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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