[Numpy-discussion] Cython numerical syntax revisited
Francesc Alted
faltet@pytables....
Thu Mar 5 10:30:12 CST 2009
A Thursday 05 March 2009, David Cournapeau escrigué:
> I don't understand your argument about Row vs Column matters: which
> one is best depends on your linear algebra equations. You give an
> example where Fortran is better, but I can find example where C order
> will be more appropriate. Most of the time, for anything non trivial,
> which one is best depends on the dimensions of the problem (Kalman
> filtering in high dimensional spaces for example), because some parts
> of the equations are better handled in a row-order fashion, and some
> other parts in a column order fashion.
Yeah. Yet another (simple) example coming from linear algebra: a matrix
multiplied by a vector. Given a (matrix):
a = [[0,1,2],
[3,4,5],
[6,7,8]]
and b (vector):
b = [[1],
[2],
[3]]
the most intuitive way to do the multiplication is to take the 1st row
of a and do a dot product against b, repeating the process for 2nd and
3rd rows of a. C order coincides with this rule, and it is optimal
from the point of view of memory access, while Fortran order is not.
--
Francesc Alted
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