array indexing with another array in multidimensional matrices

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gabriele fadanelli 2020-8-2

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评论： gabriele fadanelli 2020-8-2

Thank you in advance for helping:

I have a multidimensional matrix (let's say 4D as example) and I want to index its values through another matrix which contains all possible combinations, as an example:

A=randi(10,5,4,3,2) %%is 4D matrix

and I want to index its "submatrices" through another vector, without performing for loops. In this specific case my indexing matrix would be:

idx=[1,1;2,1;3,1;1,2;2,2;3,2] %%is indexing matrix

through which I would like to obtain my resulting matrices:

A(:,:,1,1);
A(:,:,2,1);
A(:,:,3,1);
A(:,:,1,2);
A(:,:,2,2);
A(:,:,3,2);

by adding the last two indices with my indexing matrix:

A(:,:,B(1,:));
A(:,:,B(2,:));
A(:,:,B(3,:));
A(:,:,B(4,:));
A(:,:,B(5,:));
A(:,:,B(6,:));

but the two forms don't give the same results and I would like to code a form similar to the last one to obtain the first results, i.e. singe 2D matrices.

Thanks

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Cris LaPierre 2020-8-2

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编辑：Cris LaPierre 2020-8-2

The issue here is that, when indexing, a vector of numbers acts on a single dimension. When indexing, a comma separates dimensoins, much like in a function you use a comma to separate the different inputs. You are expecting MATLAB to 'know' you want the first column to apply to one dimiension, and the second column to another. Instead, you must explicity state the input for each dimension, separating each with a comma:

A(:,:,B(:,1),B(:,2))

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gabriele fadanelli 2020-8-2

I am sorry I didn't want to get people confused in trying to address a problem I couldn't explain without an example. Your help was very appreciated, thanks.

gabriele fadanelli 2020-8-2

I actually need to extract specific 2D matrices, but through reshaping I can manage, thanks.

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Bruno Luong 2020-8-2

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编辑：Bruno Luong 2020-8-2

A=randi(10,5,4,3,2)
idx=[1,1;2,1;3,1;1,2;2,2;3,2]

Then

sz=size(A);
Aidx = A(:,:,sub2ind(sz(3:4),idx(:,1),idx(:,2)))

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Bruno Luong 2020-8-2

I have impression you do something overly complicated.

You replace the indexing of last p-dimension by all combinations possible of the p last indexes.

This is insanely redundant, you already have your data with all the combination in the original form. Simple use permute/reshape will put them in the form you want no need those combination/indexing.

gabriele fadanelli 2020-8-2