A substitute for this 'for' loop

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Vijay
Vijay 2017-5-18
评论： Vijay 2017-5-18
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!

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采纳的回答

Roger Stafford
Roger Stafford 2017-5-18
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

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