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Re: st: create a variable based on a recurring value in a varlist

From   Ulrich Kohler <>
Subject   Re: st: create a variable based on a recurring value in a varlist
Date   Thu, 13 Oct 2005 09:43:48 +0200

Derek Darves wrote:
> I have a group of variables:
> PID1 - PID15
> PID* takes on values from 1 to 8000, and many are missing.
> Basically, I would like to make a new variable, called interlock,
> that is equal to 1 if any of the variables in the list are equal to
> any other variable in the list (not including itself, of course).
> For example, if PID5==705 and PID14==705 I would like like interlock==1
> Likewise, if none of the the variables in PID* take on the value of
> any of the other variables in PID*, I would like interlock==0
> I tried this:
> egen interlock = group(pid1_a  pid1_b  pid2_a  pid2_b  pid3_a
> pid3_b  pid4_a pid4_b  pid5_a  pid5_b  pid6_a  pid6_b  pid7_a
> pid7_b  pid8_a  pid8_b  pid9_a  pid9_b  pid10_a  pid10_b  pid11_a
> pid11_b  pid12_a  pid12_b  pid13_a  pid13_b  pid14_a  pid14_b   pid15_a)
> , but it returned all missing values when I know that some share a
> common value in two of the PID* fields.
> Lastly, not that it should matter, but the above is a simplifying
> example. In my actual dataset I have about 130 PID* variables. I just
> mention this in case I am hitting some kind of memory limitation (I
> am not receiving any errors when I run the command, though, it just
> doesn't work).

There might be easier solutions, but I always think reshape-by:

// Preparations for easy reshape
local i 1
foreach var of varlist pid* {
	ren `var' pid`i++'
gen index = _n  

// Solution for Problem
reshape long pid, i(index) j(var)
by index (pid), sort: gen same = sum(pid==pid[_n-1])
by index: replace same = same[_N] >= 1
reshape wide

. by by


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