Question

What is the inverse operation to vec in Octave?

E.g. if I need to convert 12×1 vector into 3×4 matrix, what should I do?

Answer 1

I don’t know Octave, but I think you’re looking for reshape.

— Built-in Function: reshape (A, m, n, …)
— Built-in
Function: reshape (A, [m n …])
— Built-in Function:
reshape (A, …, [], …)
— Built-in Function: reshape (A, size)

Return a matrix with the specified dimensions (m, n, …) whose elements are taken from the matrix A. The elements of the matrix are accessed in column-major order (like Fortran arrays are stored).

The following code demonstrates reshaping a 1×4 row vector into a 2×2 square matrix.

      reshape ([1, 2, 3, 4], 2, 2)
           ⇒  1  3
               2  4

Note that the total number of elements in the original matrix (prod (size (A))) must match the total number of elements in the new matrix (prod ([m n …])).

A single dimension of the return matrix may be left unspecified and Octave will determine its size automatically. An empty matrix ([]) is used to flag the unspecified dimension.