# Re: st: Mata function stata() within program

 From Ulrich Kohler To statalist@hsphsun2.harvard.edu Subject Re: st: Mata function stata() within program Date Wed, 22 Mar 2006 17:06:47 +0100

```Many thanks for this. I will carefully look at it. It seems that this will
make the program much faster, which would be _very_ important. The snippet
will often run more than a million times ...

William Gould, Stata wrote:
> Ulrich Kohler <kohler@wz-berlin.de> wrote,
>
> > I have a Mata function which looks as follows:
> >
> > -------------------------------------lsq.mata--
> > (...)
> > // Mata Function to extract the substitution costs from subcost-matrix
> > void showhash(real rowvector R)
> > {
> >        string scalar key1
> >        st_local("key1",key1)
> >        key1 = strofreal(R[1,2])
> >        st_local("key1",key1)
> >        stata("local hash1 = mod(`key1',197)")
> > }
> >
> > (...)
> >------------------------------------------------
>
> and Uli notes that when he runs it, he gets an error,
>
> > : R = 2,3,5,4
> > : showhash(R)
> >
> > invalid syntax
> >                  stata():  3598  Stata returned error
> >               showhash():     -  function returned error
> >                  <istmt>:     -  function returned error
> > r(3598);
>
> Alan Riley <ariley@stata.com> has already given a solution, and suggested
> the line
>
>          stata("local hash1 = mod(`key1',197)")
>
>
>          stata("local hash1 = mod(\`key1',197)")
>
> Alan's right, but his solution is too tricky for me.  Moreover, his
> solution shows he is still thinking an ado mode rather than Mata mode.
>
> My suggested solution is
>
>          stata("local hash1 = mod("  +  key1  +  ", 197)")
>
> and, with my solution, Uli's code can be simplified to read,
>
>
>         void showhash(real rowvector R)
>         {
>                string scalar key1
>
>                key1 = strofreal(R[1,2])
>                stata("local hash1 = mod("  +  key1  +  ", 197)")
>         }
>
> or even
>
>         void showhash(real rowvector R)
>         {
>                stata("local hash1 = mod("  +  strofreal(R[1,2])  +  ",
> 197)") }
>
> Let me expound on the ado versus the Mata way of thinking.
>
> Uli wanted to run the Stata command
>
>         local hash1 = mod(_______, 197)
>
> where he substituted the value from Mata matrix R[1,2] for ______.
> Why Uli wanted to do this, we don't know, nor care.
>
> The ado way of thinking says we substitute a macro for _____, and arrange
> for the macro to contain R[1,2], so when the macro is substituted by
> Stata, we obtrain the desired result.  Good way of thinking, when you
>
> The Mata way of thinking is more direct:  we need to construct a string
>
>         "local hash1 = mod(_______, 197)"
>
> where where R[1,2] is substitued for _____, and we can just use the
> standard operators to do that,
>
>         "local hash1 = mod("  +  strofreal(R[1,2])  +  ", 197)"
>
> Here's a good rule:  It's perfectly okay to obtain input from macros, or
> post output to macros.  That is one way Mata can communicate with
> ado-files. If you have to use macros to obtain your result, however, you
> are thinking ado, not Mata.  There's a simpler, more direct way.
>
> -- Bill
> wgould@stata.com
> *
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--
kohler@wz-berlin.de
+49 (030) 25491-361
*
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*   http://www.ats.ucla.edu/stat/stata/
```