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st: RE: newbie if {} else {} question


From   "Nick Winter" <nwinter@policystudies.com>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: RE: newbie if {} else {} question
Date   Thu, 19 Sep 2002 15:51:59 -0400

> -----Original Message-----
> From: Richard Herrell [mailto:rherre2@uic.edu] 
> Sent: Thursday, September 19, 2002 3:27 PM
> To: statalist
> Subject: st: newbie if {} else {} question
> 
> 
> I've just started learning Stata syntax after long years of 
> SAS use and am
> completely stumped by a result.  I want to create a new 
> 4-level variable
> (ascertgp) based on the crosstabulation of 2 dichotomous 
> variables (probip
> and promig).  This is the syntax I wrote:
> 
> if      probip==1 & promig==1 {
>   gen ascertgp=1
> }
> else if probip==0 & promig==1 {
>   gen ascertgp=2
> }
> else if probip==1 & promig==0 {
>   gen ascertgp=3
> }
> else if probip==0 & promig==0 {
>   gen ascertgp=4
> }
> 
> When I run tab1 ascertgp I get a value of 3 for every observation.  (I
> know what the 4 frequencies should be from the crosstab.)  I 
> must be doing
> something very obviously wrong, but can see what it is. 

In Stata, the "if {} else {}" construct applies to blocks of code, not
to individual
observations.  You want to use the "if" clause within the generate
command:

	. gen ascertgp=1 if probip==1 & promig==1
	. replace ascertgp=2 if probip==0 & promig==1
	. replace ascertgp=3 if probip==1 & promig==0
	. replace ascertgp=4 if probip==0 & promig==0

The "if" here says "apply this command only to the observations
indicated".  You can use "if" in the exact same sense for most commands;
for example, to restrict the estimation sample for a command, or
whatever.

In your code above, Stata is evaluating each condition, and then either
executing or not executing the code within the {}, *for all
observations*.  Unless told otherwise, it evaluates the variables in the
expression in terms of the first observation in your dataset; apparently
for that observation, probip is 1 and promig is zero.

By the way, there are probably easier ways to generate your summary
variable:

	. egen ascertgp = group(probip promig)

will do it, albeit with a different coding.  See "help egen" for more
information on the goodies included with egen (Extended GENerate).  The
following will do it with your coding, although it is a much less
generalizable solution:

	. gen ascertgp = 2*(1-probip) + (1-promig) + 1

Welcome to Stata.  Once you get used to the syntax, you won't go back.

--Nick Winter

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