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
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