Using a vector of indices to add values to a matrix

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.

Answer 1

[Paul]

xs = N(:,1);
ys = N(:,2);
val = N(:,3);
siz = size(P);
ind = sub2ind(siz,ys,xs);
P(ind) = P(ind)+val.';

[Jos (10584)]

Note that the addition will not work properly for duplicates of (xs,ys) / ind

[Christopher]

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???

Answer 2

[Jos (10584)]

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))