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st: RE: -generate- and run time contingencies


From   "Steichen, Thomas J." <SteichT@rjrt.com>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: RE: -generate- and run time contingencies
Date   Mon, 30 Jan 2006 11:07:24 -0500

Mike Lacy writes in part:
> One feature of Stata with which I continue to experience difficulty 
> concerns how to assign a value to a variable based on a run time 
> contingency. As a simple illustration, consider the goal of assigning 
> a uniform r.v. value to a variable, while rejecting values less than, 
> say, 0.8.  If  -while- was sensitive to run time values of variables, 
> I would do:
> 
> gen x = uniform()
> while (x < 0.8) {
>    replace x = uniform()
> }
> 
> Obviously, this construct that would work in many languages does not 
> work as desired in Stata, and just as obviously, there are other ways 
> to accomplish the same end in Stata [  gen x = 0.8 + 0.2*uniform() ].

The general approach is to recognize that Stata is inherently vectorized.

Therefore, 

  . gen x = uniform()
  . replace x = uniform() if x < 0.8

accomplishes what you want.

The above says replace x only if the currrent (run time) value of x is less than 0.8.

Tom



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