如何在犰狳 C++ 中修改矩阵中的某些列

Question

I have a very simple task in an Armadillo 10x10 matrix. The task is to change the values of the columns 1,3,4 and 9 to sqrt(2) .

I already done the task using (B is only ones)

B.cols(columns) *= sqrt(2);

I did this for every column and worked.

The problem here is that reading the documentation you can make this operation using

B.cols( vector of col indices )

and I want to learn how to use it but nothing seems to work

Answer 1

Declare the "vector of col indices" as uvec and put the column indices in it.

mat B(10, 11, fill::randu);

uvec column_indices = { 1, 3, 4, 9 };

B.cols(column_indices) *= sqrt(2);

(Just to be clear, C++ has zero-based indexing, so the first column has index 0, not 1).