Indexing of unknown dimensional matrix

Question

I have a non-fixed dimensional matrix M, from which I want to access a single element. The element's indices are contained in a vector J. So for example: ``` M = rand(6,4,8,2); J = [5 2 7 1]; output = M(5,2,7,1) ``` This time M has 4 dimensions, but this is not known in advance. This is dependent on the setup of the algorithm I'm writing. It could likewise be that ``` M = rand(6,4); J = [3 1]; output = M(3,1) ``` so I can't simply use ``` output=M(J(1),J(2)) ``` I was thinking of using sub2ind, but this also needs its variables comma separated.. @gnovice this works, but I intend to use this kind of element extraction from the matrix `M` quite a lot. So if I have to create a temporary variable `cellJ` every time I access `M`, wouldn't this tremendously slow down the computation?? I could also write a separate function ``` function x= getM(M,J) x=M(J(1),J(2)); % M doesn't change in this function, so no mem copy needed = passed by reference end ``` and adapt this for different configurat…

Accepted Answer

For the general case where `J` can be any length (which I assume always matches the number of dimensions in `M`), there are a couple options you have:

You can place each entry of `J` in a cell of a cell array using the `num2cell` function, then create a comma-separated list from this cell array using the colon operator:

```
cellJ = num2cell(J);
output = M(cellJ{:});
```

You can sidestep the `sub2ind` function and compute the linear index yourself with a little bit of math:

```
sizeM = size(M);
index = cumprod([1 sizeM(1:end-1)]) * (J(:) - [0; ones(numel(J)-1, 1)]);
output = M(index);
```

Indexing of unknown dimensional matrix

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