Re: RE: st: RE: Irritating if...else question for mata vs. stata

[Thomas Jacobs <thomasjacobs@gmail.com>]

Bill,

Thanks very much.  An education, as always.

You captured my Sunday quite well, by the way, in Faulkner fashion.

Tom

On Mon, Jun 7, 2010 at 10:57 AM, William Gould, StataCorp LP
<wgould@stata.com> wrote:
>
> Thomas Jacobs <thomasjacobs@gmail.com> writes,
>
> > Here is how the final part of the mata code appears utilizing the
> > belief that mata if blocks need not be followed by empty else blocks
> > and the error it generates:
> >
> > : if (FirstCDXDay > 0) {
> > >
> > >         CDXAdjSizePowVRight = vec(CountCDXAdjSizePowRight')
> > >         CDXMktSizePowVRight = vec(CountCDXMktSizePowRight')
> > >         CDXAdjSizePowVLeft = vec(CountCDXAdjSizePowLeft')
> > >         CDXMktSizePowVLeft = vec(CountCDXMktSizePowLeft')
> > >
> > > }
> > >
> > > end
>
> unexpected end of line
> (117 lines skipped)
>
> Thomas goes on to note that when he "adds back the heretofore extraneous else
> block and the program continues on to successful completion":
>
> > : if (FirstCDXDay > 0) {
> > >
> > >         CDXAdjSizePowVRight = vec(CountCDXAdjSizePowRight')
> > >         CDXMktSizePowVRight = vec(CountCDXMktSizePowRight')
> > >         CDXAdjSizePowVLeft = vec(CountCDXAdjSizePowLeft')
> > >         CDXMktSizePowVLeft = vec(CountCDXMktSizePowLeft')
> > >
> > > }
> > > else{
> > > }
>
> Thomas concludes,
>
> > Any suggestions for what I may be missing or how I might better
> > understand what is going on here?  [...]
> > [There] certainly there must be some explanation for such behavior.
> > It has happened to me before and I make a habit of posting empty
> > else blocks all over the place and
>
>
> Why this is happening
> ---------------------
>
> Thomas is entering
>
>      : if (<exp>) {
>      >         ...
>      > }
>      > _
>
> interactively or in a do-file.  Mata is trying to figure out when
> Thomas is done entering the the line.  Mata says to itself, "Thomas
> start with -if-, entered an expression, and the body.  I understand
> that.  Possibly Thomas will have an -else- clause following this.
> Oops, Thomas hit Enter gain.  I wonder why Thomas wants all that white
> space?  Oh well, possibly Thomas will have an -else- clause following
> the -if- and white space.  Hey!  Where is Thomas?  He hasn't typed
> anything in while.  Ah, there he is, reviewing the manuals again.
> Smart.  But getting back to the problem at hand, possibly Thomas will
> have an -else- clause following the -if-, white space, and manual
> review.  What?  Even more white space?  Okay.  Perhaps Thomas will
> have an -else- clause following the -if-, white space, manual review,
> and even more white space.  Boy am I glad I don't have the microphone
> on.  Thomas looks really mad.  I wonder why?  Oh well, perhaps Thomas
> will have an -else- clause following the -if-, white space, manual
> reivew, more white space, and getting over his anger.  Oh no, Thomas
> is crying.  I've seen him do that before.  Oh well, possibly Thomas
> will have an -else- clause following the -if-, white space, manual
> review, more white space, getting over his anger, and now the bad news
> he obviously received.  I'll give him another ">" prompt.  Maybe that
> will make him feel better."
>
> Mata is a one-token look-ahead compiler, which is to say that in cases
> such as -if- possibly followed by -else-, Mata has to wait and see
> what follows before he it can interpret and compile what's typed.
>
> If Thomas had typed
>
>      : if (<exp>) {
>      >         ...
>      > }
>      > x = 3
>
>      : _
>
> Mata would have quickly figure out that there is no -else- and Thomas
> would never have known that Mata was looking ahead to determine whether
> an -else- would follow the -if-.  Usually, -if-s appear in function
> definitions, and no one notices when Mata looks ahead.  When
> you type
>
>        : function xxx()
>        > {
>        >        ...
>        > }
>
>        : _
>
> Mata does not look seriously inside the braces until you enter the
> final closing brace.  Thus, Mata can look ahead all it wants and you
> don't even know it.
>
> Most real compilers don't have a a line-by-line interactive mode.
> Mata does.
>
> In line-by-line interatactive mode, most people don't type -if-.  That's
> why we've been able to go so long with no else noticing when Mata
> looks ahead.
>
>
> Another solution
> ----------------
>
> Thomas's solution has been to type -else- Mata is watching for:
>
>
>      : if (<exp>) {
>      >         ...
>      > }
>      > else {
>      > }
>
>      : _
>
> That was a good solution in that it got Thomas through the problem.
> Mata however, is not demanding that there be an -else-, Mata is just
> waiting to see if there is an -else.  Another solution is to type
> a semicolon to the end the line:
>
>     : if (<exp>) {
>     >         ...
>     > } ;
>
>     : _
>
> Mata does not require semicolons to end lines as C does, but Mata allows
> them, and when typed, Mata takes them seriously.
>
> When Mata was locked into its loop "Possibly Thomas wll have an
> -else- clause following the -if-", had Thomas typed an -;-
> Mata would have immediately thought, "Evidently not.  This is fun".
>
>
>
> Cheers,
>
> -- Bill
> wgould@stata.com
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--
Thomas Jacobs

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