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RE: st: AW: forvalues & replace not working under two 'not equal to' conditions


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   RE: st: AW: forvalues & replace not working under two 'not equal to' conditions
Date   Tue, 10 Nov 2009 21:51:07 -0000

I've not been following this closely. It seemed to me that Martin was pointing you towards better ways to work but that you were reluctant to abandon your own code. 

Whether that's right or wrong, look at your second -forval- loop, for example. 

1. Your loop ends when the index i is the number of observations _N and the subscript firm_id[_n + `i'] is thus firm_id[_n +  _N]. 

2. That subscript is _always_ outside the range of observations, as it varies from 1 + _N to _N + _N.  

3. Thus you are referring to a value of -firm_id- that doesn't exist, but that's not illegal and Stata substitutes the empty string "". 

4. It's always going to be true that this is _not_ equal to any non-empty string. 

5. Your code does all this within -by contract_id:- but that does not change the fundamentals. Outside the range means outside the current -by:- group and the problem still bites. 

6. Actually your code also mixes the overall number of observations _N -- which is fed to it from the -forval- loop -- and the within-group observation number _n, which is used by virtue of -by:-. That's not illegal, but it doesn't seems to be what you want at all. 

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

joe j

Thanks again Martin. With your approach one could get the same results
as with my original codes (in the original post) for the first three
variables. My problem is with the last variable. I don't understand
why the following code does not work (which aims variable 'nongroup_f'
to take the value 1 only when 'firm_id' as well as 'nation_id' of
preceding and/or following observations are different ). Instead the
variable takes the value 1 for all observations:

gen nongroup_f=.

forvalues i=1/`=_N'{
bys contract_id: replace nongroup_f=1 if (firm_id~=firm_id[_n-`i']) &
(nation_id~=nation_id[_n-`i'])
}

forvalues i=1/`=_N'{
bys contract_id: replace nongroup_f=1 if (firm_id~=firm_id[_n+`i']) &
(nation_id~=nation_id[_n+`i'])
}

On Tue, Nov 10, 2009 at 6:25 PM, Martin Weiss <martin.weiss1@gmx.de> wrote:
>
> <>
>
> So a revised version for your first dummy is:
>
>
> *************
> clear*
> inp byte(contract_id firm_id)  /*
> */ nation_id:mylabel, auto
> 1   2   "US"
> 1   2   "US"
> 4   3   "UK"
> 4   3   "US"
> 8   4   "US"
> 8   4   "UK"
> 8   3   "US"
> 9   5   "US"
> 9   4   "UK"
> 9   3   "US"
> 10   5   "US"
> 10   5   "US"
> 10   6   "NL"
> 10   7   "NL"
> 10   4   "UK"
> 10   4   "CH"
> end
>
> bys contract_id firm_id nation_id:  /*
> */ egen mycount=count(nation_id)
>
> gen byte group_d=mycount>1 & !mi(mycount)
> drop mycount
>
> l, sepby(contract_id) noo
>
> *************

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