incrementing along a diagonal

[Travis Oliphant]

David Novakovic wrote:
> Hi,
>
> i'm moving some old perl PDL code to python. I've come across a line
> which changes values in a diagonal line accross a matrix.
>
> matrix.diagonal() returns a list of values, but making changes to these
> does not reflect in the original (naturally).
>
> I'm just wondering if there is a way that i can increment all the values
> along a diagonal?
>   

You can refer to a diagonal using flattened index with a element-skip 
equal to the number of columns+1.

Thus,

a.flat[::a.shape[1]+1] += 1

will increment the elements of a along the main diagonal.

-Travis




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