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RE: st: assignment by indexing


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   RE: st: assignment by indexing
Date   Wed, 4 Feb 2004 13:13:54 -0000

Now I'm confused. As was emphasised yesterday, 

gen newvar = x[indexvar] 

is legal and useful. If you want the syntax 

gen newvar = x(indexvar) 

to select from variables x1, x2, ... according to 
whether indexvar is 1, 2, ..., then (1) that
syntax is not supported; (2) there's no 
neat way of doing exactly that in one step 
(although it could easily be programmed 
as an -egen- function); (3) there are
other ways to do the same thing. 

Your Markov problem should definitely not
need 107 equations. You could set it up 
a matrix problem or as a variables problem. 

Nick 
n.j.cox@durham.ac.uk 

> -----Original Message-----
> From: owner-statalist@hsphsun2.harvard.edu
> [mailto:owner-statalist@hsphsun2.harvard.edu]On Behalf Of Ben Pelzer
> Sent: 04 February 2004 12:41
> To: statalist@hsphsun2.harvard.edu
> Subject: RE: st: assignment by indexing 
> 
> 
> Nick, Ernest and Allan,
> 
> Thanks for your comments and tips. I am surprised that a 
> statement like "gen
> newvar = x(indexvar)" is not available in Stata, since in 
> other packages,
> this is everyday meat. In my application, the indexvariable 
> is "time" which
> is used in a Markov model that links state probabilities a 
> timepoint t to
> those at t-1 and to the transition probabilities to "stay in 1" or to
> "switch from 0 to 1", i.e.:
> 
> p(y=1|time)  =    p(y=1|time-1) * p(stay in 1)    +    p(y=0|time-1) *
> p(switch from 0 to 1)
> 
> This equation is to be evaluated for 107 timepoints, so using 
> time as an
> index, would be very handsome. In the past I implemented the model in
> several other packages, (which I dare not mention here), and 
> I now would
> like to develop a Stata version as well. In Stata I suppose I 
> have to use
> 107 equations, then? Ok, if that's all it takes, I'll go for 
> it. 

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