add elements in nested list and output a list python

Question

I want to add the

distances
[array([2.0786226e-03, 3.9023757e-03, 3.4480095e-03, 5.1999092e-04], dtype=float32), array([0.0031136 , 0.00344056, 0.00739563, 0.0311079 ],
      dtype=float32), array([1.8880665e-03, 3.0295253e-03, 4.0802956e-03, 2.6324868e-02], dtype=float32), array([0.00330418, 0.00431347, 0.00802791, 0.00426304],
      dtype=float32)]

Desired Output

[0.00994899872, 0.04505769, 0.0353227554, 0.0199086]

I tried the following but it adds all the elements and gives on scalar value

print(sum( sum(x) if isinstance(x, list) else x for x in L ))

What else would you expect sum to do?

Scott Hunter
– Scott Hunter

2021-07-09 02:04:58 +00:00
Commented Jul 9, 2021 at 2:04 — Scott Hunter
– Scott Hunter, Commented Jul 9, 2021 at 2:04

Bruno Magacho · Accepted Answer · 2021-07-09 13:43:37Z

3

You could use numpy to do this, as

import numpy as np

distances = np.array([[2.0786226e-03, 3.9023757e-03, 3.4480095e-03, 5.1999092e-04],
                     [0.0031136 , 0.00344056, 0.00739563, 0.0311079 ],
                     [1.8880665e-03, 3.0295253e-03, 4.0802956e-03, 2.6324868e-02],
                     [0.00330418, 0.00431347, 0.00802791, 0.00426304]],dtype=np.float32)

print(np.sum(distances,axis=1))

which gives the desired output

edited Jul 9, 2021 at 13:43

answered Jul 9, 2021 at 2:05

Bruno Magacho

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4 Comments

Khaned Over a year ago

My array has dtype=float32 which is not present in your distances array

Khaned Over a year ago

All sub_arrays have dtype=np.float32

Bruno Magacho Over a year ago

There also, they just doesn't stay explicity outside as yours, but you could check that out by yourself taking distances[0], that is a float32 array, as all the others.

Bruno Magacho Over a year ago

note that you could also define each of the arrays as a 4 dimensional array with dtype=np.float32 and in the distances you could assembly them all together and do the same operation using np,.sum, but this of course it is up to you.

Scott Hunter · Accepted Answer · 2021-07-09 02:09:31Z

1

Assuming L is distances, you were almost there:

print([sum(x) if isinstance(x, list) else x for x in L ])

answered Jul 9, 2021 at 2:09

Scott Hunter

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Comments

Aarav Dave · Accepted Answer · 2021-07-09 02:06:17Z

1

Before we continue, I would like to mention you should specify that it is with numpy.

Why make a one-liner? Use a simple for loop. Take the first item in each and every list and add them up separately. To then store it, append it in a list and move on to the second item. After you find a working code, then only try shortening it. This is a good python practice.

answered Jul 9, 2021 at 2:06

Aarav Dave

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Comments

Gustavo Alves · Accepted Answer · 2021-07-09 02:21:20Z

0

As you already have numpy objects inside your list you can sum these objects.

The numpy object array have the element wise implementation, so you can sum:

a = np.array([a1, a2, a3])
b = np.array([b1, b2, b3])
a + b

The result of a + b will be:

[a1+b1, a2+b2, a3+b3]

As @bruno mentioned you can use the built in method numpy.sum to do this automatically.

answered Jul 9, 2021 at 2:21

Gustavo Alves

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