Vectorization fail with sub2ind?
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I have the for loop code
for m=1:numel(N,1)
y = N(m,1); % y matrix index
x = N(m,2); % x matrix index
dy = N(m,4); % codfficient
dx = N(m,3); % coefficient
wts(y,x) = wts(y,x)+dx.*dy;
end
I attempted to vectorize this as follows
ind = sub2ind(size(wts),N(:,2),N(:,1));
wts(ind) = wts(ind)+N(:,3).*N(:,4);
Unfortunately, for some reason the result is totally wrong.
This is a followup to a previous question : http://www.mathworks.com/matlabcentral/answers/119845-using-a-vector-of-indices-to-add-values-to-a-matrix
Someone later said that repeated values of "(x,y) / ind" will be erroneous. I don't know what that means, but there are many repeated values of the sub2ind index in my code (lots of addition).
How can this be fixed?
4 个评论
per isakson
2014-3-5
编辑:per isakson
2014-3-5
Why not show an executable example based on a for-loop?
采纳的回答
Roger Stafford
2014-3-5
编辑:Roger Stafford
2014-3-5
Here's a simple example to show you how repeated values of 'ind' will produce different results from your for-loop:
wts = zeros(3,1);
ind = [2;2;2];
wts(ind) = wts(ind) + [3;7;19];
wts
ans =
0
19
0
In your vectorization you would have been expecting [0;29;0] as in a for-loop, but instead only the last addition is performed, the other two results having been overwritten.
To do what you want, use matlab's 'accumarray' function:
wts = wts + accumarray([N(:,1),N(:,2)],N(:,3).*N(:,4),size(wts));
It will perform all the additions, even with repetitions.
(Note: You appear to have N(:,2) and N(:,1) in reversed order in your call to 'sub2ind' which would produce further errors.)
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