Armadillo C++:- Efficient access of columns in a cube structure

Question

Using Armadillo matrix library I am aware that the efficient way of accessing a column in a 2d matrix is via a simply call to .col(i). I am wondering is there an efficient way of extracting a column stored in a "cube", without first having to call the slice command? I need the most efficient possible way of accessing the data stored in for instance (using matlab notation) A(:,i,j) . I will be doing this millions of times on a very large dataset, so speed and efficiency is of a high priority.

Accepted Answer

I think you want

```
 B = A.subcube( span:all, span(i), span(j) );
```

or equivalently

```
 B = A.subcube( span(),  span(i),  span(j) );
```

where `B` will be a row or column vector of the same type as `A` (e.g. containing `double` by default, or a number of other available types).

Armadillo C++:- Efficient access of columns in a cube structure

Solution

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