Using a vector of indices to add values to a matrix

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Christopher
Christopher 2014-3-2
回答： Jos (10584) 2014-3-5
I am trying to vectorize a piece of code which has this form:
for m = 1:1:numel(N)
y = N(m,1);
x = N(m,2);
val = N(m,3);
P(y,x) = P(y,x)+val;
end
So, there is the matrix of column vectors N which hold coordinate information and a value. Using the coordinates the corresponding values are added to the matrix P.
How can this be done efficiently? Thanks.

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采纳的回答

Paul
Paul 2014-3-2
编辑：Paul 2014-3-2
xs = N(:,1);
ys = N(:,2);
val = N(:,3);
siz = size(P);
ind = sub2ind(siz,ys,xs);
P(ind) = P(ind)+val.';
  2 个评论
Jos (10584)
Jos (10584) 2014-3-2
Note that the addition will not work properly for duplicates of (xs,ys) / ind
Christopher
Christopher 2014-3-5
编辑：Christopher 2014-3-5
I think I have found that you are right. The result for the loop and the vectorized operation are completely different. Perhaps because there is a lot of repeated indices (thus addition) in my code. Does anyone know of a workaround???

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更多回答（1 个）

Jos (10584)
Jos (10584) 2014-3-5
ACCUMARRAY is, by far, the easiest approach:
N = [1 2 7 ; 1 2 4 ; 2 3 5 ; 2 1 6]
P = accumarray(N(:,1:2),N(:,3))

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