# st: RE: RE: Short program to "collapse (# unique elements)": Use of nested loops and a "weights not allowed" message

 From "Nick Cox" To Subject st: RE: RE: Short program to "collapse (# unique elements)": Use of nested loops and a "weights not allowed" message Date Tue, 30 Sep 2003 13:12:33 +0100

```Chih-Mao Hsieh

> > I have a
> > data file with three columns: citing, cited, nclass.  For
> > every "citing", there are multiple "cited", and for each
> > "cited" there is a "nclass".  The file is sorted by citing,
> > then nclass.  I need a program to count the number of
> > unique "nclass" strings associated to each "citing".
> >
> > As a simple example, given the following data file "data.dta":
> >
> > citing     cited         nclass
> > 100         20            12
> > 100         22            15
> > 100         23            15
> > 101         32            14
> > 101         33            15
> > 101         34            15
> > 101         40            17
> >
> > I need the following output file:
> >
> > citing    numpatclass
> > 100            2             [12 and 15 are unique, 15 is
> repeated]
> > 101            3             [14, 15, 17 are unique, 15
> is repeated]

> Phil Ryan gave excellent advice explaining how
> this can be done, without loops, by using -by:-.
>
> In addition, note the FAQ
> How do I compute the number of distinct observations?
> http://www.stata.com/support/faqs/data/distinct.html
> which explains approaches using -by:-, similar in
> spirit to Phil's solution, and also gives manual
> references and references to user-written software
> in this area.
>
> Thus, a canned solution here is
>
> bysort citing : egen numpatclass = nvals(nclass)
> by citing : keep if _n== 1

Another approach is a double -contract-:

contract citing nclass
contract citing, freq(numpatclass)

After the first -contract-, the number
of observations for each value of -citing-
is the number of distinct values of -nclass-
observed for each;
so the second -contract- immediately yields
the desired count variable.

That this solution using -contract- makes
no use of -by:- or -_N- is pure illusion.
Look inside -contract- at the Stata code
-- -contract- is implemented as an .ado --
and you will see that it is based on
exactly the same machinery.

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

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```