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RE: st: Recode slow


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   RE: st: Recode slow
Date   Tue, 25 Oct 2005 10:43:10 +0100

I agree strongly. I never use -recode-, which is not 
to deny its usefulness to those familiar with it. 
But if speed is an issue its overhead should be 
considered. 

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

Ulrich Kohler replied to Brendan Halpin

> > Is it normal that recode should be very slow with large numbers of
> > "rules"? I find a recode statement with >400 value assignments adds
> > something of the order of a minute to a job.
> >
> > N is moderately large (75k) but I wonder if recode is linear in N
> > but non-linear in the number of rules or assignments.
> >
> > If so, any tips for efficiency? Break up the command into several
> > smaller recodes? Ship out the equivalences to a lookup table and
> > merge?
> 
> I don't know how it depends on N and the number of rules, but 
> be aware that 
> -recode- is merely a wrapper for -generate- and -replace-. 
> Hence -recode- 
> interprets what the user says, constructs the -generate- and 
> -replace- 
> commands that are equivalent and let Stata process thru these 
> -generate- and 
> -replace- statements. Stata it is always faster if you write down the 
> equivalent -generate- and -replace- commands -- but you will 
> probably want  
> to add the writing-time to the processing-time. 
> 
> Personally I never use -recode-. Instead I build 
> -generate/replace statements- 
> with  -inlist()- and -inrange()-, which are fast in writing 
> _and_ processing. 
> Also note Stata-Tip 16 "Using input to generate variables" 
> (SJ 5,1 134-135) 
> and Kantor/Cox "Depening on conditions: a tutorial on the cond() 
> function" (SJ 5,3 405-412) 
> 
> Shipping out the equivalences to a lookup table and merging 
> is always worth thinking about. 

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