Mata declarations and conditional (if/else) statements

[kkranker]

I could use some help with if/else statements and Mata declarations: I have a situation where my first inclination was to do something like this:

if (something) real scalar x
else           real matrix x

But when I do this I get the error "symbol x multiply defined". I am pretty sure this is something that is simply not allowed in Mata. But I'm not sure what the reason is, or how I would so something like this that is allowed.

In practice, I'm doing something a little more complicated, along the lines of

class myclass1 {
  void new()
  real scalar x
}
class myclass2 extends myclass1 {
  void new()
  real scalar x
}
void myclass1::new() {
  "hello from 1"
}
void myclass2::new() {
  "hello from 2"
}
void testA() {
  if (0) class myclass1 scalar C
  else   class myclass2 scalar C
}
testA()

This gives the same error message as before.

I thought about using pointers as a workaround, as in the following example. You can see that the new() functions for both C1 and C2 are executed -- when the code is compiled -- even though I have these if/else statements in the code.

: void test2() {
>   "start"
>   if (0) class myclass1 scalar C1
>   else   class myclass2 scalar C2
>   pointer C
>   "declarations done"
>   if (0) C = &C1
>   else   C = &C2
>   "end"
> }
note: variable C set but not used

: test2()
  hello from 1
  hello from 1
  hello from 2
  start
  declarations done
  end

However, I can't figure out how to use this pointers approach (beyond using it to reveal when the new() functions are run) although maybe the it could work somehow. I have two concerns. First, I can't figure out the syntax for reading or writing members of a class instance when using the pointer. For example, all these things I tried inside test2() produced error messages:

*C.x = 2
(*C).x = 2
*C->x = 2
C->x = 2

The second concern is that the pointers approach would still run both new() functions instead of just running the one that is needed. That's not a problem for me, but it could be costly (computationally) if the new() function does something non-trivial.

Any help or tips would be much appreciated. Thanks!

- Keith

[Niels Henrik Bruun]

Hi
I think this is what you want:

cls
capture mata mata clear
mata
    class myclass1 {
      void new()
      string scalar x
    }
    class myclass2 extends myclass1 {
      void new()
      string scalar x
    }
    void myclass1::new() {
      x = "hello from 1"
    }
    void myclass2::new() {
      x = "hello from 2"
    }
    void myclass1::new() {
      x = "hello from 1 again"
    }
    void testA() {
      "C1 Defined"
      class myclass1 scalar C1
      "C2 Defined"
      class myclass2 scalar C2
      "End definition"
      if (0) C1.x
      else C2.x  
    }
    testA()
end

It returns

  C1 Defined
  C2 Defined
  End definition
  hello from 2

[kkranker]

Thanks for your suggestion. I should have mentioned that I realize that I could have written if/else statements throughout the rest of the test() program. My question is really all about finding a way to avoid that. The reason is that I have a complex class definition and I want the class(es) to do all the work to control which functions are used when. That is, I want to insert a bunch of code that simply refers to C, rather than effectively doubling the size of my program by writing "if ...C1...; else ...C2...;" over and over. Make sense?

Thanks again,
Keith

P.S. I think your solution is still calling both new() functions when the code is compiled--before the "C1 Defined" is printed to the screen--but it just so happens that your new() functions don't print anything so you can't tell that it's happening.

[Joseph Coveney]

Couldn't you use something like a conventional simple factory?

