incrementing along a diagonal

Travis Oliphant oliphant.travis at
Wed Oct 11 21:44:54 CDT 2006

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.


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

will increment the elements of a along the main diagonal.


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