Low computation efficiency of simple sentence in MATLAB

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Albert
Albert 2016-10-20
编辑： Thorsten 2016-10-21
Hi, everyone
I have an computation efficiency problem here, I wish someone could help me out.
here is part of my MATLAB code (the actual code is much bigger than the following):
lambda_new1=ones(32*32*32,1);
W1=zeros(32*32*32,t);
W2=zeros(32*32*32,t);
W3=zeros(32*32*32,t);
W4=zeros(32*32*32,t);
temp=reshape(sum(repmat(lambda_old1,1,4).*[W1(:,i),W3(:,i),W2(:,i),W4(:,i)]),2,2);
How come that the last sentence in above code is running so slow, which accounts for 35% total running time? Actually my code is much bigger than the above and there are more complicated computation, but why such a short and concise computation takes so much time?
Is there any better way to improve computation efficiency?
Also the line numbers in my MATLAB (2016a and 2016b) would disappear after a while, if I relaunch the software, the line number would come back. Is this an intrinsic bug in MATLAB? How can I fix it?
Greatly appreciate your help
Thank you
Albert Liew

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Thorsten
Thorsten 2016-10-20
编辑：Thorsten 2016-10-21
Instead of
X = repmat(lambda_old1,1,4).*[W1(:,i),W3(:,i),W2(:,i),W4(:,i)];
if might be faster to use
X = bsxfun(@times, [W1(:,i),W3(:,i),W2(:,i),W4(:,i)], lambda_old1);
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Albert
Albert 2016-10-20
Hi, Thorsten
Thank you very much for your answer. It does work much faster. The reason why is that, I guess, by using 'bsxfun', we simply circumvent the process of copying and pasting a large matrix, which is time consuming.
Thank you again.

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