), "s1_y": scipy.randn(10), "s2_x": scipy.randn(10), "s2_y": scipy.randn(10)})
In [24]: df
Out[24]:
s1_x s1_y s2_x s2_y
0 0.913462 0.525590 -0.377640 0.700720
1 0.723288 -0.691715 0.127153 0.180836
2 0.181631 -1.090529 -1.392552 1.530669
3 0.997414 -1.486094 1.207012 0.376120
4 -0.319841 0.195289 -1.034683 0.286073
5 1.085154 -0.619635 0.396867 0.623482
6 1.867816 -0.928101 -0.491929 -0.955295
7 0.920658 -1.132057 1.701582 -0.110299
8 -0.241853 -0.129702 -0.809852 0.014802
9 -0.019523 -0.578930 0.803688 -0.881875
s1_x and s1_y are the x/y values for sample 1, s2_x, s2_y are the sample values for sample 2, etc. How can this be reshaped into a DataFrame containing only x, y columns but that contains an additional column sample that says for each row in the DataFrame whether it's from s1 or s2? E.g.
x y sample
0 0.913462 0.525590 s1
1 0.723288 -0.691715 s1
2 0.181631 -1.090529 s1
3 0.997414 -1.486094 s1
...
5 0.396867 0.623482 s2
...
This is useful for plotting things with Rpy2 later on, since many R plotting features can make use of this grouping variable, so that's my motivation for reshaping the dataframe.
I think the answer given by Chang She doesn't translate to dataframes that have a unique index, like this one:
In [636]: df = pandas.DataFrame({"s1_x": scipy.randn(10), "s1_y": scipy.randn(10), "s2_x": scipy.randn(10), "s2_y": scipy.randn(10), "names": range(10)})
In [637]: df
Out[637]:
names s1_x s1_y s2_x s2_y
0 0 0.672298 0.415366 1.034770 0.556209
1 1 0.067087 -0.851028 0.053608 -0.276461
2 2 -0.674174 -0.099015 0.864148 -0.067240
3 3 0.542996 -0.813018 2.283530 2.793727
4 4 0.216633 -0.091870 -0.746411 -0.421852
5 5 0.141301 -1.537721 -0.371601 -1.594634
6 6 1.267148 -0.833120 0.369516 -0.671627
7 7 -0.231163 -0.557398 1.123155 0.865140
8 8 1.790570 -0.428563 0.668987 0.632409
9 9 -0.820315 -0.894855 0.673247 -1.195831
In [638]: df.columns = pandas.MultiIndex.from_tuples([tuple(c.split('_')) for c in df.columns])
In [639]: df.stack(0).reset_index(1)
Out[639]:
level_1 x y
0 s1 0.672298 0.415366
0 s2 1.034770 0.556209
1 s1 0.067087 -0.851028
1 s2 0.053608 -0.276461
2 s1 -0.674174 -0.099015
2 s2 0.864148 -0.067240
3 s1 0.542996 -0.813018
3 s2 2.283530 2.793727
4 s1 0.216633 -0.091870
4 s2 -0.746411 -0.421852
5 s1 0.141301 -1.537721
5 s2 -0.371601 -1.594634
6 s1 1.267148 -0.833120
6 s2 0.369516 -0.671627
7 s1 -0.231163 -0.557398
7 s2 1.123155 0.865140
8 s1 1.790570 -0.428563
8 s2 0.668987 0.632409
9 s1 -0.820315 -0.894855
9 s2 0.673247 -1.195831
The transformation worked but in the process the column "names" was lost. How can I keep the "names" column in the df while still doing the melting transformation on the columns that have _ in their names? The "names" column just assigns a unique name to each row in the dataframe. It's numeric here for example but in my data they are string identifiers.
thanks.
回答1:
I'm assuming you already have the DataFrame. In which case you can just turn the columns into a MultiIndex and use stack then reset_index. Note that you'll then have to rename and reorder the columns and sort by sample to get exactly what you posted in the question:
In [4]: df = pandas.DataFrame({"s1_x": scipy.randn(10), "s1_y": scipy.randn(10), "s2_x": scipy.randn(10), "s2_y": scipy.randn(10)})
In [5]: df.columns = pandas.MultiIndex.from_tuples([tuple(c.split('_')) for c in df.columns])
In [6]: df.stack(0).reset_index(1)
Out[6]:
level_1 x y
0 s1 0.897994 -0.278357
0 s2 -0.008126 -1.701865
1 s1 -1.354633 -0.890960
1 s2 -0.773428 0.003501
2 s1 -1.499422 -1.518993
2 s2 0.240226 1.773427
3 s1 -1.090921 0.847064
3 s2 -1.061303 1.557871
4 s1 -1.697340 -0.160952
4 s2 -0.930642 0.182060
5 s1 -0.356076 -0.661811
5 s2 0.539875 -1.033523
6 s1 -0.687861 -1.450762
6 s2 0.700193 0.658959
7 s1 -0.130422 -0.826465
7 s2 -0.423473 -1.281856
8 s1 0.306983 0.433856
8 s2 0.097279 -0.256159
9 s1 0.498057 0.147243
9 s2 1.312578 0.111837
You can save the MultiIndex conversion if you can just create the DataFrame with a MultiIndex instead.
