Inverse of -vec()- function?

[Alfonso Sánchez-Peñalver]

Hi everyone,

I was wondering if there is a function in Mata that does the opposite of vec(), taking the number of columns as an argument. Consider the following example

: A = 1 , 4 \ 2 , 5 \ 3 , 6

: A
       1   2
    +---------+
  1 |  1   4  |
  2 |  2   5  |
  3 |  3   6  |
    +---------+

: vec(A)
       1
    +-----+
  1 |  1  |
  2 |  2  |
  3 |  3  |
  4 |  4  |
  5 |  5  |
  6 |  6  |
    +-----+

: colshape(vec(A),2)
       1   2
    +---------+
  1 |  1   2  |
  2 |  3   4  |
  3 |  5   6  |
    +---------+

: rowshape(vec(A),3)
       1   2
    +---------+
  1 |  1   2  |
  2 |  3   4  |
  3 |  5   6  |
    +---------+

You can see that neither colshape() nor rowshape() return the original matrix. I have found the following workaround by transposing the vector into a row one and using rowshape:

: a = vec(A)'

: rowshape(a,2)'
       1   2
    +---------+
  1 |  1   4  |
  2 |  2   5  |
  3 |  3   6  |
    +---------+

But I was wondering if there was direct way. If not, this workaround could be the base for a built-in method invvec(), please?

[Scott Merryman]

You can just directly transpose the rowshape or colshape results:

mata
    A = 1 , 4 \ 2 , 5 \ 3 , 6
    A
    B = vec(A)
    B
    rowshape(B,2)'
    colshape(B,3)'
end