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st: re: matrix manipulation


From   "Susan Olivia" <olivia@primal.ucdavis.edu>
To   statalist@hsphsun2.harvard.edu
Subject   st: re: matrix manipulation
Date   Fri, 12 Dec 2008 07:28:32 -0800

Thanks for the tips, Kit. I would like to use this
opportunity to ask 3 follow up questions:

1) For each cell (i,j) of matrix B, I would like to replace
its value with 1/(A[ij]) instead of dummy [0 1] as in
before. In other words, matrix B[i,j] = 1/A[i,j] if A[i,j] <
THRESHOLD. I seem having problem in doing this.

2) The syntax you suggested "underbar", I seem couldn't find
this entry in the STATA manual. Can you please explain what
is this doing? Perhaps once I know what "underbar" is really
doing, I can tackle #1.

3) Also, when the threshold condition met, the diagonal
element of matrix B is replaced to 1 or 1/aij. Ideally, I
want the diagonal element to be zero. Is there an efficient
way to do this? What I currently do is transform the matrix
into variables and use the loop to change the diagonal
element and then transform the variables into the matrix.
I.e.

svmat B

local i=1
     while `i'<=_N {
     qui recode B`i' 1=0 in `i'
     local i=`i'+1
     }

mkmat B*, matrix(B)


Thanks in advance for any programming tips.

Cheers,

Susan
On Thu, Dec 11, 2008 at 8:00 AM, Kit Baum <baum@bc.edu>
wrote:

    <>
    Susan said

    I would like to create a binary matrix (say matrix B)
using
    the existing information from current matrix A.

    Specifically, for each cell (i,j) of matrix B, I want to
    replace its value with 1 if the value of the
corresponding
    cell (i,j) of matrix A is less than the specified
threshold
    value. In other words, matrix B[i,j] = 1 if A[i,j] <
    THRESHOLD.


    No subscripting required:

    --------------------------------
    mata: mata clear
    mata:
    void underbar(string scalar mat,
                 real scalar thresh)
    {
           st_matrix("B", (st_matrix(mat) :< thresh))
    }
    end

    sysuse auto,clear
    qui reg price trunk weight length turn
    mat list e(V)
    mata: underbar("e(V)",0.0001)
    mat list B
    -----------------------------------

    You could pass the name of the result matrix to Mata as
well if you wanted this to be a bit more general.

    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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