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

[annoporci <annoporci@gmail.com>]

you write faster than I think Nick!

Thank you very, very much.


> scalar newscalar = open[1]
>
> scalar newscalar = open[2]
>
> would work fine.


I see. Thanks. That's actually the first thing I tried, but I need to be  
able to specify the date in a "human readable" way, so I tried:


scalar newscalar = open[mdy(1,3,2012)]


It doesn't work. I'll use the method below then:


> su open if date == mdy(1,3,2001), meanonly
> scalar newscalar = r(min)
>
> would also work.
>
> But
>
> scalar <name> = <varname>
>
> will always be interpreted as
>
> scalar <name> = <varname>[1]
>
> Nick
>
> On Tue, Jan 15, 2013 at 1:28 PM, Patrick Toche <annoporci@gmail.com>  
> wrote:
> > 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
>
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--
Patrick Toche.

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