re: st: Identify combinations without respect to the order of the elements

["Airey, David C" <david.airey@vanderbilt.edu>]

.

oops, I gave the wrong subject header...

> I wonder if this message would help?
> 
> <http://www.stata.com/statalist/archive/2011-06/msg01320.html>
> 
> You might find others in the archives.
> 
> -Dave
> 
>> Hello everyone,
>> 
>> I have a dataset with the number of people moving from one
>> city to another city (or stay where they live). We have records
>> for 342 cities. The data set looks like this:
>> (just showing the structure with the three cities A, B and C)
>> 
>> from | to | n
>> ..............
>> A    | A  | 29
>> A    | B  | 1
>> A    | C  | 3
>> B    | A  | 8
>> B    | B  | 90
>> B    | C  | 0
>> C    | A  | 80
>> C    | B  | 70
>> C    | C  | 800
>> 
>> Is there an easy way to generate an identifier variable that
>> identifies pairs of cities?
>> In pseudo code it should be something like:
>> 
>> gen identifier = .
>> local counter = 0
>> forval i = 1/_N {
>>  if "reverse city-combination in the current row i exists in previous rows" {
>>     local ident_value = value of the identifier in that previous row
>>     replace identifier = `ident_value'
>>  }
>>  else {
>>     local counter = `counter' + 1
>>     replace identifier = counter
>>  }
>> }
>> 
>> The result should look like this:
>> 
>> from | to | n   | identifier
>> .........................
>> A    | A  | 29  | 1
>> A    | B  | 1   | 2
>> A    | C  | 3   | 3
>> B    | A  | 8   | 2
>> B    | B  | 90  | 4
>> B    | C  | 0   | 5
>> C    | A  | 80  | 3
>> C    | B  | 70  | 5
>> C    | C  | 800 | 6
>> 
>> 
>> Best
>> Oliver

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