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

[Nick Cox <njcoxstata@gmail.com>]

See Stata Tip 71 in Stata Journal 8(4) 2008.

Nick

On 12 Jul 2011, at 13:41, Oliver Jones <ojones@wiwi.uni-bielefeld.de>  
wrote:

> 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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