Counting distinct values for a variable by other variable

[davidsantino]

~~Hi,
I have a problem counting distinct values. Do you know if you can count the number of unique values in a variable by other variable? I mean, I want to count the number of distinct companies within a specific gegographical area, so I have a variable containing gegographical data (a categorical variable), and a variable containing different companies, and I want to count how many different companies are in the different areas.
Thanks in advance.
David

[Nick Cox]

The question of counting distinct (not unique) values was reviewed in nauseating detail in

SJ-8-4 dm0042 . . . . . . . . . . . . Speaking Stata: Distinct observations
(help distinct if installed) . . . . . . N. J. Cox and G. M. Longton
Q4/08 SJ 8(4):557--568
shows how to answer questions about distinct observations
from first principles; provides a convenience command

.pdf at http://www.stata-journal.com/sjpdf.h...iclenum=dm0042

Many ways to do it: here is one.

egen tag = tag(company area) 
egen count = total(tag), by(area)

Here is another:

bysort area company: gen count = _n == 1 
by area: replace count = sum(count) 
by area: replace count = count[_N]