Re: st: RE: RE: RE: product function

[David Kantor <dkantor@jhu.edu>]

At 06:55 PM 5/25/2004 +0100, Nick Cox wrote:
> I don't think it is explicit anywhere in the manual, but
> you can assume that -generate- and -replace- follow the
> current sort order of the observations.
> [...]
Indeed, it is not explicitly stated, and it does work that way; Bill Gould 
assured me that it is something you can depend on (I'm referring to a 
message from Bill Gould to Statalist on Sept 20, 2001).

While we usually think of Stata as operating on the whole dataset at once, 
there is an implicit and usually invisible sequential aspect to it.  This 
is one place where the sequential aspect is significant.

This feature is very useful in propagating a value "down the list", and 
especially with -by-.

Suppose p signifies present in a given year, values 0 or 1.
sort personid year
by personid: replace p=1 if p==0 & p[_n-1] & _n>1

will change p so it now signifies present in the current or any prior 
year.  You could also, more generally, propagate any non-zero value thus:
by personid: replace p=p[_n-1] if p==0 & _n>1

And there are many variations on this theme -- very useful in certain 
situations.

I hope this is interesting.
-- David

David Kantor
Institute for Policy Studies
Johns Hopkins University
dkantor@jhu.edu
410-516-5404

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