# RE: st: dropping even observations

 From "Nick Cox" To Subject RE: st: dropping even observations Date Fri, 6 Oct 2006 14:03:35 +0100

```Interesting question.

Stata does process in observation order, but
in effect _n is a pseudovariable
that is not re-calculated until the -drop-
is completed. Otherwise the advice given would
have been incorrect. (I did emphasise that no new
variable was needed.)

An even simpler example is

drop if _n == 24 | _n == 42

If _n were re-evaluated once 24 has been
dropped, then the observation -drop-ped
would not be the one you want to keep.

Although there would be an understandable logic
to what you fear, here Stata behaves as you would
hope.

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

Jeph Herrin

> 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.

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?
> >
> >
> > 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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```