Count unique values in Stata

Question

codebook is a great command in Stata. It describes data contents but also simply identifies unique values

sysuse auto, clear
codebook mpg, compact

Number of unique values of mpg is 21. Looking at the help of the command, it does not seem possible to store this value. Am I wrong?

I am aware of other ways to compute the number of unique values in Stata, but it would be so convenient to add this feature to the codebook command.

Answer 1

You can easily write a wrapper for codebook that uses Nick's distinct command from SSC to store the info you want as scalar(s).

In my experience, this wrapper approach has proven to be much more effective than asking the nice folks at StataCorp to change their command on an internet forum that they do not participate in.

Here's an example:

* (1) You can stick this into a file called mycodebook.ado in
* /ado/personal (use adopath to see exact location)
capture program drop mycodebook
program mycodebook, rclass
syntax [varlist] [if] [in][, *]
codebook `varlist' `if' `in', `options'
capture ssc install distinct
foreach var of varlist `varlist' {
    qui distinct `var' `if' `in'
    return scalar nv_`var' = r(ndistinct)
}
end

* (2) example with mycodebook
sysuse auto, clear
mycodebook price mpg rep78 if foreign==0, compact
return list

This last part will give you:

. mycodebook price mpg rep78 if foreign==0, compact

Variable   Obs Unique      Mean   Min    Max  Label
----------------------------------------------------------------------------------
price       52     52  6072.423  3291  15906  Price
mpg         52     17  19.82692    12     34  Mileage (mpg)
rep78       48      5  3.020833     1      5  Repair Record 1978
----------------------------------------------------------------------------------

. return list

scalars:
           r(nv_rep78) =  5
             r(nv_mpg) =  17
           r(nv_price) =  52

You can then do things like (or whatever it is you want to do with these):

gen x=r(nv_rep78)

Answer 2

A convenient alternative is provided by the "unique" package. Here's a quick example:

* Install the unique package
ssc inst unique

* Load toy dataset
sysuse auto, clear

* Get a quick report of unique (and total) values for a variable
unique mpg

* The result will be available as r(unique)
return list