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Re: avoiding for loop when calculating median [message #10936]

Mon, 02 February 1998 00:00

Alex Schuster
Messages: 124
Registered: February 1997

Senior Member

George McCabe wrote:

> Thanks for your inputs, Alex.
>
> following an earlier hint on the group I wrote the loop like you
> describe, but without REFORM'ing the matrix. To be honest my matrix is

Actually, it's TRANSPOSE, not REFORM!

> a large data cube, but I chose a 2D example to make the description less
> opaque. The reduction in execution time was measured - 20%, which on 45
> seconds is significant. When you say HUGE is that the scale of the
> increase you experienced.

I did something like that:

IDL> n = 2000L
IDL> m = randomu( seed, n, n )
IDL> m2 = transpose( m )
IDL> t=systime(1) & for i = 0, n-1 do c(i) = median(m(i,*)) & print,
systime(1)-t, format='(F4.1)'
3.0
IDL> t=systime(1) & for i = 0, n-1 do c(i) = median(m2(*,i)) & print,
systime(1)-t, format='(F4.1)'
0.6

That's a factor of five, and this is HUGE. Of course, the TRANSFORMing
has to be done, too, this also takes a second or so.
With 3d data it's not that easy. Does the data need to be in this form?
Changing the x, y, and z direction could speed it up.

Using 1d only, I get this:

IDL> index = lindgen( n )
IDL> t=systime(1) & for i = 0, n-1 do c(i) = median(m(index+i*n)) &
print, systime(1)-t, format='(F4.1)'
2.2

Whoops, this time it's faster than the original routine.

Alex
--
Alex Schuster Wonko@weird.cologne.de PGP Key available
alex@pet.mpin-koeln.mpg.de

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[Message index]

		Re: avoiding for loop when calculating median By: Alex Schuster on Mon, 02 February 1998 00:00
		Re: avoiding for loop when calculating median By: George McCabe on Mon, 02 February 1998 00:00
		Re: avoiding for loop when calculating median By: Alex Schuster on Fri, 30 January 1998 00:00
		Re: avoiding for loop when calculating median By: George McCabe on Fri, 30 January 1998 00:00
		Re: avoiding for loop when calculating median By: a2652099 on Fri, 30 January 1998 00:00
		Re: avoiding for loop when calculating median By: Michael Werger on Fri, 30 January 1998 00:00

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