Using cell array for indexing

Question

Lionel Pöffel 2021-8-2

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https://ww2.mathworks.cn/matlabcentral/answers/890542-using-cell-array-for-indexing

评论： Lionel Pöffel 2021-8-2

采纳的回答： Rik

I have matrices of varying dimensionality I need to index.

So just writing A(i1,...,in) is not an option because n can vary in my application.

What is known to work for getting a single element out of A is using a cell array:

IndArray={i1,...,in}

Elem=A(IndArray{:})

However I'd like to do something like SubMatrix= A(i1,i2,...,im,:,:,...,:), where m<n. Is there a way to do that?

The following does not work:

IndArray={i1,...,im}

SubMatrix=A(IndArray{:})

Thanks in advance for any hints.

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Answer 1

Rik 2021-8-2

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I would suggest something like this:

IndArray={i1,...,im}
for dim=(numel(IndArray)+1):ndims(A)
    IndArray{dim}=1:size(A,dim);
end
SubMatrix=A(IndArray{:})

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Answer 2

Stephen23 2021-8-2

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编辑：Stephen23 2021-8-2

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No need for a loop, here is the simple and efficient MATLAB approach:

IndArray = {i1,..,im};
IndArray(1+end:ndims(A)) = {':'};
SubMatrix = A(IndArray{:})

How it works: MATLAB always allows a scalar element on the RHS to be allocated to any number of elements on the LHS. This applies to any array class: numeric, cell (as above), char, string, struct, etc.

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Lionel Pöffel 2021-8-2

Hello Stephen,

thanks for your answer. That is, of course, an additional improvement.

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Answer 3

Lionel Pöffel 2021-8-2

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Hi Rik, that seems to work, thanks.

DGM posted an answer that surprisingly also seems to work but unfortunately deleted it (thanks to him/her as well).

For completeness I will note it here:

IndArray={i1,...,im}
for dim=(numel(IndArray)+1):ndims(A)
    IndArray(dim)={':'};
end
SubMatrix=A(IndArray{:})

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DGM 2021-8-2

Sorry about that. I tried reading your other posts to try to figure out what exactly you were trying to do and had the feeling that I had misunderstood the intent.

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