Substract numpy array column from other column

Question

Can I use numpy broadcasting to achieve one column (col + 1) subtraction with other column (col -1) and dividing them by constant (n).

arr = [[2, 4, 6, 5, 8],
       [5, 3, 7, 4, 4],
       [3, 9, 5, 2, 1]]

result = [
          [ 2, # (6 - 2)/2
           .5, # (5 - 4)/2
            1, # (8 - 6)/2
          ],
          [ 1, # (7 - 5)/2
           .5, # (4 - 3)/2
           -1.5 # (4 - 7)/2
          ],
          [ 1, # (5 - 3)/2
           -3.5, # (2 - 9)/2
           -2 # (1 - 5)/2
          ]
        ]

Or what is the efficient way to achieve it on GBs of data?

You should look into slicing - docs.scipy.org/doc/numpy/reference/… — Divakar
– Divakar, Commented May 1, 2020 at 19:51
What is the issue, exactly? Have you tried anything, done any research? Please see How to Ask, help center. — AMC
– AMC, Commented May 1, 2020 at 22:04

nonin · Accepted Answer · 2020-05-01 20:02:22Z

3

By considering a single column col, the operation can be performed simply with a numpy's array.

col = 1
0.5 * (arr[:, col+1] - arr[:, col-1])
[Out]: array([2., 1., 1.])

And finally, to do the whole operation:

0.5 * (arr[:, 2:] - arr[:, :-2])

answered May 1, 2020 at 20:02

nonin

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1 Comment

Manvi Over a year ago

Thanks so much for help.

Kanzey · Accepted Answer · 2020-05-01 20:19:32Z

1

I believe you have some misconception about what broadcasting is, you should get clear about it. You can use slices to do what you want, slices will give you access to views which will not take memory, but subtraction product will need to be stored in memory. If you want it to be more efficient than this, you should probably write it in c with multithreading.

https://repl.it/repls/YawningAcceptableLivecd

take a look that b and c have their base as a.

import numpy as np
a = np.array([[2, 4, 6, 5, 8],
       [5, 3, 7, 4, 4],
       [3, 9, 5, 2, 1]])
b = a[:,2::]
c = a[:,:-2:]
print b
print b.base is a
print c
print c.base is a
res = (b-c)/2.0
print res

answered May 1, 2020 at 20:19

Kanzey

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