| Re: Adding x,y events to a 2d array (quickly) [message #86435 is a reply to message #86433] |
Thu, 07 November 2013 15:18   |
Michael Galloy
Messages: 1114
Registered: April 2006
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Senior Member |
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On 11/7/13, 3:20 PM, Dick Jackson wrote:
> Phillip Bitzer wrote, On 2013-11-07, 12:16pm:
>> On Thursday, November 7, 2013 1:27:00 PM UTC-6, Dick Jackson wrote:
>>
>>> I seem to recall someone explaining this behaviour before, and thanks to
>>>
>>> Russell, I realize one good way of getting *part* of what you
>>> (reasonably!) want
>>>
>>> to do. If all of your 'e' values were equal, then you can find how
>>> many counts
>>>
>>> of each (x,y) pair exist by using Hist_ND:
>>>
>>> (http://tir.astro.utoledo.edu/idl/hist_nd.pro)
>>>
>>> IDL> Print, Hist_ND(Transpose([[1,1,2],[1,1,2]]), 1, Min=0)
>>>
>>> But, in general, to add a varying set of 'e' values to those (x,y)
>>> locations...
>>>
>>> I have to think a bit...
>>>
>>
>> I've got you covered....
>>
>> Oliver, reverse indices are your friend here, as Russell alluded to.
>> Get the two-dimensional histogram, slightly modified from Dick's version:
>>
>> h = HIST_ND( [ TRANSPOSE(x), TRANSPOSE(y) ], 1, MIN=0,
>> REVERSE_INDICES=ri )
>>
>> Since you said you have large arrays, I transpose each individually,
>> and then concatenate.
>>
>> Now, go through the reverse indices:
>>
>> totalE = FLTARR(SIZE(h, /DIM))
>> FOR i=0, N_ELEMENTS(h)-1 do if h[i] GT 0 THEN totalE[i]= TOTAL(
>> e[ri[ri[i]:ri[i+1]-1]])
>>
>> print, totalE
>> 0.00000 0.00000 0.00000
>> 0.00000 20.0000 0.00000
>> 0.00000 0.00000 10.0000
>>
>> This is the basic idea. It can be sped up by only looping over the
>> elements of h with non-zero counts (as opposed to "skipping" them as I
>> did here).
>>
>> Here's some highly recommended reading on histograms:
>> http://www.idlcoyote.com/tips/histogram_tutorial.html
>
> Histograms and reverse-indices are amazingly powerful and the right way
> to go in many tough problems, but I think Oliver is looking for a
> solution avoiding loops (I am too!). If a loop solution were OK, the
> last block here would be more direct, with no need for histograms:
>
> x=[1,1,2]
> y=[1,1,2]
> e=[10,11,12]
>
> counts=fltarr(3,3)
> counts(x,y)++
> Print, 'counts:'
> Print, counts ; Shows that three increments by 1 were done
>
> totalenergy=fltarr(3,3)
> totalenergy(x,y)+=e
> Print, 'totalenergy:'
> Print, totalenergy ; It appears that only two increments by 10 were done
>
> totalenergy2=fltarr(3,3)
> FOR i=0, N_Elements(x)-1 DO totalenergy2(x[i],y[i])+=e[i]
> Print, 'totalenergy2:'
> Print, totalenergy2 ; All three increments were done
>
> ... which gives us:
>
> counts:
> 0.000000 0.000000 0.000000
> 0.000000 2.00000 0.000000
> 0.000000 0.000000 1.00000
> totalenergy:
> 0.000000 0.000000 0.000000
> 0.000000 11.0000 0.000000
> 0.000000 0.000000 12.0000
> totalenergy2:
> 0.000000 0.000000 0.000000
> 0.000000 21.0000 0.000000
> 0.000000 0.000000 12.0000
>
> Still looking for the "IDL way" (read: "ideal way") to do this...
>
Phillip's method loops over the bins in the histogram, so should be
reasonable. My MG_HIST_ND does the same thing:
IDL> x = [1, 1, 2]
IDL> y = [1, 1, 2]
IDL> weights = [10., 11., 12.]
IDL>
IDL> h = mg_hist_nd([transpose(x), transpose(x)], weights=weights,
min=0, bin_size=1, unweighted=unweighted)
IDL> print, h
0.00000 0.00000 0.00000
0.00000 21.0000 0.00000
0.00000 0.00000 12.0000
IDL> print, unweighted
0 0 0
0 2 0
0 0 1
Get MG_HIST_ND on GitHub:
https://github.com/mgalloy/mglib/blob/master/src/analysis/mg _hist_nd.pro
Mike
--
Michael Galloy
www.michaelgalloy.com
Modern IDL: A Guide to IDL Programming (http://modernidl.idldev.com)
Research Mathematician
Tech-X Corporation
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