Re: st: dropping even observations

[Jeph Herrin <junk@spandrel.net>]

One related point, not readily apparent to even regular users,
is whether in

 drop if [exp]

expression [exp] is evaluated sequentially for each obs, or
all at once as a vector. My first thought in reading the
question was that the user was asking whether

 drop if !mod(_n,2)

would reset _n after each dropped observation; that is, after
dropping _n==2, is the next observation going to be _n==3 or
_n==2?

Obviously, it's not a big deal to

 gen byte even = !mod(_n,2)
 drop if even

but it's still worth knowing whether one needs the extra step.

Jeph



Nick Cox wrote:
> The following question and reply arose privately. 
> I have been asked this various times before, so the 
> discussion should be of some wider interest. 
> Nick
> n.j.cox@durham.ac.uk 
> Question: Do you have an expression or do file I could 
> use or adapt to drop even rows (_n = 2,4,6 etc) or 
> dropping every other row?  
> Reply: 
> Sure. The remainder on dividing integers 
> by 2 is either 1 or 0 depending on whether 
> those integers are odd or even. In Stata 
> with observation numbers _n this remainder
> is simply 
> mod(_n,2) 
> Logical negation ! flips 0 and 1 the 
> other way round. 
> Thus try 
> sysuse auto, clear 
> list mpg if mod(_n,2) 
> list mpg if !mod(_n,2)
>
> and so forth. 
> Note that 
> 1. you do not need to create any extra 
> variables. 
> 2. the technique generalises easily
> to related problems: e.g. every 5th 
> observation is selected by 
> if !mod(_n,5) 
> Your query adds support to my longstanding
> view that there are useful functions that
> people persistently overlook, although 
> their usefulness can be blindingly obvious
> once pointed out. Sooner or later I will
> write a Stata Journal Tip on -mod()-.
>
>  
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