Convert 1d array to lower triangular matrix

Question

I would like to convert a 1 dimensional array into a lower, zero diagonal matrix while keeping all the digits.

I am aware of numpy.tril function, but it replaces some of the elements with zeros. I need to expand the matrix to contain all the original digits.

For example:

[10,20,40,46,33,14,12,46,52,30,59,18,11,22,30,2,11,58,22,72,12]

Should be

0
10 0
20 40 0
46 33 14 0
12 46 52 30 0
59 18 11 22 30 0
2 11 58 22 72 12 0

Answer 1

You could also use the numpy function np.tril_indices:

arr = np.array([10,20,40,46,33,14,12,46,52,30,59,18,11,22,30,2,11,58,22,72,12])
n = int(np.sqrt(len(arr)*2))+1

Generate zeros matrix and fill the lower part with your values:

idx = np.tril_indices(n, k=-1, m=n)
matrix = np.zeros((n,n)).astype(int)
matrix[idx] = arr
array([[ 0,  0,  0,  0,  0,  0,  0],
       [10,  0,  0,  0,  0,  0,  0],
       [20, 40,  0,  0,  0,  0,  0],
       [46, 33, 14,  0,  0,  0,  0],
       [12, 46, 52, 30,  0,  0,  0],
       [59, 18, 11, 22, 30,  0,  0],
       [ 2, 11, 58, 22, 72, 12,  0]])