| Re: position matching [message #54005 is a reply to message #54004] |
Tue, 15 May 2007 07:36   |
cmancone
Messages: 30
Registered: May 2007
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Member |
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Yes, I read that article. However, it doesn't quite translate well
into what I need. It presents two methods, one using arrays, the
other using a Delaunay triangulation (DT). For my purposes (20,000
stars) the array method won't work - it requires way too much memory
(I pondered a similar solution myself). That leaves the DT method.
There's two problems with this. First, I don't just need the closest
neighbor, I need the closest neighbor within a certain distance.
Persumably, this is easily solved with a properly placed WHERE or IF
statement. The bigger problem, however, is that I am matching up two
separate lists, and I can't have stars on one list matching up stars
on the same list. The DT doesn't make any distinction between stars
as far as I can tell. You give it one combined list, and it finds the
closest stars no matter where they come from, which is a problem.
I've been trying to figure out just how the DT works, so I can
determine if it is possible to disentangle the two star lists or not.
It's a bit confusing though, and I have yet to determine if it will
work for my purposes.
On May 15, 10:14 am, Paolo Grigis <pgri...@astro.phys.ethz.ch> wrote:
> Seems to be a recurring theme... here's a nice article:
>
> http://www.dfanning.com/code_tips/slowloops.html
>
> Ciao,
> Paolo
>
> cmanc...@ufl.edu wrote:
>> Hi everyone,
>> A common task I have to do is take two lists of stars with x & y
>> positions and match up the closest stars within a certain radius (so
>> that each star has at most one match, that one being the best match).
>> A long time ago I wrote some code to do this that gets the job done,
>> but probably not in the fastest way. It just uses a for loop over one
>> of the lists and uses a where to search for the closest star to each
>> star on the other list. Most of the time this is more than adequate,
>> but anytime my star lists get around 10000-20000 stars each (which
>> happens on a not-so irregular basis) the program turns into quite a
>> beast and takes its sweet time (i.e. a minute or two). Granted, this
>> isn't exactly research-stopping time delays, but I'm sure that with a
>> well thought-out algorithm, the execution time could be pulled down to
>> a handful of seconds. The problem is, I have yet to come up with a
>> well thought-out algorithm. I'm sure I'm not the only one who has run
>> into this, so I was hoping there might be someone else out there that
>> has dealt with the same thing, and knows a better way.
>> -Conor
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