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st: RE: forvalues and scalar: easy question


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: RE: forvalues and scalar: easy question
Date   Thu, 27 May 2004 12:07:51 +0100

How to do this was mentioned in passing in my posting 
"counting unique values across a set of variables"
on Tuesday. You don't need a scalar here, or 
a prior -generate-, nor a prior -summarize-. 
You can do it directly with 

forval i = 1/`=_N' { 

Please see that posting for more explanation, 
and embedding in a real example. 

Your approach is correct in principle, except 
for trying to refer to the scalar m as if
it were in a local macro m. So a first fix would 
be 

gen obs = _n 
sum obs
local m = r(max) 
forval i = 1/`m' { 

Because small or occasionally large problems with various 
details here could arise in other contexts, I will 
spell out various issues. 

1. -gen obs = _n- is a useful idiom for many problems. 
In really big datasets, you would be better off 
specifying a -long-:  

gen long obs = _n 

2. -sum obs- will take up more effort than is needed
just to get a maximum. Use the -meanonly- option 
(which despite its name yields the maximum as well). 

3. We could pick up the result of the -summarize- 
directly. 

So your approach could be condensed to 

gen long obs = _n 
su obs, meanonly 
forval i = 1/`r(max)' { 

However, as said, I advise just using _N directly. 

This still leaves open the question of whether 
you really need a loop over observations. Most of 
the time, you don't. 

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

Justina.Fischer@unisg.ch

since I have already disturbed a colleague who could not help 
me out I am going to confront you with my small problem:

I want Stata to run a loop from 1 to the maximum of observations in my dataset.
I tried:

gen obs2 = _n

sum obs2

scalar m = r(max)

forvalues i=1/`m'{
....
}


Which did not work. There is probably a very easy way to do that 
but I just do not know it.


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