Write vector to variable, svmat with if condition

[Oleg Badunenko]

Dear stata-users,

I would like to write a stata matrix (actually a column vector) into a variable. Its number of rows (k) is smaller then number of observations (n) in the data. I also have a sample mark `touse'.

Since svmat does not allow "if `touse'", when I use svmat, stata writes from the first observation and leaves n-k missing at the end.

I have written a small C code (vectovar.c):

#include "stplugin.h"


STDLL stata_call(int argc, char *argv[])
{
int i, j, n, m, nn;

n = SF_row(argv[0]);
m = SF_col(argv[0]);

// allocate memory depending on whether row- or colvector
if(n==1){
nn = m;
} else {
nn = n;
}
double * tmp = (double *) malloc(nn * sizeof(double));

if(n==1){
for(i=0; i<m; i++){
SF_mat_el(argv[0], 1, i+1, &tmp[i]);
}
} else {
for(i=0; i<n; i++){
SF_mat_el(argv[0], i+1, 1, &tmp[i]);
}
}

// store the result in stata variable
i = 0;
for(j=1; j <= SF_nobs(); j++){
if(SF_ifobs(j)){
SF_vstore(1, j, tmp[i]);
i++;
}
}
return 0;
}

and when I call it using

plugin call vectovar `myvarname' if `touse', `mymatrixname'

it does what I need.

I think that using C for this simple task is an overkill. Is there a stata/mata solution.

Thanks,
Oleg

[Christophe Kolodziejczyk]

You can use st_matrix() and st_store() mata functions to achieve what you are doing in your C-plugin.I am not sure though how the marksample touse fit into your program. Does each row of the vector correspond to a specific observation in your dataset?

[Oleg Badunenko]

Suppose mymatrix is

1
2
3


and `touse' is

1
0
1
1

so myvarname in the data should be
1
.
2
3

while svmat would produce missing for the last, not the second observation

1
2
3
.

I think I need to use st_view to write the values directly into the variable, but I am not sure.

Oleg

[Christophe Kolodziejczyk]

I think the code below is more effective than using st_view and looping. It supposes that the number of rows of your matrix is equal to the number of ones in touse.

mata: b = st_matrix(st_local("mymatrixname"))
mata: st_store(.,st_local("myvarname"),st_local("touse"),b)

[Christophe Kolodziejczyk]

with st_view

gen `myvarname' = .
mata:
st_view(c=.,.,,st_local("myvarname"),st_local("touse"))
for (i=1;i<=rows(b); i++)
{
c[i,1]=b[i,1]
}

[Oleg Badunenko]

Thanks Christophe, the solution

quietly generate `myvarname' = .
mata: temp1 = st_matrix("mymatrixname")
mata: st_store(., st_local("myvarname"), st_local("touse"), temp1)

seems to work OK. I did not know st_store had the third option: now I see it in help file.

Cheers,
Oleg

[Oleg Badunenko]

I can even save one line:

mata: st_store(., st_local("myvarname"), st_local("touse"), st_matrix("mymatrixname"))

[Christophe Kolodziejczyk]

I have tested the other solution and it should give the same result. It is one nice feature of views to be able to change them directly.

[Oleg Badunenko]

As it looks, it should work, I have not tried it. I prefer the one-line solution
Thanks