| Re: Cumulative max() in *arbitrary* dimension? [message #79508 is a reply to message #79382] |
Thu, 08 March 2012 13:15   |
Lajos Foldy
Messages: 176
Registered: December 2011
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Senior Member |
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On Mar 8, 7:33 pm, JDS <jdtsmith.nos...@yahoo.com> wrote:
>
> IDL> a=byte(randomu(sd,300,400,3000)*256)
> IDL> t=systime(1) & b=max(a,DIMENSION=3) & print,systime(1)-t
> IDL> t=systime(1) & b2=a[*,*,0] & for i=1,3000-1 do b2>=a[*,*,i] & print,systime(1)-t
> IDL> print,array_equal(b,b2)
>
> I can only presume the built-in MAX has some design limitations for final dimension looping. I presume this all works the same way for MIN, BTW.
b=max(a,DIMENSION=3) is equivalent to
b=a[*,*,0]
for j1=0,299 do begin
for j2=0,399 do begin
b[j1,j2]=a[j1,j2,0]
for j3=1,2999 do b[j1,j2]>=a[j1,j2,j3]
endfor
endfor
Your solution is equivalent to
b2=a[*,*,0]
for j3=1,2999 do begin
for j2=0,399 do begin
for j1=0,299 do b2[j1,j2]>=a[j1,j2,j3]
endfor
endfor
The big difference comes from the memory access pattern.
regards,
Lajos
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