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st: Re: Programming: Capturing bysort-parameters for use within a program


From   "Martin Weiss" <martin.weiss1@gmx.de>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: Re: Programming: Capturing bysort-parameters for use within a program
Date   Tue, 24 Mar 2009 23:15:10 +0100

<>

Before you dig too deep, make sure you read
http://www.stata.com/support/faqs/data/missing.html



HTH
Martin
_______________________
----- Original Message ----- From: "Steinar Fossedal" <steinar.fossedal@gmail.com>
To: <statalist@hsphsun2.harvard.edu>
Sent: Tuesday, March 24, 2009 10:53 PM
Subject: st: Programming: Capturing bysort-parameters for use within a program


Hi, listers

I'm having trouble capturing the parameters for bysort used when
calling a program. The following simple (and untested) program
illustrates the problem. It replaces missing values with the
previously known value, using only a subset of the data.

capture program drop missing_like_last
program missing_like_last, byable(onecall)
     syntax varlist [if]
     marksample touse
     bysort `touse' `_byvars' (`varlist2-reference'): replace `var' =
`var'[_n-1] if `var'==. & `touse'
end

bysort id (sortvar): missing_like_last myvar if somevar==1

(syntax for bysort is: bysort varlist1 (varlist2): ...)

As you can see, I'm trying to issue commands using the exact same
bysort-parameters as used when calling the program, just adding the
`touse' variable. I need the order to be correct as I am issuing
commands with references between rows ( [_n-1] ).

To do this, I need to capture the -bysort- parameters varlist1 and
varlist2 containing the values id and sortvar. -_byvars- takes care of
varlist1, but how can I capture varlist2? Without specifying varlist2,
the sort order within id will be scrambled.

I hope you can help.

Cheers,
-Steinar
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