How to create a diagonal array from an existing array in numpy
I am trying to make a diagonal numpy array from:
[1,2,3,4,5,6,7,8,9]
Expected Result:
[[ 0, 0, 1, 0, 0],
[ 0, 0, 0, 2, 0],
[ 0, 0, 0, 0, 3],
[ 4, 0, 0, 0, 0],
[ 0, 5, 0, 0, 0],
[ 0, 0, 6, 0, 0],
[ 0, 0, 0, 7, 0],
[ 0, 0, 0, 0, 8],
[ 9, 0, 0, 0, 0]]
What would be an efficient way to do this?
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2 answers
You can use integer array indexing to set the specified output elements:
>>> import numpy as np
>>> a = [1,2,3,4,5,6,7,8,9]
>>> arr = np.zeros((9, 5), dtype=int) # create empty array
>>> arr[np.arange(9), np.arange(2,11) % 5] = a # insert a
>>> arr
array([[0, 0, 1, 0, 0],
[0, 0, 0, 2, 0],
[0, 0, 0, 0, 3],
[4, 0, 0, 0, 0],
[0, 5, 0, 0, 0],
[0, 0, 6, 0, 0],
[0, 0, 0, 7, 0],
[0, 0, 0, 0, 8],
[9, 0, 0, 0, 0]])
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Inspired np.fill_diagonal
, which can wrap but not displace:
In [308]: arr=np.zeros((9,5),int)
In [309]: arr.flat[2:45:6]=np.arange(1,10)
In [310]: arr
Out[310]:
array([[0, 0, 1, 0, 0],
[0, 0, 0, 2, 0],
[0, 0, 0, 0, 3],
[0, 0, 0, 0, 0],
[4, 0, 0, 0, 0],
[0, 5, 0, 0, 0],
[0, 0, 6, 0, 0],
[0, 0, 0, 7, 0],
[0, 0, 0, 0, 8]])
(although for some reason this has the 4th all null string).
def fill_diagonal(a, val, wrap=False):
...
step = a.shape[1] + 1
# Write the value out into the diagonal.
a.flat[:end:step] = val
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