CODEWITHSUNDEEP
performancematlab
Felipe
Felipe

Reputation: 196

MATLAB scaled repmat

Is there an easy way to have a scaled repmat in matlab?

The idea is, given matrices A and B

result = [A*B(1, 1) A*B(1, 2) A*B(1, 3) ... 

          A*B(1, n); A*B(2, 1)...];

Right now I accomplish this by using permute and reshape, but the permutation operation is 90% of my script time.

Upvotes: 1

Views: 119

Answers (2)

Divakar
Divakar

Reputation: 221624

You are probably performing the elementwise multiplications that results in a huge array and then permuting. Now, permute on a big array would incur heavy data transfers, which won't be efficient. So, I am guessing you are doing something along these lines :

reshape(permute(bsxfun(@times,A,permute(B,[3,4,1,2])),[1,3,2,4]),N,[]) 
           ^ (kills performance)

The alternative is to perform permute on A and B before performing elementwise multiplications between the input arrays, such that finally we would just need a reshape. Permuting on comparatively A and B should be much more efficient.

So, one efficient solution would be -

n = size(A,1)*size(B,1);
out = reshape(bsxfun(@times,permute(A,[1,3,2]),permute(B,[3,1,4,2])),n,[]);

Runtime test

Let's time the two approaches to verify our performance improvements and also include kron based method as suggested in @Shai's post, which seems like a clean solution -

% Max sized input arrays that my system could handle
A = randi(9,70,70);
B = randi(9,70,70);

disp('-------------- With permute on A and B and finally reshaping')
tic
n = size(A,1)*size(B,1);
out0 = reshape(bsxfun(@times,permute(A,[1,3,2]),permute(B,[3,1,4,2])),n,[]);
toc, clear out0 n

disp('-------------- With permute on elementwise result')
tic
n = size(A,1)*size(B,1);
out1 = reshape(permute(bsxfun(@times,A,permute(B,[3,4,1,2])),[1,3,2,4]),n,[]);
toc, clear out1 n

disp('-------------- With kron from @Shai post')
tic,
result1 = kron(B, A);
toc

Timings -

-------------- With permute on A and B and finally reshaping
Elapsed time is 0.161598 seconds.
-------------- With permute on elementwise result
Elapsed time is 0.413746 seconds.
-------------- With kron from @Shai post
Elapsed time is 0.612825 seconds.

Upvotes: 2

Shai
Shai

Reputation: 114866

I think you are looking for kron:

result = kron(B, A);

Upvotes: 3

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