Is there a more efficient way to extract a column from a cube in Armadillo than Q.slice(a).col(b)?
Something like tube, but for the last 2 dimensions instead of the first 2.
More efficient way to do `Q.slice(a).col(b)` using Armadillo
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1 Answers
Obtain a pointer to the first element of the desired column of the desired slice. Then pass that pointer to one of the advanced constructors of the vec class. Example:
cube C(6, 5, 4, fill::randu); // cube with 4 slices
vec v1 = C.slice(2).col(3); // normal way of extracting a vector
vec v2( &C(0,3,2), C.n_rows, false, false); // alternative way
Note that while this works, it's not safe. If C is resized or deleted, v2 will be using unallocated memory, or memory from a different object. In the first case it may result in a segfault. In the second case it's an information leak.
If C is a const cube, use const_cast to strip away the const from the obtained pointer. Again, this is not safe.
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