How to sort a numpy array into a specific order, specified by a separate list, based on the relative sizes of the values in the array

Question

Within a piece of code I have a numpy array already constructed, and I want to sort this 1st array given a specific order specified in a list. The result will be a 3rd array (new_values).

The 1st array has the values to be sorted.

values = numpy.array([10.0, 30.1, 50, 40, 20])

The list provides the order given by the indices of the values in new_values which should be in descending order where 0 corresponds to the largest number.

order=[0, 3, 1, 4, 2].

So,

new_values[0] > new_values[3] > new_values[1] > new_values[4] > new_values[2]

I tried to find a specific sort function to accomplish this such as argsort or with a key, however I did not understand how to adapt these to this situation.

Is there a simple quick method to accomplish this as I will be doing this for many iterations. I am willing to change the 2nd array, order, to specify the indices in another way if this was advantageous to sorting with a better method.

Currently I am using the following for loop.

size=len(values)
new_values = np.zeros(size)
values = sorted(values,reverse=True)

    for val in range(0, size):
        new_values[int(order[val])] = values[val]

Thank you in advance!

Kasravnd · Accepted Answer · 2015-06-01 17:19:03Z

4

You can simply use indexing for that :

>>> import numpy as np
>>> values = np.array([10.0, 30.1, 50, 40, 20])
>>> order=[0, 3, 1, 4, 2]
>>> sorted_array=values[order]
>>> sorted_array
array([ 10. ,  40. ,  30.1,  20. ,  50. ])

Also as @Divakar mentioned in comment if you want the following condition :

new_values[0] > new_values[3] > new_values[1] > new_values[4] > new_values[2]

You can do :

>>> values[order]=np.sort(values)[::-1]
>>> values
array([ 50. ,  30.1,  10. ,  40. ,  20. ])

edited Jun 1, 2015 at 17:19

answered Jun 1, 2015 at 16:26

Kasravnd

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7 Comments

Divakar Over a year ago

Think, it should be the other way: sorted_array[order]=sorted(values,reverse=True) ?

Kasravnd Over a year ago

@Divakar what you mean?

Divakar Over a year ago

I mean if you are trying to get new_values as the final output, that comes out to be array([ 50. , 30.1, 10. , 40. , 20. ]), which I am assuming to be equivalent to your sorted_array, you might have to index it like sorted_array[order] = values, where values = sorted(values,reverse=True).

Divakar Over a year ago

@Kasra No worries! Don't forget to initialize that array before indexing.

NCLee Over a year ago

Thank you to @Kasra for the posted answer. The code above in the final part of Kasra 's answer worked fine and I have implemented it in my exisiting code. It is a simpler and quicker method than that which I was using and I can use this lesson in future code. Perfect! Also thanks for working out the answer so quickly.

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