Avoid for loop on sum on given indexes, use accumarray?

Question

0 个投票

Hi,

I'm trying to get rid of the for loop on the follwoing code:

S=zeros(100,50);
V=rand(1,50);
Index=round(50*rand(1,50))
for i=1:50
S(:,Index(i))=S(:,Index(i))+V(i);
end

I probably need to use accumarray but I cannot figure how.

Thanks a lot

2 个评论
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Walter Roberson 2019-7-10

Why are you creating 100 rows when you do the same thing to all of them? Why not create one row with the values and later replicate to 100 copies?

Lenaic Couedel 2019-7-10

It is a simplified version of my problem.

A more accurate one would be:

S=zeros(100,50);
V=rand(100,50);
Index=round(50*rand(1,50))
for i=1:50
    S(:,Index(i))=S(:,Index(i))+V(:,i);
end

Thanks

Answer 1

Walter Roberson 2019-7-10

0 个投票

Idx1 = repmat(1:size(V,1), 1, length(Index)) ;
Idx2 = repelem(Index, 1, size(V, 1));
S = accumarray([Idx1(:), Idx2(:)], V(:), size(V)) ;

Lenaic Couedel 2019-7-10

Great,

Thanks