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RE: st: creating count within id & date


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   RE: st: creating count within id & date
Date   Sun, 27 Feb 2005 16:50:05 -0000

bysort id (date) : gen visitno = sum(date != date[_n-1]) 

does it in one line.  

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

Joseph Coveney replied to Richard Hiscock
 
> I have the following data structure
> 
>     id       date        var1        var2
> 
>     1        16267         x         53
>     1        16267         y        106
>     1        16267         y        84
>     1        16267         y        90
>     1        16267         z         76
>     1        16267         z         65
>     1        16298         x         87
>     1        16298         y        76
>     2        15678         x        98
>     2        15678         y        98
>     2         16541        x        150
> 
> etc
> I wish to create identifier (visitno) 1,2,3,... for each 
> sequential date
> within id.
> I have tried to use :
> sort id date
> by id date: gen visitno = [_n]  which obviously doesnt produce the
> required variable
> I would be greatful for solution & ?or direction to FAQ dealing with
> this

> How about:
> 
> -by id date: generate byte visitno = _n == 1-
> 
> and then
> 
> -by id: replace visitno = sum(visitno)-
> 
> Or, in one line:
> 
> -bysort id (date): generate byte visitno = ///
>   sum( (_n == 1) | (date > date[_n-1]) )-
> 
> As I recall, Nick Cox wrote a posted a more elegant one-line 
> command for
> this task recently on the list, but I cannot seem to find it.
> 
> You mention that your final aim is to generate summary 
> statistics each date
> within id.  For this, check out -egen-.

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