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Re: arithmetic operation on array [message #85479 is a reply to message #85478]

Mon, 12 August 2013 15:22

Phillip Bitzer
Messages: 223
Registered: June 2006

Senior Member

OK, I'll bite. There are three ways I can think to do this off the top of my head:
1) Do a loop, like Phillip said (what a fantastic name :-) )
2) Rebin, like David said
3) Use the mysterious "add an extra dimension" method ( https://groups.google.com/d/msg/comp.lang.idl-pvwave/Vu9rzqc kBNQ/HvkK_QnJrsgJ and more recently https://groups.google.com/d/msg/comp.lang.idl-pvwave/dM8XXas Eio0/d3__pvX7svMJ)

Here are the sample code I used:

data = RANDOMU(1L, 360, 180, 456)
avg = MEAN(data, DIM=3)
mData1 = FLTARR(360, 180, 456)

tic & for i=0, 455 do mData1[*,*,i] = data[*,*,i] - avg & toc

mData2 = FLTARR(360, 180, 456)

tic & mData2 = data - Rebin(avg, 360, 180, 456) & toc
tic & data = TEMPORARY(data) - Rebin(avg, 360, 180, 456) & toc ;just for completeness

data = RANDOMU(1L, 360, 180, 456) ;redefine data - we changed it above
mData3 = FLTARR(360, 180, 456)

tic & mData3 = data - avg[*, *, 0] & toc

I used mData (modified data) arrays so I can check I get the same answer, regardless of the method.

The four times I get are, in order relative to the above:
% Time elapsed: 0.12749481 seconds.
% Time elapsed: 0.33231401 seconds.
% Time elapsed: 0.33304191 seconds.
% Time elapsed: 0.019619942 seconds.

So, it seems the mysterious extra dimension method is the fastest, by an order of magnitude. Whoa.

For prosperity,
IDL> print, !VERSION
{ x86_64 darwin unix Mac OS X 8.2.2 Jan 23 2013 64 64}

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[Message index]

		arithmetic operation on array By: Phillip Miller on Mon, 12 August 2013 13:59
		Re: arithmetic operation on array By: David Fanning on Mon, 12 August 2013 14:24
		Re: arithmetic operation on array By: Phillip Bitzer on Mon, 12 August 2013 15:22
		Re: arithmetic operation on array By: Phillip Miller on Mon, 12 August 2013 16:08
		Re: arithmetic operation on array By: Phillip Miller on Mon, 12 August 2013 16:17
		Re: arithmetic operation on array By: David Fanning on Mon, 12 August 2013 16:18
		Re: arithmetic operation on array By: Phillip Bitzer on Mon, 12 August 2013 16:37
		Re: arithmetic operation on array By: Phillip Miller on Mon, 12 August 2013 15:51
		Re: arithmetic operation on array By: John Correira on Wed, 14 August 2013 12:24
		Re: arithmetic operation on array By: wlandsman on Wed, 14 August 2013 12:54

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