CODEWITHSUNDEEP
c++matrixarmadillo
oracle3001
oracle3001

Reputation: 1100

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

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.

Upvotes: 4

Views: 7582

Answers (2)

mtall
mtall

Reputation: 3620

.slice() should be pretty quick. It simply provides a reference to the underlying Mat class. You could try something along these lines:

cube C(4,3,2);
double* mem = C.slice(1).colptr(2);

Also, bear in mind that Armadillo has range checks enabled by default. If you want to avoid the range checks, use the .at() element accessors:

cube C(4,3,2);
C.at(3,2,1) = 456;

Alternatively, you can store your matrices in a field class:

field<mat> F(100);
F(0).ones(12,34);

Corresponding element access:

F(0)(1,2);      // with range checks
F.at(0).at(1,2); // without range checks

You can also compile your code with ARMA_NO_DEBUG defined, which will remove all run-time debugging (such as range checks). This will give you a speedup, but it is only recommended once you have debugged all your code (ie. verified that your algorithm is working correctly). The debugging checks are very useful in picking up mistakes.

Upvotes: 1

Dirk is no longer here
Dirk is no longer here

Reputation: 368231

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).

Upvotes: 5

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