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RE: st: RE: Re: Like %GL(r,n) in GLIM


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   RE: st: RE: Re: Like %GL(r,n) in GLIM
Date   Wed, 28 Feb 2007 11:59:58 -0000

-egen, seq()- was published in STB-50 in 1999
and added to Stata 7. 

An earlier program -seq- was published in STB-37 in 1997. 
It is now obsolete. 

I wanted to construct such sequences often enough 
myself and people used to post questions like Herb's 
on Statalist at the time. I know also that in writing 
the code I looked at books on GLIM and also on S 
to see what was available to users of those programs. 

There is much two-way traffic here. I understand that 
R now has reshape commands similar to -reshape-. For
that matter, many programs have say square root 
or log functions, all with remarkably similar names.

A more interesting general question is that users may
want to do something quite simple and may be unclear 
whether they 

0. Need to program it themselves. 

1. Can find a way of doing it with functions. 

2. Can find a way of doing it with an inbuilt 
command. 

If it's really simple it is unlikely that you are
the first to want it! 

-egen, seq()- is really just a convenience function
as probably everything it does could be done in 
one line with more basic functions and _n. It is
just that many of us would need to fiddle a bit
to get there. You need to subtract 1 and add 1
in the right places and getting it slightly wrong
is getting it wrong. 

For sequences like 1, 2, 3, 4, 1, 2, 3, 4, ...
note first that the remainder on dividing 
integers 1 up by 4 goes 

1, 2, 3, 0, 1, 2, 3, 0, ...

and the remainder on dividing integers 0 up
by 4 goes 

0, 1, 2, 3, 0, 1, 2, 3, ...

from which one solution is 1 + mod(_n - 1, 4). 

For sequences 1, 1, 1, 1, 2, 2, 2, 2, 
there is a solution 

1 + int((_n - 1) / 4) 

with some similarity of structure, but 
more direct is 

ceil(_n / 4) 

There is more on -floor()-, -ceil()- and -mod()-
in the series on Tips in the Stata Journal. 

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

Herb Smith
 
> Thanks all, and some clever solutions, too.
> 
> I know that back in the day, Stata had not had such a function, and as
> Nick's original e-mail suggests, someone probably wrote one up --egen
> seq()- for one of us GLIM aficianados!
> 
> As I said previously, I was having the hardest time trying to 
> think of a good keyword to search on...

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