How to add matrices with Python

Question

How to add matrices of different sizes in python? I have a 4 by 4 matrix of zeros and want to add a 2 by matrix at [0,0] and a 2 by 2 matrix at [1,1]

input:
Kglobal = np.zeros([4, 4])
Keq1 = np.array([2, -2], [-2, 2])
Keq2 = np.array([3, -3], [-3, 3])
Keq3 = np.array([6, -6], [-6, 6])

i tried:

Kglobal[0,0], Kglobal[1,1], Kglobal[2,2] = Keq1, Keq2, Keq3

The result needs to be

[2   -2   0   0
-2  2+3  -3   0 
0    -3 3+6  -6
0    0   -6   6]

Thanks for the help!!

BrokenBenchmark · Accepted Answer · 2022-09-17 19:06:08Z

1

Using the fact that we can assign to slices of NumPy arrays, we can do the following:

import numpy as np
Kglobal = np.zeros([4, 4])
Keq1 = np.array([[2, -2], [-2, 2]])
Keq2 = np.array([[3, -3], [-3, 3]])
Keq3 = np.array([[6, -6], [-6, 6]])

for idx, arr in enumerate([Keq1, Keq2, Keq3]):
    row_shift, col_shift = arr.shape
    Kglobal[idx:idx+row_shift, idx:idx+col_shift] += arr

print(Kglobal)

This outputs:

[[ 2. -2.  0.  0.]
 [-2.  5. -3.  0.]
 [ 0. -3.  9. -6.]
 [ 0.  0. -6.  6.]]

answered Sep 17, 2022 at 19:06

BrokenBenchmark

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2 Comments

user12581768 Over a year ago

Thank you, it works. Do you also know how to convert it into a 'for i in range' loop? With Kglobal being a matrix of (i+1, i+1) and there are Keqi arrays to add up

BrokenBenchmark Over a year ago

I suppose it's possible, but I think that isn't good style. If this is for practice, I will leave it as an exercise for you. :)

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