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st: Problem with tempvar __000000 (still confused) -- Addendum


From   "Renzo Comolli" <renzo.comolli@yale.edu>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: Problem with tempvar __000000 (still confused) -- Addendum
Date   Tue, 22 Jun 2004 11:52:22 -0400

Sorry if I send two messaged in a row but I realized something as soon as I
clicked send.
There is a way to cause the errors that makes them much more likely to
happen.
Follow the same instruction detailed here below, but instead of fumbling
around the command line, just do the following:

Step 1: In the Stata Do-file-Editor write
. use http://www.stata-press.com/data/r8/quad1.dta, clear
. replace id=_n // this line is not part of the problem, it is there only to
create a suitable dataset 
. tempvar num_miss
. gen `num_miss'=0
. save debugreshape, replace
. use debugreshape, clear
. reshape long x, i(id) j(number)

Step 2: highlight the following part only and click to button -do-
. use http://www.stata-press.com/data/r8/quad1.dta, clear
. replace id=_n 
. tempvar num_miss
. gen `num_miss'=0
. save debugreshape, replace

Step 3: highlight the following part only and click to button -do-
. use debugreshape, clear
. reshape long x, i(id) j(number)

Stata should stop and you should get the error message
__000000 already defined
variable __000000 not found

Best,
Renzo



-----Original Message-----
Thanks to both Tom and Nick.

This message shows how to see the error message (part 1) and how this
"feature" can cause -reshape- to fail (part 2)

1. To see the "error" message (which is not a true error message because the
operation is carried on successfully, it is more  a "warning message") do
the following
Step 1: Put the following instructions in a do file
. sysuse auto, clear
. tempvar num_miss
. gen `num_miss'=0
. save debugauto, replace
Step 2: do the do file
Step 3: put the following instructions in the command line
. use debugauto, clear
. twoway (lfit  price  mpg) (scatter  price mpg), by( foreign)

You should now be able to see the "error" message. 
Notice also that the __000000 is there when you -use debugauto- but it has
disappeared after -twoway-. This disappearence is probably intentional and
gives no problem to me, but I thought we need to remark on it "for future
generations".


2. Here is how you can cause reshape to fail
Step 1: Put the following instructions in a do file
use http://www.stata-press.com/data/r8/quad1.dta, clear
replace id=_n // this line is not part of the problem, it is there only to
create a suitable dataset 
tempvar num_miss
gen `num_miss'=0
save debugreshape, replace

Step 2: do the do file
Step 3: put the following instructions in the command line
use debugreshape, clear
reshape long x, i(id) j(number)

I get 
__000000 already defined
variable __000000 not found

Best Regards,
Renzo Comolli


----------------------------------------------------------------------------
----
*From   "Nick Cox" <n.j.cox@durham.ac.uk> 
To   <statalist@hsphsun2.harvard.edu> 
Subject   st: RE: Re: Problem with tempvar __000000 
Date   Mon, 21 Jun 2004 09:29:51 +0100 

----------------------------------------------------------------------------
----

I concur with Tom's analysis. Like him, 
I can't reproduce the "already defined" message. 

Note two further details. 

The fact that a temporary variable does not persist 
beyond the end of a do file is explicit within [P] 
macro but may come as a surprise.  

The particular temporary variable name __000000 can only 
I think be reproduced if you have not created 
temporary variables earlier in the same session. More 
generally, Stata avoids names previously 
used in a session even if variables with those names 
no longer exist. Presumably always using a new name 
is the easiest and safest general solution. 

A little experiment you can try is 

forval i = 1/10 { 
	tempvar foo 
	gen `foo' = 1 
	di "`foo'" 
	drop `foo' 
} 

This incidentally illustrates one use of tempvars 
within a main program, to elucidate some programming 
puzzle without writing a named program. 

Nick 
n.j.cox@durham.ac.uk 

Tom Steichen
 
> For the situation you describe below, retention of the temp 
> variable in the
> saved dataset is the expected behavior.
> 
> Temp variables are deleted automatically only when their 
> creating program
> (or do file) ends. Thus they remain in existence until dropped or the
> program (or do file) that created them ends.  It follows that 
> if you create
> a temp variable in the "main" program level of Stata, the 
> temp variable
> remains in existence until either dropped or you exit from 
> Stata. If you do
> a -save- without specifying which variables, all variables, 
> including temp
> variables, get saved.
> 
> I tried your example, exactly as you show below but on Stata SE (not
> Intercooled), born the same date you show, and I do not get any error
> message even though __000000 exists.  Thus the entanglement 
> of Graph with
> the temp variable may be an Intercooled problem.

Renzo Comolli
 
> > A variable created by -tempvar- is hanging around inside the saved
> dataset.
> > Its name is __000000
> > Is that normal behavior? I had never noticed this behavior 
> before, but maybe I am just confused.
> >
> > See the (reproducible) code below, taken from my results window.
> > The case presented here is benign, but soemtimes malign cases occur:
> > A. the code gives an error message and still performs the 
> operation, as in
> > the exaple of -twoway- here below
> > B. I believe that at least in one case, it actually broke 
> the code. It was
> > in a -reshape long-. (I can't be sure though, because I was 
> tearing a huge
> > file apart to figure out where the problem was coming from).
> >
> > If, after the code below, you actually issue once again
> > . use debugauto, clear
> > then variable __000000 appears in the variable window.
> >
> > Any clarification? Is it a bug? I am missing something?
> >
> > Best Regards,
> > Renzo Comolli
> >
> >
> >
> > Here is the code to reproduce (most) of the behavior described.
> >
> > . about
> > Intercooled Stata 8.2 for Windows
> > Born 18 May 2004
> >
> > . sysuse auto, clear
> > (1978 Automobile Data)
> >
> > . tempvar num_miss
> >
> > . gen `num_miss'=0
> >
> > . save debugauto, replace
> > file debugauto.dta saved
> >
> > .
> > . use debugauto, clear
> > (1978 Automobile Data)
> >
> > . describe __000000
> >
> >               storage  display     value
> > variable name   type   format      label      variable label
> > 
> --------------------------------------------------------------
> ------------
> --
> > ---
> > __000000        float  %9.0g
> >
> > . tab __000000, mi
> >
> >    __000000 |      Freq.     Percent        Cum.
> > ------------+-----------------------------------
> >           0 |         74      100.00      100.00
> > ------------+-----------------------------------
> >       Total |         74      100.00
> >
> > . twoway (lfit  price  mpg) (scatter  price mpg), by( foreign)
> > __000000 already defined
> >
> > * but the graph still appears correctly


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