Re: st: data extraction by date

[David Kantor <dkantor@jhu.edu>]

At 10:49 AM 12/2/2003 -0500, Mike Carney wrote:
> I have a series of stock returns sorted by Stata's standard date form 
> (that is, when 0 = 1Jan 1960). Is it possible for me to create a data set 
> with 1 for all dates ranging from 1 jan to 10 jan (including all years) 
> and zero otherwise. I've tried
>
> .   gen var2 = var1 if doy(1/4), month(1)
>
> but it doesn't seem to work. Any help would be appreciated immensely!!!
>
> regards,
> Mike
A couple of people have already replied while I was writing this, but here 
is my advice anyway.

A few things about your question and your alleged Stata command don't make 
sense.

First, I believe that you want to create a variable -- not a dataset -- 
valued 1 for all dates ranging from  ranges 1 jan to 10 jan.  (If you want 
a dataset, you can subsequently filter on the variable.)

Secondly, the argument to the doy function is a constant expression, so 
there is no real sense to using it that way.  (Also, that constant 
expression is 1/4 which rounds to 0 because doy likes integer arguments.)

Third, where is your date variable?  (Is it var1? If so, then, if the rest 
of your formula were okay, the values would be var1 in the targeted cases 
-- not 1.)

Fourth, what is the supposed significance of  -, month(1)- ?  I should note 
that -generate- does not allow any options as far as I know.

Fifth, if you write,
 gen var2 = some_expression if some_other_expression

then the cases where some_other_expression is false will receive a missing 
value -- not zero.

Here is what I would do. Suppose that your date variable is called 
date.  Then code as follows.

gen int doy = doy(date)
gen var2 = doy >=1 & doy <=10

You can subsequently drop doy.  You could have, alternatively done, in one 
command, ...
 gen var2 = doy(date) >=1 & doy(date) <=10

and one can argue about which is better.

I hope this helps.
-- David

David Kantor
Institute for Policy Studies
Johns Hopkins University
dkantor@jhu.edu
410-516-5404

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