how to sort only some of the columns in a data frame in pandas?

问题

Is there a way to sort only some elements of a list in a user-defined manner?

import pandas as pd
import numpy as np
df = pd.DataFrame(np.random.rand(5, 6), columns=['x','a','c','y','b','z'])

I'd like to sort the columns of df in a way that the first 3 columns are [x, y, z] (in this order), and it doesn't matter where the remaining columns are placed.

For this example I could do it manually, but as the list gets bigger it would be convenient to use a more appropriate method.

I thought of using l = df_r.columns.tolist() but I can't figure out how to this it even with a single list...

回答1:

If you know that you want a few columns in a specific order, just do a set difference between all columns and the pre-ordered columns, then call reindex:

In [13]: cols = list('xacybz')

In [14]: df = DataFrame(randn(10, len(cols)), columns=cols)

In [15]: preordered = list('xyz')

In [16]: new_order = preordered + list(df.columns - preordered)

In [17]: new_order
Out[17]: ['x', 'y', 'z', 'a', 'b', 'c']

In [18]: df.reindex(columns=new_order)
Out[18]:
       x      y      z      a      b      c
0 -0.012  0.949 -0.276 -0.074 -0.054  0.541
1  0.994  1.059 -0.158  0.267 -0.590  0.263
2 -0.632 -0.015 -0.097 -1.904 -1.351 -1.105
3 -0.730 -0.684 -0.226  2.664 -0.385  1.727
4  0.891 -0.602  3.426  1.529  0.853 -0.451
5 -0.471  0.689  1.170 -0.635 -0.663  0.180
6  1.536  0.793  1.461  0.723 -0.795 -1.094
7  0.417  0.787  1.676  1.563  1.412  0.398
8  0.378  1.436 -0.024  0.293  0.655 -0.113
9 -0.159 -0.416 -1.526  0.633 -0.780 -0.613

It won't matter what order the elements of preorder occur in:

In [25]: shuffle(df.columns.values)

In [26]: df
Out[26]:
       b      a      z      c      x      y
0 -0.054 -0.074 -0.276  0.541 -0.012  0.949
1 -0.590  0.267 -0.158  0.263  0.994  1.059
2 -1.351 -1.904 -0.097 -1.105 -0.632 -0.015
3 -0.385  2.664 -0.226  1.727 -0.730 -0.684
4  0.853  1.529  3.426 -0.451  0.891 -0.602
5 -0.663 -0.635  1.170  0.180 -0.471  0.689
6 -0.795  0.723  1.461 -1.094  1.536  0.793
7  1.412  1.563  1.676  0.398  0.417  0.787
8  0.655  0.293 -0.024 -0.113  0.378  1.436
9 -0.780  0.633 -1.526 -0.613 -0.159 -0.416

In [27]: new_order = preordered + list(df.columns - preordered)

In [28]: new_order
Out[28]: ['x', 'y', 'z', 'a', 'b', 'c']

回答2:

First build your new columns:

new_cols = ['x', 'y', 'z'] + [c for c in df.columns if c not in ['x', 'y', 'z']]

Then do:

new_df = df.reindex(columns=new_cols)

来源：https://stackoverflow.com/questions/18579871/how-to-sort-only-some-of-the-columns-in-a-data-frame-in-pandas

标签

python

sorting

pandas

dataframe