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I have a file with a matrix: let's say matrix.dat and I want to extract the diagonal, subdiagonal and upper diagonal from it, to create a new tridiagonal matrix M. I know that with numpy you can get the diagonal, but is there any way to extract the data of the subdiagonal or upper diagonal.

Vadim Kotov
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Jack MR
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1 Answers1

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You can construct a tridiagonal matrix in general like this. 

from scipy.sparse import spdiags
import numpy as np
data = np.array([[1, 2, 3, 4], [1, 2, 3, 4], [1, 2, 3, 4]])
diags = np.array([0, -1, 1])
array = spdiags(data, diags, 4, 4).toarray()

array([[1, 2, 0, 0],
       [1, 2, 3, 0],
       [0, 2, 3, 4],
       [0, 0, 3, 4]])