Python, sort array with respect to a sub-array

Question

I have the following array:

master_array = [[1. 2. 3. 4. 5.]
                [9. 8. 4. 5. 1.]]

I would like to sort the master_array with respect to the second sub-array so that the relationship between first sub-array and second sub-array is maintained

master_array = [[5. 3. 4. 2. 1.] 
                [1. 4. 5. 8. 9.]]

Thank You

Why is this tagged with numpy? What you are showing here is a list. — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Jan 24, 2018 at 8:18
So, I tried searching how to arrange an array with respect to a sub-array, was not able to find anything substantial yet. Sorry, forgot the dots after the numbers. — Isaac A
– Isaac A, Commented Jan 24, 2018 at 8:21

Albin Paul · Accepted Answer · 2018-01-24 09:32:25Z

2

Convert list to numpy array

>>> import numpy as np
>>> master_array = [[1.,2.,3.,4.,5.], [9.,8.,4.,5.,1.]]
>>> n=np.array(master_array)
>>> n
array([[ 1.,  2.,  3.,  4.,  5.],
       [ 9.,  8.,  4.,  5.,  1.]])

Assign index values for the second array so take n[1]

>>> temp=list(enumerate(n[1]))
>>> temp
[(0, 9.0), (1, 8.0), (2, 4.0), (3, 5.0), (4, 1.0)]

sort the array with respect to array elements

>>> list1=sorted(temp,key=lambda x:x[1])
>>> list1
[(4, 1.0), (2, 4.0), (3, 5.0), (1, 8.0), (0, 9.0)]

Take all the indexes from sorted result and store in a seperate array

>>> a=[i[0] for i in list1 ]
>>> a
[4, 2, 3, 1, 0]

Use indexing of columns in numpy on the numpy array

>>> n[:,a]
array([[ 5.,  3.,  4.,  2.,  1.],
       [ 1.,  4.,  5.,  8.,  9.]])

edited Jan 24, 2018 at 9:32

answered Jan 24, 2018 at 9:05

Albin Paul

3,4292 gold badges17 silver badges34 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Isaac A Over a year ago

That's beautiful! Do you mind explaining what each of the steps does?

Albin Paul Over a year ago

@IsaacA i have modified the answer pls check

Isaac A Over a year ago

Hello Paul, that's quite a brilliant solution. Thank you for your help. Explanations make sense. I found the last part to be somewhat challenging, but after some thinking I understood what is happening.

Collectives™ on Stack Overflow

Python, sort array with respect to a sub-array

1 Answer 1

3 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related