# 2D matrix multiply with 3D array

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Qiu Xu on 23 Oct 2021
Commented: Qiu Xu on 24 Oct 2021
Hi,
I have two 2D matrix u and v, and a 3D array B. I want to perform the following calculations, for example
u=rand(200,300);
v=rand(200,300);
B=rand(60,60,300)
c=0;
for j=1:300
u1=0;
v1=0;
for k=1:200
u1=u1+u(k,j)*B(:,:,k);
v1=v1+v(k,j)*B(:,:,k);
end
c=c+u1.*v1;
end
Due to two for loops, it is very slowly for the larger matrix u, v and B. Are there some good methods to compute c without using any for loop?.
Thanks a lot.
##### 1 CommentShowHide None
Matt J on 23 Oct 2021
Are you sure the size of B shouldn't be 60x60x200? If not, your code example ignores the final 100 pages of B.

Matt J on 23 Oct 2021
Edited: Matt J on 23 Oct 2021
[m,n,p]=size(B);
Br=reshape(B,[],p);
Br=Br(:,1:200); %might not be necessary
U1=Br*u;
V1=Br*v;
c=reshape( sum(U1.*V1,2) , m,n);
##### 1 CommentShowHide None
Qiu Xu on 24 Oct 2021
Thanks, I have make a mistake that the size of B should be 60x60x200. Your method is very good.