Why bother?
That loop takes less than seven 10,000ths of a second on my system. You could run this loop billions of times in the time it might take you to possibly gain a 10000th of a second by vectorizing.
More
Even more reason to not vectorize since memory requirements will be huge and slow it down more than a for-loop.
Your numbers didn't scale up but here's the loop with comparable numbers run the same amount of times:
indexes = randi(100,3000,150);
K = rand(3000,47);
Q = zeros(3000,1000+46);
tic
for jj = 1:10
for k = 1:100
Q(:,k:k+46) = Q(:,k:k+46) + K(indexes(:,k),:);
end
end
toc
Elapsed time is 1.330083 seconds.
I would guess any vectorization, if possible, would be significantly slower due to the memory requirements.
