MATLAB: How to remove for loops

Question

Hello. I would like to remove the for loops from this script. Could anyone provide me information on how to do it? The loop takes a long time to run. Thank you!

n=12000;
resamples=5000;
      Z=zeros(1,5);
      Y=zeros(resamples,5);
      for j=1:resamples
          for k=1:n
              r=randi(n);
              Z=Z+B(r,:);
          end
          Y(j,:)=Z;
      end

Answer 1

 idx=randi(n,n*resamples,1);
 Y=sum(reshape(B(idx,:),n,[]),1);
 Y=reshape(Y,resamples,[]);