[SciPy-Dev] scipy.sparse spdiags/dia_matrix additional calling mode
Tony Stillfjord
tony@maths.lth...
Thu Oct 6 03:59:07 CDT 2011
Dear all,
Given that Python is not MATLAB, would it be possible to add an additional,
in some
cases more intuitive, way of calling the spdiags function? This function
creates a sparse
matrix given its non-zero diagonals.
Currently, the first input has to be an array 'data' where every row
corresponds to a
diagonal - which one is specified in the next input, 'diags'. However, as
the diagonals
have different length, the actual values have to be zero-padded on either
the left or
right end to fit the array. This is not a big problem if all the elements in
a diagonal are
the same, for example
spdiags([ones(4), -2*ones(4), ones(4)], (-1,0,1), 4, 4)
works nicely as some values are just ignored.
But when that is not the case, and all the values are already stored in some
arrays, it
does not make sense that the user should have to pad them with a number of
zeros
(remembering on which end to put them!) and then also construct one more
temporary
array to hold them.
I suggest that spdiags accepts a list/tuple as data input, where every list
element is
supposed to be an array holding a diagonal of the correct size. That is,
instead of doing
something like
spdiags(row_stack(r_[zeros(2), a], b, r_[c, zeros(3)]), (-2, 0, 3), n, n )
one would write
spdiags([a,b,c], (-2, 0, 3), n, n)
Does that make sense? What it boils down to is that you should not have to
know
how the matrix type is implemented to use it - right? The diagonals could
just as well
be stored in different ways than they are currently.
When I had a look at the source code, it seems that all the work is done in
the
dia_matrix __init__ method, but the same argument applies there.
Currently, you -can- send in a list [a,b,c] where a, b and c are arrays of
different
length, but the function atleast_2D() is applied to the list which turns it
into an array
of objects. This obviously causes problems.
Kind regards,
Tony Stillfjord
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