insert singelton dimension for broadcasting
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Let A be of size (M x N x P) and B be of size (M x N x K). What's the most conveniant way to broadcast C = A.*B such that C is of size (M x N x P x K) ?
In python it's
C = A[:,:,np.newaxis,:]*B[:,:,:,np.newaxis]
I can get it working by using reshape so that A is of (M x N x 1 X P), but it's a bit annoying because
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Réponses (3)
Catalytic
le 23 Mar 2023
You could also create your own specialized function that does it -
[M,N,P,K]=deal(2,3,4,5);
A=rand(M,N,P);
B=rand(M,N,K);
C=tensortimes(A,B);
size(C)
function C=tensortimes(A,B)
[m,n,p]=size(A);
[mm,nn,k]=size(B);
assert(mm==m && nn==n, 'First 2 dimensions don''t match');
C=A.*reshape(B,m,n,1,k);
end
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Catalytic
le 23 Mar 2023
Modifié(e) : Catalytic
le 23 Mar 2023
Inserting dimensions seems like as much a pain as reshape, but if you must do it that way, here's an approach closer to the Python style -
[M,N,P,K]=deal(2,3,4,5);
A=rand(M,N,P);
B=rand(M,N,K);
C=A.*newdim(B,3); size(C)
function A=newdim(A,n)
%insert a singleton dimension at one or more locations in size(A),
%designated by vector n.
n0=ndims(A);
n=unique(n);
N=max(n0+numel(n),max(n));
dims=nan(1,N);
dims(n)=1;
nanlocs=find(isnan(dims));
dims(nanlocs(1:n0))=size(A);
dims(isnan(dims))=1;
A=reshape(A,dims);
end
4 commentaires
Catalytic
le 27 Mar 2023
If MATLAB by default had the newdim.m functionality you proposed, that'd be I think very intuitive
This seems to imply that the newdim function does what you want. If so, I wonder if you'd consider clicking Accept. From the standpoint of your convenience, it shouldn't matter whether the function is provided by MathWorks or by me.
Catalytic
le 23 Mar 2023
Modifié(e) : Catalytic
le 23 Mar 2023
You didn't complete your description of why reshape() is "a bit annoying". If you're going to be doing the same operation repeatedly, this can cut down on the syntax somewhat (by reusing I) -
[M,N,P,K]=deal(2,3,4,5);
A=rand(M,N,P);
B=rand(M,N,K);
I=reshape(1:M*N*K, M,N,1,K);
C=A.*B(I);
size(C)
5 commentaires
DGM
le 29 Mar 2023
Modifié(e) : DGM
le 29 Mar 2023
You are correct. What's frustrating to automation is that despite MATLAB arrays implicitly having infinite trailing singleton dimensions, you can't borrow arbitrarily from them in a call to permute(). The permutation vector must contain no gaps or repetition, so you can effectively only borrow from the dims(X)+1 dimension.
Still, it's important to the novice to get familiar with both reshape() and permute(), especially if they're starting to learn how to use one without realizing the power of using both.
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