3d Matrix - Extract Subarray and Multiply by Conjugate Transpose without Forloops

4 次查看（过去 30 天）

Science Machine 2022-9-6

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1797040-3d-matrix-extract-subarray-and-multiply-by-conjugate-transpose-without-forloops

编辑： Science Machine 2022-9-8

I have a 3d matrix A(i,j,k) of size [1:100,1:10000,1:989]. On the kth index I want to extract the 989 elements into a vector u and form the product u*ctranspose(u), for each of the

indices.

Using a double for loop, which is not something one should do in Matlab,

% this is an evil double for loop - I want to avoid doing this.
for ii=1:100
    for jj=1:10000
        u=squeeze(A(ii,jj,:));
        % somehow compute and store uu in a vectorized way, but how?
        uu=u*ctranspose(u); % note that uu is a 989x989 matrix, not a vector.
    end %ii
end %jj

This would be really slow. Is there a vectorized way to the above, so I am not doing a double for loop?

17 个评论
显示 15更早的评论隐藏 15更早的评论

Matt J 2022-9-8

Probably, but you haven't explained what t and r are.

Science Machine 2022-9-8

编辑：Science Machine 2022-9-8

t and ras parameters are the ii and jj indices from above u of size 1:100 and 1:10000, and the vector $$ has parameters jj also.

请先登录，再进行评论。

请先登录，再回答此问题。

采纳的回答

Matt J 2022-9-7

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1797040-3d-matrix-extract-subarray-and-multiply-by-conjugate-transpose-without-forloops#answer_1045435

编辑：Matt J 2022-9-7

在 MATLAB Online 中打开

[m,n,p]=size(A);
Ar=reshape(A,[],p).';
UU = reshape(Ar,p,1,[]).*conj(reshape(Ar,1,p,[]));
UU=reshape(UU,p,p,m,n); %obtain the (i,j)-th outer product as UU(:,:,i,j)