[Numpy-discussion] offset in fill diagonal

[Julian Taylor]

On 21.01.2017 16:10, josef.pktd at gmail.com wrote:
> Is there a simple way to fill in diagonal elements in an array for other
> than main diagonal?
> 
> As far as I can see, the diagxxx functions that have offset can only
> read and not inplace modify, and the functions for modifying don't have
> offset and only allow changing the main diagonal.
> 
> Usecase: creating banded matrices (2-D arrays) similar to toeplitz.
> 

you can construct index arrays or boolean masks to index using the
np.tri* functions.
e.g.

a = np.arange(5*5).reshape(5,5)
band = np.tri(5, 5, 1, dtype=np.bool) & ~np.tri(5, 5, -2, dtype=np.bool)
a[band] = -1