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Re: Cumulative max() in *arbitrary* dimension? [message #79502 is a reply to message #79382] Thu, 08 March 2012 16:17 Go to previous messageGo to previous message
Heinz Stege is currently offline  Heinz Stege
Messages: 189
Registered: January 2003
Senior Member
On Thu, 8 Mar 2012 10:33:39 -0800 (PST), JDS wrote:

> I've since tuned this up a bit more, saving 1/2 of the index computation
> during each step of the loop by incrementing a running index array. It's
> now (rather remarkably) >5x faster than MAX(DIMENSION=3) for me with
> large 3D arrays. And of course it gives all the intermediate cumulative
> max values.
>
The loop can be tuned up even more. Replacing the array of indices by
two scalars for the subscript range makes the loop faster and also
saves memory. I replaced the following 2 lines of your code
inds=lindgen(off)
for i=1,s[d]-1 do a[i*off]=a[inds]>a[(inds+=off)]
by the following 3 lines:
i1=0
i2=off-1
for i=1,s[d]-1 do a[i*off]=a[i1:i2]>a[(i1+=off):(i2+=off)]

In my examples max_cumulative is about 2 to 3 times faster than
before:
~2.5 times for a 60x400x3000 byte array
~3.1 times for a 60x400x300 byte array
~2.1 times for a 60x40x3000 byte array

Heinz

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