.ÿ
.ÿversionÿ15.1

.ÿ
.ÿclearÿ*

.ÿ
.ÿlocalÿline_sizeÿ`c(linesize)'

.ÿsetÿlinesizeÿ72

.ÿ
.ÿmata:
-------------------------------------------------ÿmataÿ(typeÿendÿtoÿexit
>ÿ)ÿ--------------------------------------------------------------------
:ÿmataÿsetÿmatastrictÿon

:ÿ
:ÿclassÿMyClassÿ{
>ÿÿÿÿÿÿÿÿÿprotected:
>ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿrealÿscalarÿx
>ÿ}

:ÿ
:ÿclassÿMyClass1ÿextendsÿMyClassÿ{
>ÿÿÿÿÿÿÿÿÿprivate:
>ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿvoidÿnew()
>ÿ}

:ÿvoidÿfunctionÿMyClass1::new()ÿ{
>ÿÿÿÿÿÿÿÿÿ"helloÿfromÿ1"
>ÿ}

:ÿ
:ÿclassÿMyClass2ÿextendsÿMyClassÿ{
>ÿÿÿÿÿÿÿÿÿprivate:
>ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿvoidÿnew()
>ÿ}

:ÿvoidÿfunctionÿMyClass2::new()ÿ{
>ÿÿÿ"helloÿfromÿ2"
>ÿ}

:ÿ
:ÿclassÿFactoryÿ{
>ÿÿÿÿÿÿÿÿÿprivate:
>ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿpointer(classÿMyClassÿscalar)ÿscalarÿget1()
>ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿpointer(classÿMyClassÿscalar)ÿscalarÿget2()
>ÿÿÿÿÿÿÿÿÿpublic:
>ÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿpointer(classÿMyClassÿscalar)ÿscalarÿgetMyClass()
>ÿ}

:ÿpointer(classÿMyClassÿscalar)ÿscalarÿfunctionÿFactory::get1()ÿ{
>ÿÿÿÿÿÿÿÿÿclassÿMyClass1ÿscalarÿmy_class
>ÿÿÿÿÿÿÿÿÿreturn(&my_class)
>ÿ}

:ÿpointer(classÿMyClassÿscalar)ÿscalarÿfunctionÿFactory::get2()ÿ{
>ÿÿÿÿÿÿÿÿÿclassÿMyClass2ÿscalarÿmy_class
>ÿÿÿÿÿÿÿÿÿreturn(&my_class)
>ÿ}

:ÿpointer(classÿMyClassÿscalar)ÿscalarÿfunctionÿFactory::getMyClass(real
>ÿÿscalarÿwhich)ÿ{
>ÿÿÿÿÿÿÿÿÿifÿ(whichÿ==ÿ1)ÿreturn(get1())
>ÿÿÿÿÿÿÿÿÿelseÿifÿ(whichÿ==ÿ2)ÿreturn(get2())
>ÿÿÿÿÿÿÿÿÿelseÿÿ_error(3300,ÿ"Argumentÿneedsÿtoÿbeÿeitherÿ1ÿorÿ2")
>ÿ}

:ÿ
:ÿ
:ÿvoidÿfunctionÿtestA(realÿscalarÿwhatever)ÿ{
>ÿ
>ÿÿÿÿÿÿÿÿÿpointer(classÿMyClassÿscalar)ÿscalarÿm
>ÿ
>ÿÿÿÿÿÿÿÿÿclassÿFactoryÿscalarÿf
>ÿ
>ÿÿÿÿÿÿÿÿÿpragmaÿunsetÿm
>ÿÿÿÿÿÿÿÿÿpragmaÿunusedÿm
>ÿÿÿÿÿÿÿÿÿifÿ(!whatever)ÿmÿ=ÿf.getMyClass(1)
>ÿÿÿÿÿÿÿÿÿelseÿmÿ=ÿf.getMyClass(2)
>ÿ
>ÿ}

:ÿ
:ÿtestA(0)
ÿÿhelloÿfromÿ1

:ÿ
:ÿtestA(1)
ÿÿhelloÿfromÿ2

:ÿ
:ÿend
------------------------------------------------------------------------

.ÿ
.ÿsetÿlinesizeÿ`line_size'

.ÿ
.ÿexit

endÿofÿdo-file


.

[kkranker]

Thanks Joseph! I had never seen this approach before, but it looks really promising. Thanks so much!