| Some histogram magic help required - gridding/counting large dataset [message #88153] |
Tue, 25 March 2014 03:11  |
rjp23
Messages: 97
Registered: June 2010
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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.
Cheers
Rob
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| Re: Some histogram magic help required - gridding/counting large dataset [message #88171 is a reply to message #88156] |
Tue, 25 March 2014 07:59  |
rjp23
Messages: 97
Registered: June 2010
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Member |
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It's two 1D datasets with e.g. 20,000 values.
How I've used hist_nd before was to put these into a 2D dataset [20,000, 2] and then run hist_nd on that data but that doesn't seem to work and is only returning 1 bin, even though I'm specifying e.g. [100,2] bins.
On Tuesday, March 25, 2014 12:56:48 PM UTC, AMS wrote:
> If it is a 2D (x/y) dataset you could use the built-in function hist_2d: http://www.exelisvis.com/docs/HIST_2D.html
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> What exactly is the issue you are having? You can specify bin sizes and min/max values with keywords min1, max1, bin1, min2, max2, bin2.
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> Andy
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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
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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.
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> 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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