st: looping with geodist

["Frederick Guy" <f.guy@bbk.ac.uk>]

Robert Picard sent the code below, which works as advertised - many thanks, Robert! Now I have a slightly different problem: I have two kinds of locations in the data, i and j. For each location of type i, I need to compute the distances to every location of type j. If I just stack observations type i on top of observations type j, geodist doesn't like the missing values (observations type i have missing values for type j, and vice versa). Can anybody suggest a solution?

-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Robert Picard
Sent: 30 April 2010 17:09
To: statalist@hsphsun2.harvard.edu
Subject: Re: st: RE: RE: RE: AW: Creating index relative to other observations

Perhaps the following example is close to what you are trying to do.
It loops through all observations. Each time, it calculates the
distance from observation `i' to all others (distance will be missing
for the observation `i'). Values for variable x1 are adjusted
according to the distance to `i' and summed. The observation `i' of x3
is then updated with the value of the sum plus the value of x2 for
observation `i'.

Hope this helps,

Robert
http://robertpicard.com/

*--------------------------- begin example -----------------------
version 11

* This example require my -geodist- program available on SSC
* To install: ssc install geodist

clear all
set obs 5
set seed 1234
gen lat = 37 + (41 - 37) * uniform()
gen lon = -109 + (109 - 102) * uniform()
gen x1 = round(uniform()*100)
gen x2 = round(uniform()*100)
gen x3 = .

forvalues i = 1/`c(N)' {
	geodist lat lon `=lat[`i']' `=lon[`i']' if _n != `i', gen(d)
	gen xtemp = x1 / d
	sum xtemp, meanonly
	qui replace x3 = r(sum) + x2 in `i'
	list
	drop d xtemp
}
*--------------------- end example --------------------------


On Fri, Apr 30, 2010 at 7:49 AM, Frederick Guy <f.guy@bbk.ac.uk> wrote:
> Many thanks. Now for a crash-course in MATA...
>
> -----Original Message-----
> From: owner-statalist@hsphsun2.harvard.edu
> [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Nick Cox
> Sent: 29 April 2010 19:22
> To: statalist@hsphsun2.harvard.edu
> Subject: st: RE: RE: AW: Creating index relative to other observations
>
> I'd do this in Mata. Mata has a -for- loop.
>
> Nick
> n.j.cox@durham.ac.uk
>
> Frederick Guy
>
> Thanks, I guess I was unclear on this aspect of the problem. For each
> observation, the sum I'm talking about is of measurements made relative
> to all other observations (or more generally, to some set of other
> observations) in the sample.
>
> Martin Weiss
>
> ".. sum up the results of these computations,".
>
> Creating sums can mean different things in Stata. It may sound trite,
> but
> the easiest is simply to -generate- a sum by adding values with a "+"
> sign.
> If you want the total of a variable, look at -egen, total()-. If you
> want a
> running sum, take a look at -help sum()-.
>
> Frederick Guy
>
> I have need to use information from all observations (about 1800 of
> them) to create a new variable.
>
> The variable created is a weighted sum of the inverse of geographical
> distances between observation i and all j n.e. i. I have longitude and
> latitude for each observation, and computation of the distance from any
> i to any j is straightforward. What I don't know is how to get Stata to
> loop over all observation and sum up the results.
>
> For every observation i, I think I need to
>
> (a) loop through all j n.e. I, doing computations involving variables
> x1, x2(i) and x1, x2(j), and then
>
> (b) sum up the results of these computations, returning a value which
> becomes variable x3 for that i.
>
> I expect there's a straightforward way to do this. Any suggestions?
>
> *
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