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# Re: st: Efficient way to create adjacency matrix in Mata

 From "Lacy,Michael" To "statalist@hsphsun2.harvard.edu" Subject Re: st: Efficient way to create adjacency matrix in Mata Date Thu, 30 Jun 2011 18:55:43 +0000

```On Wed, Jun 29, 2011 at 10:01 PM, Benjamin Allaire wrote:
> Question: what is the most efficient way to create an adjacency matrix in mata,
> given a unique identifier ("id") and a club id ("club_id") for folks in the same club.
>
> So the data looks like this:
>
> id    club_id
> 1       1
> 2       1
> 3       2
> 4       2
> 5       2
>
> Is there a clever way to do it without looping over the people?

Here's another approach:

clear
// create example data
set seed 12579
set obs 10
local nclub = 3
gen int id = _n
gen club_id = 1 + trunc(`nclub' * runiform())
//
// Restructure data so that observations are clubs, variables are member ids w/in clubs
sort club_id
by club_id: gen int nmem = _N if _n == 1
quiet {
summ nmem
forval i = 1/`=r(max)' {
by club_id: gen mem`i' = id[`i'] if (_n ==1)
}
}
//
// Make some stuff nicer for Mata
local nsubj = _N
keep if !missing(nmem)
keep nmem mem*
//
// Mata loops over all non-duplicate pairs within club
mata
mata clear
M = J(0,0,.)
st_view(M, ., .)
for (nc = 1; nc <= rows(M); nc++) {     // each club
nmem = M[nc,1]
for (i = 2; i <= nmem; i++) {       // all non-duplicating pairs of members w/in club
for (j = i + 1; j <= nmem + 1 ; j++) {
ego = trunc(M[nc,i])
alter = trunc(M[nc,j])
}
}
}
end

Regards,

Mike Lacy
Dept. of Sociology