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Re: Faster way to "shift" array? [message #84802 is a reply to message #84785]

Tue, 11 June 2013 14:39

Yngvar Larsen
Messages: 134
Registered: January 2010

Senior Member

On Tuesday, 11 June 2013 14:50:43 UTC+2, rj...@le.ac.uk wrote:
> I have a longitude array which ranges from 0 to 360 but I want it to range from -180 to 180.
>
> Currently I'm doing this:
>
> lon=shift(lon, n_elements(lon)/2.)
> lon[where(lon GT 180)]=lon[where(lon GT 180)]-360.
> The lon array is 3600 elements and the shift command is taking around 1 second.

Huh? That cannot be correct. On my machine:

IDL> lon = 360*lindgen(3600)/3599
IDL> t=systime(/sec)& lon=shift(lon, n_elements(lon)/2.) & lon[where(lon GT 180)]=lon[where(lon GT 180)]-360. & print, systime(/sec)-t
0.00010800362

The full operation took 0.1 millisecond.

IDL> lon = 360*lindgen(3600)/3599
IDL> t=systime(/sec)&for n=0,9999 do lon=shift(lon, n_elements(lon)/2.)&print, systime(/sec)-t
0.044115782

So the shift operation alone took 0.04 seconds for 10000 iterations, i.e. 4 microseconds per iteration.

> When multiplied by the thousands of files I need to handle this becomes quite a considerable time component. Is there a faster way to do this?

Most likely.

N = n_elements(lon)/2
lon = shift(lon, N)
lon[0:N-1] -= 360.

But I'm 100% sure that this is not your bottleneck. I assume you also have to shift your 2D data array, not just the 1D longitude array? In that case, I would look there first.

--
Yngvar

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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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