Re: st: Unexpected end of file in mata.

[Amadou DIALLO <stata.diallo@gmail.com>]

Dear Bill,
Indeed this is really awesome! Many thanks for taking on your scarce
time to answer these questions. I really learn a lot and will keep
improving my mata knowledge.
I wish you a nice day.

2014-01-27, William Gould, StataCorp LP <wgould@stata.com>:
> Amadou Diallo <stata.diallo@gmail.com> has yet more questions on Mata,
> and good ones.
>
>> I am a bit surprised to learn that mata/stata interprets "if (i=2) {"
>> as "whether 2 is not equal to 0."  (I'd have thought equal to 2!).
>
> Think like this:
>
>         a = b = 2
>
> assigns the value 2 to a and b.  That happens because
>
>         a = b = 2  is equivalent to typing a = (b=2)
>
> Thus, (b=2) returns 2, and that's how a is set equal to 2.
>
> Okay, we've established that the expression (b=2) returns 2.
> So now consider
>
>         if (b=2) {
>                 ...
>         }
>
> The statement (b=2) is true because b=2 returns 2 and Mata interprets
> 0 to mean false and all other values to mean true.
>
>
>> I couldn't find a way to ask mata to check whether x was indeed a
>> colvector...
>
>      A colvector has one column.  Ergo,
>
>            if (cols(a)==1) {
>                  /* a is a column vector */
>            }
>
> Other useful functions in checking what a variable contains
> include rows(), length(), eltype(), isreal(), iscomplex(), isstring(),
> and ispointer().
>
>
>> More generally, when a mata program aborts (with a error code as it
>> seems to me it is almost always the case), how to capture that error
>> code, so that we use that _rc as in stata mode?
>
> When somethiung goes wrong, Mata aborts the execution and presents
> an ugly traceback log, a process we summarizes by saying that the
> subroutine aborts with error.  Abort with error is useful for
> debugging, but not so useful for professional code where you want to
> present an error message and show the return code.
>
> The functions you want to learn about are errprintf(), error(), and
> exit().
>
> Let's consider the following code
>
>         ... mysubroutine(...)
>         {
>                 ...
>                 stata("summarize + " user_specified_varname)
>                 ...
>         }
>
> In this program, we want to run Stata's -summarize- command.  Let's
> imagine what happens when the user_specified_varname is not found.
> The way stata() works, if the commnd fails, it aborts with error.
> Here's alternative code that exits gracefully in the error case:
>
>         ... mysubroutine(...)
>         {
>                 real scalar  rc
>                 ...
>                 if ((rc=_stata("summarize + " user_specified_varname)) {
>                         exit(rc)
>                         /*NOTRECHED*/
>                 }
>                 ...
>         }
>
> _stata() is the function that does what stata() does, but rather than
> aborting with error if the Stata command fails, returns the return
> code from the Stata command in all cases.  _stata() returns 0 when
> the command executed without error.  It returns nonzero when there
> was a problem.
>
> exit() is the function that terminates execution (without traceback log)
> and sets the return code to the specified argument.
>
> Oh yes, and notice that I used expression (rc=...) with the single equal
> sign.  I could have coded the expression  ((rc=...)!=0).
>
> I didn't have to call Stata to get a return code.  Here's another
> example:
>
>         ... myothersubroutine(...)
>         {
>                 ...
>                 if (cols(x)!=1) {
>                      exit(error(503))
>                 }
>                 ...
>         }
>
> error(503) presents the error message associated the return code 503,
> and it returns 503.  exit() exits cleanly -- stops execution -- with
> the specified return code.  Thus, the user will see
>
>         . whatever ...
>         conformability error
>         r(503);
>
> Here's a version that's more customized:
>
>         ... myothersubroutine(...)
>         {
>                 ...
>                 if (cols(x)!=1) {
>                      (void) error(503)
>                      errprintf("{p 4 4 2}\n")
>                      errprintf("%d x %d found where column vector
> expected.\n",
>                                rows(x), cols(x))
>                      errprintf("{p_end}\n")
>                      exit(503)
>                 }
>                 ...
>          }
>
> The user will see
>
>
>         . whatever ...
>         conformability error
>             2 x 4 found where column vector expected.
>         r(503);
>
> -- Bill
> wgould@stata.com
> *
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>


-- 
Amadou B. DIALLO, PhD.
Senior Economist, AfDB.
a.b.diallo@afdb.org
+21671101789
*
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