undo or reverse argsort(), python

Question

Given an array 'a' I would like to sort the array by columns sort(a, axis=0) do some stuff to the array and then undo the sort. By that I don't mean re sort but basically reversing how each element was moved. I assume argsort() is what I need but it is not clear to me how to sort an array with the results of argsort() or more importantly apply the reverse/inverse of argsort()

Here is a little more detail

I have an array a, shape(a) = rXc I need to sort each column

aargsort = a.argsort(axis=0)  # May use this later aSort = a.sort(axis=0) 

now average each row

aSortRM = asort.mean(axis=1) 

now replace each col in a row with the row mean. is there a better way than this

aWithMeans = ones_like(a) for ind in range(r)  # r = number of rows     aWithMeans[ind]* aSortRM[ind] 

Now I need to undo the sort I did in the first step. ????

Answer 1

There are probably better solutions to the problem you are actually trying to solve than this (performing an argsort usually precludes the need to actually sort), but here you go:

>>> import numpy as np >>> a = np.random.randint(0,10,10) >>> aa = np.argsort(a) >>> aaa = np.argsort(aa) >>> a # original array([6, 4, 4, 6, 2, 5, 4, 0, 7, 4]) >>> a[aa] # sorted array([0, 2, 4, 4, 4, 4, 5, 6, 6, 7]) >>> a[aa][aaa] # undone array([6, 4, 4, 6, 2, 5, 4, 0, 7, 4])