A substitute for this 'for' loop

Could anyone shed some light on how this 'for' loop can be replaced by a single command in MATLAB?

for i=1:size(w,3)

     x=w(:,:,i);

    w1(i,:)=x(B(i),:);

 end

 clear x

Here, w is 3D (x by y by z) matrix and B (1 by z) is a vector containing rows pertaining to each layer in w. This 'for' loop takes about 150 seconds to execute when w is 500000 layers deep. I tried using,

 Q = w(B,:,:);

 Q = reshape(Q(1,:),[500000,2])';

This creates a matrix Q of size 500000 X 2 X 500000 and MATLAB threw me an error saying memory out of bound. Any help would be appreciated!

Answer 1

[Roger Stafford]

It is important for efficiency’s sake that you initially allocate a sufficient amount of space for the w1 array before entering the for-loop:

[~,n2,n3] = size(w);
w1 = zeros(n3,n2); % <-- Initial memory allocation
for i=1:n3
  w1(i,:)=w(B(i),:,i); 
end

[Vijay]

Thank you very much for the answer! Cheers!