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How to replace for loop with vectorization?

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Is there any way I can replace this for loop with a vectorized approach?
v = zeros(4, 3);
ii = [1 ; 2 ; 3 ; 1 ; 3 ; 4];
res = [-1 -1 1 ; -1 -1 1 ; -1 -1 1 ; 1 1 -1 ; 1 1 -1 ; -1 1 -1];
for i = 1:3
v(:,i) = accumarray(ii , res(:,i));
end
I know if res was just scalar values, I could use this:
v= accumarray(ii , res);
But is it posible to remove the loop knowing that res has more than one columns?

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Walter Roberson
Walter Roberson on 17 Oct 2019
You can repmat(ii, size(res,2),1) and use res(:)
Sai Bhargav Avula
Sai Bhargav Avula on 23 Oct 2019
repmat may not be the right way as the accumarray is used here.

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Accepted Answer

Sai Bhargav Avula
Sai Bhargav Avula on 23 Oct 2019
Hi,
You can eliminate the for loop by using the code below.
ii = [1 ; 2 ; 3 ; 1 ; 3 ; 4];
res = [-1 -1 1 ; -1 -1 1 ; -1 -1 1 ; 1 1 -1 ; 1 1 -1 ; -1 1 -1];
[x, y] = ndgrid(ii,1:size(res,2));
v=accumarray([x(:) y(:)],res(:));
But I would recommend to use the for loop if the size of the matrix is small.
In the above case
The elapsed time in the case of for loop is 0.000248 sec. Where as in the case of where for loop is eliminated the elapsed time is 0.000750 sec.
Hope this helps!

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More Answers (1)

Bruno Luong
Bruno Luong on 23 Oct 2019
[iii,jj]=ndgrid(ii,1:size(res,2));
v=accumarray([iii(:) jj(:)], res(:))

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