MATLAB: How to replace for loop with vectorization

for loopvectorization

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?

Best Answer

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!
Related Question