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Re: find max in 3D array -- slow [message #70462 is a reply to message #70378]

Mon, 12 April 2010 09:20

Juggernaut
Messages: 83
Registered: June 2008

Member

On Apr 10, 12:03 pm, "Timothy W. Hilton" <hil...@meteo.psu.edu> wrote:
> Hello IDL users,
>
> I have a 1200x1200x2900 array of floats. The dimensions correspond to
> latitude x longitude x time. I need to find the maxium at each
> location -- that is, I need the 1200x1200 array containing the max
> along the 3rd dimsion. IDL takes almost 3 minutes to do this on my
> system. This seemed slow. I compared it with Matlab, which took ten
> seconds. Is there a better way to search for the maxima using IDL?
>
> The demo code I used to compare IDL and Matlab is below (with output).
>
> I'm wondering if I ought to switch to Matlab. I just spent a couple
> of days writing IDL code to read my data, so I'd rather not.
>
> Many thanks,
> Tim
>
> --
>
> Timothy W. Hilton
> PhD Candidate, Department of Meteorology
> The Pennsylvania State University
> 503 Walker Building, University Park, PA 16802
> hil...@meteo.psu.edu
>
> ========
> scratch.pro:
>
> foo = randomu(0, 1200, 1200, 2920)
> PRINT, systime()
> foo_max = max(foo, DIMENSION = 3)
> PRINT, systime()
> END
>
> IDL> .run scratch
> % Compiled module: $MAIN$.
> Sat Apr 10 10:44:44 2010
> Sat Apr 10 10:47:36 2010
> IDL>
>
> ========
> scratch.m:
>
> foo = rand(1200,1200,2920);
> fprintf('%s\n', datestr(now()));
> foo_max = max(foo, [], 3);
> fprintf('%s\n', datestr(now()));
>
>>> scratch
>
> 10-Apr-2010 10:42:45
> 10-Apr-2010 10:42:55

In my experience for large arrays it's best to simply do the following
foo = randomu(0, 1200, 1200, 2920)
maxArray = fltarr(1200,1200)
for i = 0, 2919 do maxArray = maxArray > foo[*,*,i]

Using IDLs routines is sloooow for this.

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[Message index]

		find max in 3D array -- slow By: Timothy W. Hilton on Sat, 10 April 2010 09:03
		Re: find max in 3D array -- slow By: JDS on Mon, 12 April 2010 10:48
		Re: find max in 3D array -- slow By: Juggernaut on Mon, 12 April 2010 09:20
		Re: find max in 3D array -- slow By: Steve[5] on Mon, 12 April 2010 04:15
		Re: find max in 3D array -- slow By: Maxwell Peck on Mon, 12 April 2010 03:36

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