Building a custom array from zeros and a given 1-d array

Question

I have a list that I want to build a kernel from [90,50,10]

What I want to do is to get a kernel that looks like -

[[90,0,0]
 [50,0,0]
 [10,0,0]
 [0,90,0]
 [0,50,0]
 [0,10,0]
 [0,0,90]
 [0,0,50]
 [0,0,10]]

I can implement this using a loop, AND I tried doing something in the lines of this -

k = np.array([90,50,10])
for i in k:
    d = np.zeros((3,3))
    np.fill_diagonal(d, i)
    print(d)

[[90.  0.  0.]
 [ 0. 90.  0.]
 [ 0.  0. 90.]]
[[50.  0.  0.]
 [ 0. 50.  0.]
 [ 0.  0. 50.]]
[[10.  0.  0.]
 [ 0. 10.  0.]
 [ 0.  0. 10.]]

The issue is that to reorder the rows, I will need another iteration and np.fill_diagonal is an in-place method so I cant use list comprehensions.

I am looking for a way I can solve this in a vectorized manner without any loops, using only numpy and in a single line of code.

EDIT:

The preferred solution should be able to build a 25X5 kernel if I use a 5X1 array like [90,50,40,30,10]

Ayoub ZAROU · Accepted Answer · 2020-08-14 11:38:14Z

2

you could do:

s = np.array([90,50,10])
size = s.size
o = np.zeros((size ** 2, s.size))
o[np.arange(size**2), np.repeat(np.arange(size), size)] = np.tile(s, size)
print(o)

[[90.  0.  0.]
 [50.  0.  0.]
 [10.  0.  0.]
 [ 0. 90.  0.]
 [ 0. 50.  0.]
 [ 0. 10.  0.]
 [ 0.  0. 90.]
 [ 0.  0. 50.]
 [ 0.  0. 10.]]

answered Aug 14, 2020 at 11:38

Ayoub ZAROU

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Comments

Mateen Ulhaq · Accepted Answer · 2020-08-14 11:04:30Z

1

Here's one possible one-liner for, e.g. k = np.array([90, 50, 10]):

np.concatenate([np.pad(k[:, None], ((0, 0), (i, 2-i))) for i in range(k.size)])

answered Aug 14, 2020 at 11:04

Mateen Ulhaq

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Comments

ansev · Accepted Answer · 2020-08-14 13:53:27Z

1

IIUC,

#k = np.array([90, 50, 10])
kernel = np.tile(np.identity(k.shape[0])*k, k.shape[0]).reshape(k.shape[0]**2, k.shape[0])
print(kernel)

[[90.  0.  0.]
 [90.  0.  0.]
 [90.  0.  0.]
 [ 0. 50.  0.]
 [ 0. 50.  0.]
 [ 0. 50.  0.]
 [ 0.  0. 10.]
 [ 0.  0. 10.]
 [ 0.  0. 10.]]

edited Aug 14, 2020 at 13:53

answered Aug 14, 2020 at 11:15

ansev

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2 Comments

Akshay Sehgal Over a year ago

Wish we could select 2 answers as top.

Mateen Ulhaq Over a year ago

Isn't the output here different from the question?

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