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Re: st: copying a string variable to all rows within a group


From   <Seyi.Soremekun@lshtm.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   Re: st: copying a string variable to all rows within a group
Date   Wed, 29 Feb 2012 11:31:03 +0000

Thanks Nick. I was using search terms (phrases including egen, copy, string, row, etc) that didn't bring up that particular FAQ.
 
These two do as I expected, however they don't do exactly the same thing (discrepancy of 2-3 in total number of changes made):
1. bysort groupvar (oldvar) : replace oldvar = oldvar[_N]
2. bysort groupvar : replace oldvar = oldvar[_n-1] if missing(oldvar) & _n > 1

This one on the other hand appears to do something completely different - including returning many of the original (single) observations to missing:
3. bysort groupvar : replace oldvar = oldvar[1]
I've used the first option as this results in the largest number of changes (3 more than second option), and intuitively seems more accurate - maybe not the most robust of reasons but no other more sensible ones come to mind I'm afraid.
 
thanks,
Seyi

>>> Nick Cox <njcoxstata@gmail.com> 2/29/2012 9:01 AM >>>
For "row" read "observation". We know what you mean, but that is the
correct Stata terminology.

bysort groupvar (oldvar) : replace oldvar = oldvar[_N]

Alternatively,

bysort groupvar : replace oldvar = oldvar[1]

will probably work too. Alternatively,

bysort groupvar : replace oldvar = oldvar[_n-1] if missing(oldvar) & _n > 1

will probably work too. This last is an FAQ

FAQ     . . . . . . . . . . . . . . . . . . . . . . . Replacing missing values
        . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  N. J. Cox
        2/03    How can I replace missing values with previous or
                following nonmissing values?
                http://www.stata.com/support/faqs/data/missing.html 

On Wed, Feb 29, 2012 at 8:28 AM,  <Seyi.Soremekun@lshtm.ac.uk> wrote:
,
> If I have a numeric variable "oldvar" which appears on only the first row of a set of rows defined by a group variable "groupvar" (rest of oldvar rows are blank), it is easy enough to copy oldvar down all rows within groupvar:
>
> bysort groupvar: egen newvar=max(oldvar) // can equally use =min(oldvar) as there is only 1 value of oldvar.
>
> How might I do the same to copy a string oldvar to all rows - again it only appears once within the group, but not necessarily on the first row.
>
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