st: AW: What's the added value of having -in- subset the data before -if- does?

["Martin Weiss" <martin.weiss1@gmx.de>]

<> 



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bys for: list  if foreign &_n<=10
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HTH
Martin


-----Ursprüngliche Nachricht-----
Von: owner-statalist@hsphsun2.harvard.edu
[mailto:owner-statalist@hsphsun2.harvard.edu] Im Auftrag von Dan Blanchette
Gesendet: Mittwoch, 4. Februar 2009 16:28
An: statalist@hsphsun2.harvard.edu
Betreff: st: What's the added value of having -in- subset the data before
-if- does?

Have you ever wanted to list a selection of observations based on a 
condition but only list say a subset of 10 obs of that condition? 
If so, perhaps you've been frustrated with the fact that:
. sysuse auto
. list  if foreign == 1 in 1/10
lists no observations because in the first 52 observations foreign == 0.
The -in- subsets the data before the -if- condition subsets the data. 
This is the opposite in SAS:

/* WHERE subsets the data before OBS subsets the data */

PROC PRINT DATA= SASHELP.SHOES(WHERE=(STORES < 10)  OBS = 10);
RUN;

So, the above code lists the first 10 observations where (STORES < 10).

I can't think of any situation where I would want to know how many
times a certain condition exists in the first X observations.  Do others
ever need to know that?

I figured out a solution where Stata will subset the data to the 
condition and then only list the range of observations I'm interested in:
. list  if sum((foreign == 1)) <= 10

The "(foreign == 1)" inside the sum() creates a value equal to 1 when the 
condition is true and then sum() creates a running sum of that.  You can 
use the sum() function to subset your data for other Stata commands.

You could get a range of observations as well:

. list  if inrange(sum((foreign == 1)),2,11)

I may decide always to use this since:

. list  if sum((foreign == 1)) <= 100

will also work despite the fact there aren't 100 observations in the data.
I'll never again get the error message:
  Obs. nos. out of range
  r(198);

My previous solution was to:

preserve
keep  if foreign == 1
local nobs = 10
if  _N  < `nobs'  local nobs = _N
list in 1/`nobs'
restore

Thoughts?

Dan Blanchette
Research Associate
Center of Entrepreneurship and Innovation
Duke University's Fuqua School of Business
Durham, NC USA
dan.blanchette@duke.edu


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