st: re: interaction of column matrix

[Kit Baum <kitbaum@mac.com>]

Abdel wrote

I've written this mata function as a solution to my own question  
yesterday.
horizontal direct product (hdp).
--------begin code------------
version 9.2
local mydir "."
mata:
mata clear
real matrix  hdp(real matrix A, real matrix B)
{
assert(rows(A)==rows(B))
real scalar n
real matrix C
n=rows(A)
C=J(n,1,.)
for (i=1; i<=cols(A); i++) {
C=C,(A[.,i]:*B)
}
C=C[.,2..cols(C)]
return(C)
}


I've never heard of a 'horizontal direct product' but in a 2x2  
example it seems to be the first and last rows of a conventional  
Kronecker product:

: a
       1   2
    +---------+
  1 |  1   2  |
  2 |  3   4  |
    +---------+

: b
       1   2
    +---------+
  1 |  5   6  |
  2 |  0   1  |
    +---------+

: c
        1    2    3    4
    +---------------------+
  1 |   5    6   10   12  |
  2 |   0    3    0    4  |
    +---------------------+

: a#b
        1    2    3    4
    +---------------------+
  1 |   5    6   10   12  |
  2 |   0    1    0    2  |
  3 |  15   18   20   24  |
  4 |   0    3    0    4  |
    +---------------------+


For a 3x3 example, it is the 1st, 5th and 9th rows of the Kronecker  
product. Thus it appears that whatever this construction might be it  
could be generated from a Kronecker.




Kit Baum, Boston College Economics and DIW Berlin
http://ideas.repec.org/e/pba1.html
An Introduction to Modern Econometrics Using Stata:
http://www.stata-press.com/books/imeus.html


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