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Pair Counts in an Annulus, for large data sets [message #51262]

Fri, 10 November 2006 21:52

fatcat3131
Messages: 6
Registered: November 2006

Junior Member

Hi There

I have a large data set (~350,000 galaxies) of x,y points. For a given
radius R [R=sqrt(x^2+y^2)], I need to count the total number of pairs
in the annulus R+deltaR. That is, I choose a given data point as my
center, then count the number of points that lie inside that annulus. I
then do this for each of my data points to get the total number of
pairs. A simplified version of the code I'm using now is as follows:

****************
n = n_elements(x) ; number of data points
seperation = fltarr(n) ; the seperation between data points

for i=0L,n-1 do begin
seperation = sqrt((x - x[i])^2 + (y - y[i])^2) ;distance between the
two points, centering on point "i"
seperation[i] = 999 ;simply because I don't want it to count itself
as a pair

if_inside = ((seperation gt (r)) and (seperation lt (r + deltar_r))
;has value "1" for points which lie inside, "0" for those outside
counter = counter + total(if_sep) ;count up the number of pairs
endfor

num_pairs = counter / 2 ; since I don't want to count everything twice
*****************

I've tried my best to avoid the urge to put lots of for loops
everywhere (you should have seen it before!), but I just don't know how
to make it drastically more efficient. There must be a way though,
because the computations for my code are just ridiculous.... Is there a
way to eliminate that nasty loop I have, which would help things?

Any help you can give would be greatly appreciated. I'm very new to
IDL, as you surely know. I'm and undergrad, too.

Tara.

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[Message index]

		Pair Counts in an Annulus, for large data sets By: fatcat3131 on Fri, 10 November 2006 21:52
		Re: Pair Counts in an Annulus, for large data sets By: David Fanning on Thu, 23 November 2006 19:20
		Re: Pair Counts in an Annulus, for large data sets By: fatcat3131 on Thu, 23 November 2006 18:47
		Re: Pair Counts in an Annulus, for large data sets By: David Fanning on Thu, 23 November 2006 07:58

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