How to pass the matrix arguments to the mata function in the foreach loop?

[Kazuto Sumita]

Dear stata users:

I always enjoy looking this forum and learn a lots!
Let me ask the question I have today.

I have several matrices from years 2003-2016 named as:
W_2003, X_2003, …, W_2016, X_2016.
“W_”s are spatial weight matrices and matrices “X_” include variables related to the place.
I would like to make products WX for each year.
Following is the mock example of my code:

/*** mata function ***/
mata
void wx(string matrix A, string matrix B) {
A_mat = st_matrix(“A”)
B_mat = st_matrix(“B”)
AB_mat = A_mat*B_mat
st_matrix("AB_mat", AB_mat)
}
end
/*** stata session****/
/* matrices W_2003, X_2003, … are already defined as “matrix” in the stata */
forvalues Y=2003/2016 {
mata: wx(“W_`Y’”, “X_`Y’”)
}

However, this code did not work and following errors/notes appeared:

note: argument A unused
note: argument B unused

I suppose I probably have problems of passing the matrices to my Mata function, wx.
Although I know there are several similar questions had already been posted,
I couldn’t find the cases using matrices for arguments as far as I know.
Could anyone suggest remedies to deal with the problem?

Thanks in advance.

Kazuto

[Christophe Kolodziejczyk]

Maybe you should consider a thrid argument for the name of the new matrix. I guess you want to keep the results of the products.
You pass "A" instead of A, which triggers the message since the matrix A is not used during the program. Moreover the Stata matrices A and B do not exist which will trigger an error when you run your program.
The argument you pass to st_matrix is a string scalar. In the first argument it should be A, which is a string scalar (= "W_2003" for example) and not "A".

mata
void wx(string scalar A, string scalar B) {
A_mat = st_matrix(A)
B_mat = st_matrix(B)
AB_mat = A_mat*B_mat
st_matrix("AB_mat", AB_mat)
 }
end
/*** stata session****/
/* matrices W_2003, X_2003, … are already defined as “matrix” in the stata */
forvalues Y=2003/2016 {
mata: wx(“W_`Y’”, “X_`Y’”)
 }

[Kazuto Sumita]

Dear Christophe
Thanks so much! I misunderstood the way to use "string scalar".
I could run the code successfully (I added a line to save the calculated matrix as the name with year).

 
  mata void wx(string scalar A, string scalar B) {
A_mat = st_matrix(A)
B_mat = st_matrix(B)
AB_mat = A_mat*B_mat
st_matrix("AB_mat", AB_mat)
} end
 /*** stata session****/
 /* matrices W_2003, X_2003, … are already defined as “matrix” in the stata */
forvalues Y=2003/2016 {
mata: wx(“W_`Y’”, “X_`Y’”)
matrix AB_mat_`Y' = AB_mat
}

Thanks again!