Gregor Lingl wrote:
Hi all of you,
I'm representing a 4x4 matrix as a 16-element list, e.g.
m=range(16)
first 4 elements first row, second four elements second row etc.
I want to sum rows and columns like
i-th row:
sum(m[4*i:4*i+4])
and ith column:
sum(m[i::4])
This seems to be slow because of the formation of the slices.
I wonder if there is a way using generators or generator-expressions
(which I didn't study yet) to compute these sums without copying
parts of the matrix to a new list. (I'd guess that there should exist
some canonical generator for sequences, which produces their elements
..., maybe also for slices ?)
All comments, hints, solutions are welcome.
Regards,
Gregor
The way I usually handle this is to have a 2-d array, such that every
list within the main list is a row:
>>> m = list(range(4) for i in range(4))
>>> m
[[0, 1, 2, 3], [0, 1, 2, 3], [0, 1, 2, 3], [0, 1, 2, 3]]
>>> m[0][0] = 1
>>> m
[[1, 1, 2, 3], [0, 1, 2, 3], [0, 1, 2, 3], [0, 1, 2, 3]]
>>> m = [range(4) for i in range(4)]
>>> m
[[0, 1, 2, 3], [0, 1, 2, 3], [0, 1, 2, 3], [0, 1, 2, 3]]
>>> m[0][0] = 1
>>> m
Either of the above works. Then, to some a row, you just do this:
>>> m
[[1, 1, 2, 3], [0, 1, 2, 3], [0, 1, 2, 3], [0, 1, 2, 3]]
>>> sum(m[0])
7
--
Email: singingxduck AT gmail DOT com
AIM: singingxduck
Programming Python for the fun of it.
_______________________________________________
Tutor maillist - Tutor@python.org
http://mail.python.org/mailman/listinfo/tutor
