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Re: Some histogram magic help required - gridding/counting large dataset [message #88172 is a reply to message #88153]

Tue, 25 March 2014 08:28

David Fanning
Messages: 11724
Registered: August 2001

Senior Member

rjp23@le.ac.uk writes:

> I have 2 large datasets that I want to plot as the x and y data on a scatter plot.
>
> However I'd also like to grid this data in the x and y direction and count how many datapoints fall into each grid cell.
>
> I know I should be able to do this with hist_nd but I just can't figure out exactly how to do it

I believe your thinking about this is All Wrong. :-)

Just histogram each 1D data set into the same number of bins.

h1 = cgHistogram(data1, NBins=100, Reverse_Indices=ri1)
h2 = cgHistogram(data2, NBins=100, Reverse_Indices=ri2)

When you want to find out, for example how many "hits" you have in bin
25 in the first data set and bin 45 in the second data set, you do this:

b25indices = cgReverseIndices(ri1, 24, COUNT=c1)
b45indices = cgReverseIndices(ri2, 44, COUNT=c2)

IF (c1) GT 0) && (c2 GT 0) THEN BEGIN
indices = cgSetIntersection(b25indices, b45indices, COUNT=count)
IF count GT 0 THEN BEGIN
Print, 'Matches: ', count
ENDIF ELSE Print, 'No matches.
ELSE Print, 'No matches'

If you want to plot the points in this intersection of bins:

cgPlot, data1[indices], data2[indices], PSYM=1

Cheers,

David
--
David Fanning, Ph.D.
Fanning Software Consulting, Inc.
Coyote's Guide to IDL Programming: http://www.idlcoyote.com/
Sepore ma de ni thue. ("Perhaps thou speakest truth.")

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[Message index]

		Some histogram magic help required - gridding/counting large dataset By: rjp23 on Tue, 25 March 2014 03:11
		Re: Some histogram magic help required - gridding/counting large dataset By: Andy Sayer on Tue, 25 March 2014 05:56
		Re: Some histogram magic help required - gridding/counting large dataset By: rjp23 on Tue, 25 March 2014 07:59
		Re: Some histogram magic help required - gridding/counting large dataset By: David Fanning on Tue, 25 March 2014 08:28
		Re: Some histogram magic help required - gridding/counting large dataset By: Phillip Bitzer on Tue, 25 March 2014 09:10

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