Numpy Python: Sorting columns of an array in alphabetical order

Question

I'm currently working with Python 3 and Numpy. I have a structured array created by using numpy.genfromtxt("textfile.txt", names=True) and I want to sort the columns in alphabetical order.

The first row contains the names of the columns.
The other columns contain the accompanying values per instance.

I simply want to switch the order of the columns to the alphabetical order of the column names. For instance, now the order of the columns is 'year', 'population', 'area' and I want it to become 'area', 'population', 'year'. Thanks in advance for helping out!

Thanks for helping out! I haven't managed to use argsort to order alphabetically. Do I need to tune a parameter? — Frederique
– Frederique, Commented Nov 28, 2016 at 14:13
Abdou, I think you misunderstood the question. He does not want to sort the records in the array, he wants to reorder the columns. — HiFile.app - best file manager
– HiFile.app - best file manager, Commented Nov 28, 2016 at 14:14
@V.K. I just realized. Select the arrays by name and stack or concatenate them together in the order you want. — Abdou
– Abdou, Commented Nov 28, 2016 at 14:27
@Frederiquevdb, can you please provide a toy dataset to work with? — Abdou
– Abdou, Commented Nov 28, 2016 at 15:06

Abdou · Accepted Answer · 2016-11-28 23:50:43Z

2

For the following numpy array, the answer is:

import numpy as np


x = np.array([(2015, 34, 12, 13), (2016, 41, 6, 7), (2016, 17, 5, 2),
       (2013, 21, 8, 19), (2013, 1, 81, 9)], 
      dtype=[('year', '<i8'), ('tigers', '<i8'), ('monkeys', '<i8'), ('cows', '<i8')])

x[numpy.sort(x.dtype.names)]

# Output:
array([(13, 12, 34, 2015), (7, 6, 41, 2016), (2, 5, 17, 2016),
       (19, 8, 21, 2013), (9, 81, 1, 2013)], 
      dtype=[('cows', '<i8'), ('monkeys', '<i8'), ('tigers', '<i8'), ('year', '<i8')])

edited Nov 28, 2016 at 23:50

Abdou

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answered Nov 28, 2016 at 17:57

Frederique

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