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RE: st: RE: Changing positive values to negative in Stata

From   "Nick Cox" <[email protected]>
To   <[email protected]>
Subject   RE: st: RE: Changing positive values to negative in Stata
Date   Thu, 20 Apr 2006 22:10:57 +0100

You are correct. In principle, I posted lousy code. 

When Stata does something -if-, then every observation
is tested. In a really big dataset this could 
take a while. 

On the other hand, doing something -in- will
often need a prior -sort-, and the tradeoff
can be subtle. 

With 4000 observations, the difference is 
going to be trivial.  

[email protected] 

> Thanks Nick - I neglected to state that  have over 600 
> observations with 
> all different positive values (as well as negative values and missing 
> values) within the subgroup of total observations of the variable, so 
> this code could be somewhat inefficient.  Also,  it is difficult to 
> specify which observations, for example, by id  - thus  the next best 
> thing in this instance seems to be to sort by their common 
> characteristic - which  is captured by another variable.
> Nick Cox wrote:
> >replace suzy = -suzy if suzy > 0 & <whatever> 
> >
> >So if suzy is positive, it is negated. The 
> >crunch is specifying the extra condition <whatever> 
> >that catches just the observations desired. 
> >
> >Sorting is a red herring here. So long as you 
> >can specify which observations you want, sort order
> >is immaterial. 
> >
> >Nick 
> >[email protected] 
> >
> >Suzy
> > 
> >  
> >
> >>I have a continuous variable with positive, negative, and 
> zero values 
> >>and also missing values(.). 
> >>I would like to know if anyone has an easy Stata code/command 
> >>to change 
> >>just the positive values to a negative (change the sign: 4.4567 to 
> >>-4.4567) for just a particular subgroup of the total (n=661 
> >>out of over 
> >>N=4,000). Within this subgroup (which I can sort, but not by 
> >>id) there 
> >>already are a few appropriately negative values in place.

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