Re: st: -generate- and run time contingencies

[Mike Lacy <Michael.Lacy@colostate.edu>]

[Regarding my request for generic ways to
to use while or if constructs with a run-time
value of a variable,

n j cox wrote:

>Mike wants generic ways to do itnot
>work-arounds.

>I am still on the lookout for a second
>example of his class of problems.


Actually, I seem to encounter it fairly
commonly. I've always programmed in SPSS and
other procedural languages in which something
like "while condition, calculate.. " is a
natural construct to use with a variable,
i.e, an item in an array of cases. So, FWIW,
here are some more examples that occur to me:

A classic algorithm (See Numerical Recipes...) for
generating values from Normal(0,1) would look
something like this in Stata, if -while- could
access the run-time value of a variable:

tempvar v1 v2 done
gen `done' = 0
while not `done'  {
   gen `v1' = 2.0 * uniform() - 1.0
   gen `v2' = 2.0 * uniform() - 1.0
   gen r = `v1' * `v2'
   gen `done' = (r < 1.0 ) & (r > 0.0) /*
}
etc.  more stuff

More generally, applications using rejection sampling
to generate a random variable would seem to be require
acces to the run-time value of a variable.


Another example occurs when one wants to do what
I think of a "rippling" through from 1 to N, using
the run-time value of a previous case.
For example, to consecutively groups of cases
based on some kind of shared value of a key,
a natural approach in other languages would
be something that would not fit with Stata:

* More Code that wouldn't work in Stata
sort SomeKey
generate GroupID = 1 if _n ==1
replace GroupID = GroupID[_n-1] ///
                   if (_n > 1)  & !(SomeKey == SomeKey[_n-1])

I know there are ways to accomplish this goal in
Stata, but I'm trying to adapting my existing bag
of algorithmic tricks :-}.


I like the suggested approach in a later posting of doing the following:

gen x = uniform()
gen byte OK = x >= 0.8
capture assert OK == 1
while _rc {
        replace x = cond(x < 0.8, uniform(), x)
        replace OK = x >= 0.8
        capture assert OK == 1
}

However, it does seem odd that one needs to use what looks like a 
trick to get make a value of a variable something that -while- can 
access  at run time. I wouldn't have thought of this.

Regards,




=-=-=-=-=-=-=-=-=-=-=-=-=
Mike Lacy
Fort Collins CO USA
(970) 491-6721 office






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