Re: st: generate variable versus define scalar, with conditional statement

[annoporci <annoporci@gmail.com>]

Thanks for these explanations Nick!

> if date == mdy(1,2,2001) scalar newscalar = open
>  still looks most unlikely to be what you really want _unless_ -date-
> is also a scalar.
Barely had I pressed "send" that I realized something was not quite right:

/// This one does what I intend
clear all
sysuse sp500
if date == mdy(1,2,2001) scalar newscalar = open
scalar list

/// This one doesn't do it:
clear all
sysuse sp500
if date == mdy(1,3,2001) scalar newscalar = open
scalar list

what's going on? The first command stores the value of open at the date  
02jan2001. I expected the second one to store the value of open at the  
date 03jan2001, but it doesn't ...
So I guess -date- is not a scalar or something ...

Oh wait, what I'm really asking is:

> "I have an if or while command in my program that only seems to evaluate  
> the first observation.What’s going on?"
> http://www.stata.com/support/faqs/programming/if-command-versus-if-qualifier/
Okay, I need to follow that one up.

Thanks Nick!

Patrick


----
Patrick Toche.



On Tue, 15 Jan 2013 21:02:32 +0800, Nick Cox <njcoxstata@gmail.com> wrote:

> What's intuitive is usually just what is familiar through experience
> (although that is contentious within (?) 20% of the history of
> psychology and 80% of the history of epistemology).
>
> Stata has an -if- command and an -if- qualifier and one distinction
> between them is in where they are used, not in what they look like at
> first glance.
>
> What comes first logically in either case?
>
> If something is true, you want a scalar calculated.
>
> You want a variable -generate-d using a specified rule but only -if-
> something is true in each observation.
>
> The trouble is that I think that way and explain it that way because I
> have internalised Stata's syntax.
>
> All that said,
>
> 1. I'd assert that the form you found puzzling is a common form in
> many programming languages. It is the -if- qualifier that is
> idiosyncratic. However, Stata learners are most likely to meet the
> -if- qualifier first.
>
> 2.
>
> if date == mdy(1,2,2001) scalar newscalar = open
>
> still looks most unlikely to be what you really want _unless_ -date-
> is also a scalar.
>
> To see why, see
>
> FAQ     . . . . . . . . . . . . . . . . . . . . .  if command vs. if  
> qualifier
>         . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  J.  
> Wernow
>         6/00    I have an if command in my program that only seems
>                 to evaluate the first observation, what's going on?
> http://www.stata.com/support/faqs/programming/if-command-versus-if-qualifier/
>
> Nick
>
> On Tue, Jan 15, 2013 at 12:38 PM, annoporci <annoporci@gmail.com> wrote:
>
> > I learned something today I'd like to share with Statalist.
> >
> > The way to conditionally define a variable and a scalar are different.  
> > An
> > example is given below.
> >
> > clear all
> > sysuse sp500
> >
> > /// generate a new variable with missing entries except at the given  
> > date
> > mdy(1,2,2001):
> > gen newvariable = open if date == mdy(1,2,2001)
> >
> > /// define a new scalar by the value of variable open at the given date
> > mdy(1,2,2001):
> > if date == mdy(1,2,2001) scalar newscalar = open
> > scalar list
> >
> > /// THIS IS NOT THE WAY:
> > scalar newscalar = open if date == mdy(1,2,2001)
> >
> > The above, incorrect way, is intuitive for a beginner (like myself), it  
> > is
> > similar to the syntax to generate new variables, but it won't work.
> >
> > This is trivial for 99% of Statalist users, but may be a useful tip for
> > the remaining 1%. And if not, almost surely for a future incarnation of
> > myself scanning through Statalist for hints when the following error
> > message displays:
> >
> >       if not allowed
> >
> > When I submitted *stata scalar "if not allowed"* Google returned a  
> > solution,
> > in a more complicated setting:
> >
> > http://statalist.1588530.n2.nabble.com/scalar-do-not-allow-if-td5748129.html
> >
> > --
> > Patrick Toche.
> *
> *   For searches and help try:
> *   http://www.stata.com/help.cgi?search
> *   http://www.stata.com/support/faqs/resources/statalist-faq/
> *   http://www.ats.ucla.edu/stat/stata/

--
Patrick Toche.

*
*   For searches and help try:
*   http://www.stata.com/help.cgi?search
*   http://www.stata.com/support/faqs/resources/statalist-faq/
*   http://www.ats.ucla.edu/stat/stata/