Vectorize nested for loops with indices and subscripts
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How can I vectorize these nested loops?
A=[1 2 0 2;2 1 3 0;0 3 1 3;2 0 3 1];
matArray=unifrnd(-1,1,[4,4,10]);
m=size(matArray,3);
n=size(unique(A),1)-1;
out=zeros(m,n);
for i = 1 : m
mat=matArray(:,:,i);
for j = 1 : n
out(i,j) = sum(mat(A(:)==j));
end
end
Imo, the main challenge seems to be that if I vectorize i, then I don't seem how to combine indices and subscripts for mat(A(:)==j) to work. I guess one option could be to arrange m copies of A in the same shape as matArray but I am not sure whether that is the best approach.
Accepted Answer
A = [1,2,0,2;2,1,3,0;0,3,1,3;2,0,3,1]
matArray = unifrnd(-1,1,[4,4,10])
Method one:
[X,Y] = meshgrid(1:size(matArray,3),A(:));
Z = Y>0;
B = accumarray([X(Z),Y(Z)],matArray(Z))
Method two:
[X,Y] = meshgrid(1:size(matArray,3),1+A(:));
B = accumarray([X(:),Y(:)],matArray(:));
B = B(:,2:end)
Method three:
F = @(n)reshape(sum(matArray.*(n==A),1:2),[],1);
C = arrayfun(F,1:max(A(:)),'uni',0);
B = [C{:}]
Compared against your approach:
m=size(matArray,3);
n=size(unique(A),1)-1;
out=zeros(m,n);
for i = 1:m
mat = matArray(:,:,i);
for j = 1:n
vec = mat(A(:)==j);
tmp = sum(vec);
out(i,j) = tmp;
end
end
out
More Answers (1)
Brahmadev
on 24 Apr 2024
I think we can modify the loop as follows:
for j = 1:n
mask2 = A == j; % Logical indexing for each unique value in A
out2(:, j) = squeeze(sum(sum(space .* mask2, 1), 2)); % Assuming space to be similar to matArray
end
Hope this helps!
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