We're both right on this. And I'm wrong.
In essence, I was too quick on the draw
and shot you by mistake. Sorry. Every
other time I've seen this code use
it's been based on a misunderstanding.
As I emphasised,
while b == . {
...
}
is equivalent to
while b[1] == . {
...
}
and if that's not understood, problems
almost always ensue. But in your example,
b[1] starts out as missing and the loop continues
until b[1] is no longer missing.
This gives exactly what you want, at the
cost of some inefficiency. However, I wonder
whether there are examples in which you
would exit prematurely.
Nick
[email protected]
> -----Original Message-----
> From: [email protected]
> [mailto:[email protected]]On Behalf Of
> [email protected]
> Sent: 12 July 2004 18:39
> To: [email protected]
> Subject: st: while loops [was: filling previous missing observations
> repeatedly]
>
>
> Nick,
>
> I guess I do not understand the problem with -while- (and,
> why for Dev it only worked for 1998). Maybe I have
> misunderstood the initial problem.
>
>
> Here is what I did:
>
> **Assume the initial data set is as follows:
>
> . l
>
> +--------------+
> | year c b |
> |--------------|
> 1. | 1995 1 . |
> 2. | 1996 2 . |
> 3. | 1997 3 . |
> 4. | 1998 4 . |
> 5. | 1999 5 5 |
> |--------------|
> 6. | 2000 6 . |
> 7. | 2001 7 . |
> 8. | 2002 8 . |
> +--------------+
>
> **Run the -while- loop
>
> . do "C:\DOCUME~1\SCOTT~1.MER\LOCALS~1\Temp\STD00000000.tmp"
>
> . while b == . {
> 2. replace b=b[_n+1]*c if b==. & year <1999
> 3. replace b=b[_n-1]*c if b==. & year >1999
> 4. }
> (1 real change made)
> (3 real changes made)
> (1 real change made)
> (0 real changes made)
> (1 real change made)
> (0 real changes made)
> (1 real change made)
> (0 real changes made)
>
> .
> end of do-file
>
> . l
>
> +-----------------+
> | year c b |
> |-----------------|
> 1. | 1995 1 120 |
> 2. | 1996 2 120 |
> 3. | 1997 3 60 |
> 4. | 1998 4 20 |
> 5. | 1999 5 5 |
> |-----------------|
> 6. | 2000 6 30 |
> 7. | 2001 7 210 |
> 8. | 2002 8 1680 |
> +-----------------+
>
>
> Scott
>
> ----- Original Message -----
> From: Nick Cox <[email protected]>
> Date: Monday, July 12, 2004 12:07 pm
> Subject: RE: st: filling previous missing observations repeatedly.
>
> > This is not going to work for reasons I gave
> > and for reasons I alluded to in an earlier
> > posting.
> >
> > -while- does not buy you a loop over observations
> > unless you ask for that explicitly.
> >
> > Also, Stata is going to interpret this as
> >
> > while b[1] == . {
> > ...
> > }
> >
> > so, depending on b[1], the loop is either never
> > entered or infinite.
> >
> > Nick
> > [email protected]
> >
>
>
> *
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>
*
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