| Re: array index summations [message #69189] |
Fri, 18 December 2009 10:48  |
H. Evans
Messages: 18
Registered: December 2009
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Junior Member |
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On Dec 18, 7:35 pm, David Fanning <n...@dfanning.com> wrote:
> H. Evans writes:
>> The difference is that the histogramming functions count the number of
>> points in the bins, i.e. the number of points between x and x+width.
>> Whereas the CONGRID, GRIDDATA, REBIN functions interpolate the data
>> points to an X-Y grid.
>
>> This function performs statistics on the contents of the Z vector,
>> i.e. what is the mean value of the data points in the range x->x+dx,
>> not how many data points are in the range X->x+dx, which the histogram
>> function provides. In other words, it finds the data points that are
>> in the bin, and then sums up the Z values in that bin.
>
>> If the histogram function provided a weighting function to the
>> counting, then this could be used to sum the Z values in the bin.
>
> I'm not following this closely, but I think the point is
> that HIST_ND could tell you which voxels were in each
> XY bin, and you could then perform your own statistics
> or weighting function on those values, however you liked.
>
> This would reduce the complexity of your code significantly.
Hi,
Finding the indices of the data points in a 2d bin is not the hard
part. What I'm trying to avoid is having a FOR loop that iterates over
the bins doing the statistics on the data points that are in a bin.
Ta.
Hugh
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| Re: array index summations [message #69190 is a reply to message #69189] |
Fri, 18 December 2009 10:35  |
David Fanning
Messages: 11724
Registered: August 2001
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Senior Member |
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H. Evans writes:
> The difference is that the histogramming functions count the number of
> points in the bins, i.e. the number of points between x and x+width.
> Whereas the CONGRID, GRIDDATA, REBIN functions interpolate the data
> points to an X-Y grid.
>
> This function performs statistics on the contents of the Z vector,
> i.e. what is the mean value of the data points in the range x->x+dx,
> not how many data points are in the range X->x+dx, which the histogram
> function provides. In other words, it finds the data points that are
> in the bin, and then sums up the Z values in that bin.
>
> If the histogram function provided a weighting function to the
> counting, then this could be used to sum the Z values in the bin.
I'm not following this closely, but I think the point is
that HIST_ND could tell you which voxels were in each
XY bin, and you could then perform your own statistics
or weighting function on those values, however you liked.
This would reduce the complexity of your code significantly.
Cheers,
David
--
David Fanning, Ph.D.
Fanning Software Consulting, Inc.
Coyote's Guide to IDL Programming: http://www.dfanning.com/
Sepore ma de ni thui. ("Perhaps thou speakest truth.")
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| Re: array index summations [message #69191 is a reply to message #69190] |
Fri, 18 December 2009 10:28 |
H. Evans
Messages: 18
Registered: December 2009
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Junior Member |
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On Dec 18, 6:00 pm, Gianguido Cianci <gianguido.cia...@gmail.com>
wrote:
> On Dec 18, 5:07 am, "H. Evans" <bloggs...@googlemail.com> wrote:
>
>> I have a time-series data set, effectively a 3-vector (x,y,z), where
>> the Z values are to be binned into a 2D array along X/Y.
>
> Isn't this what hist_nd.pro was invented for? or for that matter, the
> inbuilt hist_2d? or am I missing something?
>
> Gianguido
The difference is that the histogramming functions count the number of
points in the bins, i.e. the number of points between x and x+width.
Whereas the CONGRID, GRIDDATA, REBIN functions interpolate the data
points to an X-Y grid.
This function performs statistics on the contents of the Z vector,
i.e. what is the mean value of the data points in the range x->x+dx,
not how many data points are in the range X->x+dx, which the histogram
function provides. In other words, it finds the data points that are
in the bin, and then sums up the Z values in that bin.
If the histogram function provided a weighting function to the
counting, then this could be used to sum the Z values in the bin.
Ta,
Hugh
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| Re: array index summations [message #69365 is a reply to message #69189] |
Tue, 29 December 2009 20:58 |
Jeremy Bailin
Messages: 618
Registered: April 2008
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Senior Member |
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On Dec 18, 1:48 pm, "H. Evans" <bloggs...@googlemail.com> wrote:
> On Dec 18, 7:35 pm, David Fanning <n...@dfanning.com> wrote:
>
>
>
>
>
>> H. Evans writes:
>>> The difference is that the histogramming functions count the number of
>>> points in the bins, i.e. the number of points between x and x+width.
>>> Whereas the CONGRID, GRIDDATA, REBIN functions interpolate the data
>>> points to an X-Y grid.
>
>>> This function performs statistics on the contents of the Z vector,
>>> i.e. what is the mean value of the data points in the range x->x+dx,
>>> not how many data points are in the range X->x+dx, which the histogram
>>> function provides. In other words, it finds the data points that are
>>> in the bin, and then sums up the Z values in that bin.
>
>>> If the histogram function provided a weighting function to the
>>> counting, then this could be used to sum the Z values in the bin.
>
>> I'm not following this closely, but I think the point is
>> that HIST_ND could tell you which voxels were in each
>> XY bin, and you could then perform your own statistics
>> or weighting function on those values, however you liked.
>
>> This would reduce the complexity of your code significantly.
>
> Hi,
>
> Finding the indices of the data points in a 2d bin is not the hard
> part. What I'm trying to avoid is having a FOR loop that iterates over
> the bins doing the statistics on the data points that are in a bin.
>
> Ta.
> Hugh
As often seems the case, I think this article might help:
http://www.dfanning.com/code_tips/drizzling.html
-Jeremy.
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