# st: re: optional arguments in Mata

 From Kit Baum To statalist@hsphsun2.harvard.edu Subject st: re: optional arguments in Mata Date Mon, 1 Oct 2007 19:54:33 -0400

Le said

I am trying to program a Mata function with optional arguments. For
example, I want to: (1) read the data y and x (2) create a matrix
equals x and J(rows(x),1,1) (constant) (3) create a matrix x'*y

void myfunction(string scalar depvar, | string scalar varlist)
{
st_view(y=.,.,depvar,.)
st_view(x=.,.,varlist,.)

x=x,J(rows(x),1,1)

x'*y
}

The program will work fine if i specify the varlist
myfunction("y","x")

However, it won't work if I don't specify the varlist because
"st_view(x=.,.,varlist,.)" requires an argument. How should I solve
this problem?

But what should the function do if a varlist is not provided? Should it just create a vector of 1s? If so you want to make the st_view of x conditional on whether a varlist was provided, and likewise generate x = J(rows(y),1,1). I believe this is doing the right thing:

mata: mata clear
mata:
real matrix myf(string scalar depvar, | string scalar varlist)
{
st_view(y=.,.,depvar)
if (args()==1) {
x = J(rows(y),1,1)
}
else {
st_view(x=.,.,varlist)
x = x,J(rows(x),1,1)
}
return(x'*y)
}
end

See [M-2] optargs.

Kit

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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