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st: RE: RE: Re: Two "forvalues" loops working together


From   siyama@who.int
To   statalist@hsphsun2.harvard.edu
Subject   st: RE: RE: Re: Two "forvalues" loops working together
Date   Fri, 14 May 2004 13:10:34 +0200

Many thanks to Jean Ries for pointing-out the use of a macro and Nick Cox
for perfecting the necessary code. I have got exactly what I need.

Viva "stata-list"

Amani



-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu
[mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Nick Cox
Sent: 14 May 2004 12:19
To: statalist@hsphsun2.harvard.edu
Subject: st: RE: Re: Two "forvalues" loops working together

This won't work yet, as constructs like `Y+10' 
won't work the way you want them to work. 

There is, as far as I know, no undocumented syntax for looping through two
sets of values simultaneously.  

You can go `=`Y'+10' to get an evaluation on the fly. 

You can also go `X--' to get decrementing on the fly. 

But in this case, the problem is not a full-blown multiple loop problem: it
can be reduced to a single loop, as Jean showed. There is, however, a
trade-off between concision and clarity. In this case I would be driven
mostly by the consideration that I might well want to come back to this code
and understand it immediately and be able to modify it quickly. Clarity
would like win over concision in favour of something like 

local X = 43
forvalues Y = 120(10)530 {
	local Y2 = `Y' + 10 
	predict prob`Y', pr0(`Y', `Y2')
	predict exp`Y', e0(`Y', `Y2')
	local where = `samp' - `X' 
	local X = `X' - 1 
	replace prob = prob`Y' in `where'
	replace exp = exp`Y' in `where'
} 

You can squeeze this to fewer lines with 

local X = 43
forvalues Y = 120(10)530 {
	predict prob`Y', pr0(`Y', `=`Y' + 10')
	predict exp`Y', e0(`Y', `=`Y' + 10')
	local where = `samp' - `X--' 
	replace prob = prob`Y' in `where'
	replace exp = exp`Y' in `where'
} 

and even fewer 

local X = 43
forvalues Y = 120(10)530 {
	predict prob`Y', pr0(`Y', `=`Y' + 10')
	predict exp`Y', e0(`Y', `=`Y' + 10')
	replace prob = prob`Y' in `=`samp' - `X''
	replace exp = exp`Y' in `=`samp' - `X--''
}

but I think many Stata users would find this more difficult to read. 

There is a much more detailed discussion of such parallel loops problems in
Stata Journal
3(2):185-202 (2003). 

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

jean ries

> Here is a suggestion. Hope it helps.
> 
> local X 43
> 
> forvalues Y = 120(10)530 {
>                  predict prob`Y', pr0(`Y',`Y+10')
>                  predict exp`Y', e0(`Y',`Y+10')
>                  replace prob=prob`Y' in `samp-`X''
>                  replace exp=exp`Y' in `samp-`X''
>                  loc X = `X' - 1
> }

Amani Siyam 
 
> I wish to make two "forvalues" loops work together rather than nested 
> as shown by my suggested code below.  Is there a way I can do that in 
> STATA
> 
> 
> local samp=_N+43
> set obs `samp'
> foreach M of newlist prob exp {
>          gen `M'=.
> }
> intreg ptiming timing if ~npointer1, noskip intreg forvalues Y = 
> 120(10)530 ;  X = 43(-1)1 {
>                  predict prob`Y', pr0(`Y',`Y+10')
>                  predict exp`Y', e0(`Y',`Y+10')
>                  replace prob=prob`Y' in `samp-`X''
>                  replace exp=exp`Y' in `samp-`X''
> }
> local samp=_N-43
> slist prob exp in `samp'/l
> 

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