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Re: Faster way to "shift" array? [message #84792 is a reply to message #84791]

Tue, 11 June 2013 09:33

Fabzi
Messages: 305
Registered: July 2010

Senior Member

On 06/11/2013 03:44 PM, rjp23@le.ac.uk wrote:> On Tuesday, June 11, 2013
2:28:50 PM UTC+1, David Fanning wrote:
>> I'm probably doing it wrong.
> I suspect not :p
>
> I think my issue might be how I then subset a geographic
> region if it crosses the date line.
>
> Thanks for pointing me in the right direction :-)

I also agree that sometimes it is impossible to overcome a subset
problem without having to shift the data. Let's take the example of
netcdf files, which have the nice capability to be subset _without_
reading the full databox in the memory. If your subset goes something
like: [-40, 40] in longitude and your data is in [0, 360] then you can't
efficiently use the netcdf COUNT and OFFSET keywords.
Data organized in [0, 360] has the bad property of cutting Europe and
Africa in two, while [-180, 180] mostly cuts oceans. It depends on how
often you use the data...

I tried this on my laptop:

pro test
t_lon = findgen(7200) / 20.
data = (LONARR(3601)+1) ## t_lon
; Convert longs
lon = ((t_lon + 180) MOD 360) - 180
nl = n_elements(lon)/2.
print, 'Shift'
tic
new_data1 = shift(data, nl)
toc
print, 'Concatenate'
tic
new_data2 = [data[nl:*, *], data[0:nl-1,*]]
toc
end

Shift
% Time elapsed: 0.098950863 seconds.
Concatenate
% Time elapsed: 0.20306301 seconds.

It seems that shift is twice as fast as the "array concatenation" solution.

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[Message index]

		Faster way to "shift" array? By: rjp23 on Tue, 11 June 2013 05:50
		Re: Faster way to "shift" array? By: rjp23 on Tue, 11 June 2013 05:52
		Re: Faster way to "shift" array? By: David Fanning on Tue, 11 June 2013 06:04
		Re: Faster way to "shift" array? By: rjp23 on Tue, 11 June 2013 06:23
		Re: Faster way to "shift" array? By: David Fanning on Tue, 11 June 2013 06:28
		Re: Faster way to "shift" array? By: David Fanning on Tue, 11 June 2013 06:35
		Re: Faster way to "shift" array? By: rjp23 on Tue, 11 June 2013 06:44
		Re: Faster way to "shift" array? By: Fabzi on Tue, 11 June 2013 09:33
		Re: Faster way to "shift" array? By: David Fanning on Tue, 11 June 2013 10:23
		Re: Faster way to "shift" array? By: Yngvar Larsen on Tue, 11 June 2013 14:39

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