Edit: use merge to join original ids back in
In [59]: df
Out[59]:
names s1_x s1_y s2_x s2_y
0 0 0.732099 0.018387 0.299856 0.737142
1 1 0.914755 -0.798159 -0.732868 -1.279311
2 2 -1.063558 0.161779 -0.115751 -0.251157
3 3 -1.185501 0.095147 -1.343139 -0.003084
4 4 0.622400 -0.299726 0.198710 -0.383060
5 5 0.179318 0.066029 -0.635507 1.366786
6 6 -0.820099 0.066067 1.113402 0.002872
7 7 0.711627 -0.182925 1.391194 -2.788434
8 8 -1.124092 1.303375 0.202691 -0.225993
9 9 -0.179026 0.847466 -1.480708 -0.497067
In [60]: id = df.ix[:, ['names']]
In [61]: df.columns = pandas.MultiIndex.from_tuples([tuple(c.split('_')) for c in df.columns])
In [62]: pandas.merge(df.stack(0).reset_index(1), id, left_index=True, right_index=True)
Out[62]:
level_1 x y names
0 s1 0.732099 0.018387 0
0 s2 0.299856 0.737142 0
1 s1 0.914755 -0.798159 1
1 s2 -0.732868 -1.279311 1
2 s1 -1.063558 0.161779 2
2 s2 -0.115751 -0.251157 2
3 s1 -1.185501 0.095147 3
3 s2 -1.343139 -0.003084 3
4 s1 0.622400 -0.299726 4
4 s2 0.198710 -0.383060 4
5 s1 0.179318 0.066029 5
5 s2 -0.635507 1.366786 5
6 s1 -0.820099 0.066067 6
6 s2 1.113402 0.002872 6
7 s1 0.711627 -0.182925 7
7 s2 1.391194 -2.788434 7
8 s1 -1.124092 1.303375 8
8 s2 0.202691 -0.225993 8
9 s1 -0.179026 0.847466 9
9 s2 -1.480708 -0.497067 9
Alternatively:
In [64]: df
Out[64]:
names s1_x s1_y s2_x s2_y
0 0 0.744742 -1.123403 0.212736 0.005440
1 1 0.465075 -0.673491 1.467156 -0.176298
2 2 -1.111566 0.168043 -0.102142 -1.072461
3 3 1.226537 -1.147357 -1.583762 -1.236582
4 4 1.137675 0.224422 0.738988 1.528416
5 5 -0.237014 -1.110303 -0.770221 1.389714
6 6 -0.659213 2.305374 -0.326253 1.416778
7 7 1.524214 -0.395451 -1.884197 0.524606
8 8 0.375112 -0.622555 0.295336 0.927208
9 9 1.168386 -0.291899 -1.462098 0.250889
In [65]: df = df.set_index('names')
In [66]: df.columns = pandas.MultiIndex.from_tuples([tuple(c.split('_')) for c in df.columns])
In [67]: df.stack(0).reset_index(1)
Out[67]:
level_1 x y
names
0 s1 0.744742 -1.123403
0 s2 0.212736 0.005440
1 s1 0.465075 -0.673491
1 s2 1.467156 -0.176298
2 s1 -1.111566 0.168043
2 s2 -0.102142 -1.072461
3 s1 1.226537 -1.147357
3 s2 -1.583762 -1.236582
4 s1 1.137675 0.224422
4 s2 0.738988 1.528416
5 s1 -0.237014 -1.110303
5 s2 -0.770221 1.389714
6 s1 -0.659213 2.305374
6 s2 -0.326253 1.416778
7 s1 1.524214 -0.395451
7 s2 -1.884197 0.524606
8 s1 0.375112 -0.622555
8 s2 0.295336 0.927208
9 s1 1.168386 -0.291899
9 s2 -1.462098 0.250889
来源：https://stackoverflow.com/questions/14916358/reshaping-dataframes-in-pandas-based-on-column-labels