### Monday, March 06, 2006

## RE: st: macros & expanding names in mata?

Matissa Hollister <m73hollis@yahoo.com> asked about how to store and access multiple matrices in mata.

Matissa described a macro type solution the problem, while indicating that this solutions was not feasible because Mata is not a macro language.

Below, I propose a simple solution to Matissa's problem that stores and accesses the submatrices in one big matrix.

Matissa has two vectors of parameters, say -x- and -y-. For each combination of the parameters, Matissa wants to create and store a matrix.

Let's suppose that x is 1 x p, y is 1 x q and that for each (x,y) a function called dist(x,y) creates an n x m matrix. The essence of the solution below is simply to stack the p*q n x m matrices into a big matrix called D. The details of storing and retrieving the matrices are in the example code below. I have added some comments to explain the process.

clear

mata:

x = ( 1, 2, 3) y = ( 4, 5)

// this is just an example function real matrix dist( real scalar x, real scalar y, real scalar n, real scalar m) { return((1/abs(x-y))*J(n, m, 1)) }

p = cols(x) q = cols(y)

n = 3 m = 4

// make D to hold individual matrices // note that the row dimension is p*q*n because we stack the individual // matrices

D = J(p*q*n, m, .)

// This loop puts the individual matrices into D for(i=1; i<=p; ++i) { for(j=1; j<=q; ++j) { frow = ((i-1)*q +j-1)*n + 1 // first row for this matrix lrow = ((i-1)*q +j)*n // last row for this matrix printf("when i = %1.0f and j = %1.0f, the matrix is\n", i, j) dist(x[i], y[j], n, m) D[|frow,1 \ lrow,m|] = dist(x[i], y[j], n, m) } }

printf("\n\nD now contains the individual matrices\n") D

printf("\n\nNow let's retrieve them elements of D and print them out\n") for(i=1; i<=p; ++i) { for(j=1; j<=q; ++j) { frow = ((i-1)*q +j-1)*n + 1 // first row for this matrix lrow = ((i-1)*q +j)*n // last row for this matrix printf("when i = %1.0f and j = %1.0f, the matrix is\n", i, j) D[|frow,1 \ lrow,m|] } } end

--David ddrukker@stata.com * * For searches and help try: * http://www.stata.com/support/faqs/res/findit.html * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

Tag: statalist