Sorting a list using argsort in Numpy?

Question

I have a list in python and the first numbers are [[29.046875, 1], [33.65625, 1], [18.359375, 1], [11.296875, 1], [36.671875, 1], [23.578125, 1],.........,[34.5625, 1]]

The above list is given an id of listNumber. I'm trying to use numpy.argsort to sort it based on the float elements:

listNumber = np.array(listNumber)
print(np.argsort(listNumber))

But this gives me the following but not sure why:

[[1 0]
 [1 0]
 [1 0]
 ...
 [1 0]
 [1 0]
 [1 0]]

Why is this returning this? and is there another way to approach this?

Possible duplicate of How to sort (list/tuple) of lists/tuples by the element at a given index? — busybear
– busybear, Commented Sep 12, 2019 at 21:47

will · Accepted Answer · 2019-09-12 21:50:20Z

3

Ok so i think there's two things going on here:

1- Your list is a list of lists

2- The 'argsort' function:

returns the indices that would sort an array.

According to the documentation.

So what is happening is the function reads through each item of the list, which in itself is a list, say index 0 is:

[29.046875, 1]

Then it is saying, okay this is another list so let me sort it and then return a number based on where it would go if it was the new index:

[29.046875, 1] -> [1, 0]

Because 1 would come before 29 if it was sorted in ascending order. It does this for every nested list then gives you a final list containing all these 1's and 0's.

This answers the first question. Another user was able to answer the second :)

answered Sep 12, 2019 at 21:50

will

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Comments

Shahab Rahnama · Accepted Answer · 2019-09-12 21:50:12Z

2

You must set axis like:

import numpy as np

l = [[29.046875, 1], [33.65625, 1], [18.359375, 1], [11.296875, 1], [36.671875, 1], [23.578125, 1],[34.5625, 1]]
l = np.argsort(l, axis=0)  # sorts along first axis (down)
print(l)

output:

[[3 0]
 [2 1]
 [5 2]
 [0 3]
 [1 4]
 [6 5]
 [4 6]]

answered Sep 12, 2019 at 21:50

Shahab Rahnama

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Comments

Shivid · Accepted Answer · 2019-09-12 21:51:46Z

0

Try this;

sortedList = listNumber[listNumber[:,0].argsort(axis=0)]
print(sortedList)

answered Sep 12, 2019 at 21:51

Shivid

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Comments

moe asal · Accepted Answer · 2019-09-12 22:40:20Z

I don't know why people like using predone functions instead of using their own algorithm. Anyway, you are using argsort in a bad way. argsort returns an array containing the INDEXES of your elements, thos are 2 examples :

Code 1:


import numpy as geek 

# input array 

in_arr = geek.array([ 2, 0,  1, 5, 4, 1, 9]) 

print ("Input unsorted array : ", in_arr)  

out_arr = geek.argsort(in_arr) 

print ("Output sorted array indices : ", out_arr) 

print("Output sorted array : ", in_arr[out_arr])

Output :

Input unsorted array :  [2 0 1 5 4 1 9]
Output sorted array indices :  [1 2 5 0 4 3 6]
Output sorted array :  [0 1 1 2 4 5 9]

Code 2:


# Python program explaining 
# argpartition() function 

import numpy as geek 

# input 2d array 

in_arr = geek.array([[ 2, 0, 1], [ 5, 4, 3]]) 

print ("Input array : ", in_arr)  


# output sorted array indices 

out_arr1 = geek.argsort(in_arr, kind ='mergesort', axis = 0) 

print ("Output sorteded array indices along axis 0: ", out_arr1) 

out_arr2 = geek.argsort(in_arr, kind ='heapsort', axis = 1) 

print ("Output sorteded array indices along axis 1: ", out_arr2)

Output:

Input array :  [[2 0 1]
 [5 4 3]]
Output sorteded array indices along axis 0:  [[0 0 0]
 [1 1 1]]
Output sorteded array indices along axis 1:  [[1 2 0]
 [2 1 0]]

I am supposing that your data is stored in listnumber

import numpy as np
new_listnumber = listnumber[:, 0]
index_array = np.argsort(new_listnumber , axis=0)
New_val = listnumber[index_array] 
print(New_val)

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