Fill matrix diagonal with different values for each python numpy
That the docs call the fill val
a scalar is an existing documentation bug. In fact, any value that can be broadcasted here is OK.
Fill diagonal works fine with array-likes:
>>> a = np.arange(1,10).reshape(3,3)
>>> a
array([[1, 2, 3],
[4, 5, 6],
[7, 8, 9]])
>>> np.fill_diagonal(a, [99, 42, 69])
>>> a
array([[99, 2, 3],
[ 4, 42, 6],
[ 7, 8, 69]])
It's a stride trick, since the diagonal elements are regularly spaced by the array's width + 1.
From the docstring, that's a better implementation than using np.diag_indices
too:
Notes
-----
.. versionadded:: 1.4.0
This functionality can be obtained via `diag_indices`, but internally
this version uses a much faster implementation that never constructs the
indices and uses simple slicing.
You can use np.diag_indices
to get those indices and then simply index into the array with those and assign values.
Here's a sample run to illustrate it -
In [86]: arr # Input array
Out[86]:
array([[13, 69, 35, 98, 16],
[93, 42, 72, 51, 65],
[51, 33, 96, 43, 53],
[15, 26, 16, 17, 52],
[31, 54, 29, 95, 80]])
# Get row, col indices
In [87]: row,col = np.diag_indices(arr.shape[0])
# Assign values, let's say from an array to illustrate
In [88]: arr[row,col] = np.array([100,200,300,400,500])
In [89]: arr
Out[89]:
array([[100, 69, 35, 98, 16],
[ 93, 200, 72, 51, 65],
[ 51, 33, 300, 43, 53],
[ 15, 26, 16, 400, 52],
[ 31, 54, 29, 95, 500]])
You can also use np.diag_indices_from
and probably would be more idomatic, like so -
row, col = np.diag_indices_from(arr)
Note : The tried function would work just fine. This is discussed in a previous Q&A - Numpy modify ndarray diagonal too.