Armadillo C ++：如何使用来自另一个矩阵的多个元素修改矩阵的多个数组元素，特别是在立方体结构中？

Question

In MATLAB, if I have two 3x3 matrices A and B and want to copy the last two columns of B into the first two of A, I would use the following syntax:

A(:,1:2) = B(:,2:3)

I am trying to complete the same action using Armadillo in C++, specifically for a cube structure. In Armadillo, if I had two cubes A and B with nine slices (with each slice being a 3x3 matrix), I assumed I would use the following to perform the same column element update:

A(span(0,2),span(0,1),span(i)) = B(span(0,2),span(1,2),span(i))

where 'i' is just the slice index. The syntax is based on the Armadillo syntax guide.

The code compiles without error and runs; the cube slices are just not being updated. Am I using the correct Armadillo syntax here, and is this the most efficient way to perform these operations?

Answer 1

If you want a simple copy/paste (not a shift/rotation of your cube matrices), you can use this syntax that do the trick :

#include <armadillo>

int main (int argc, char* argv[])
{
    size_t num_slices = 9;

    arma::icube A (3, 3, num_slices);
    arma::icube B (3, 3, num_slices);

    A.zeros();
    B.randn();

    A.print("Cube A :\n");
    B.print("Cube B :\n");

    for (int s = 0; s < num_slices; ++s)
        A.slice(s)(arma::span::all, arma::span(0, 1)) = 
        B.slice(s)(arma::span::all, arma::span(1, 2));

    A.print("Cube A :\n");
    B.print("Cube B :\n");

    return 0;
}