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Re: multiplication [message #19620 is a reply to message #19427]

Tue, 28 March 2000 00:00

Craig Markwardt
Messages: 1869
Registered: November 1996

Senior Member

Carsten Dominik <dominik@astro.uva.nl> writes:
>
> Well, it depends very much on the size of the array. Loops in IDL are
> indeed very slow. Try the following: Set N to a large number
> (e.g. 10 000 000) and execute the following lines:
>
> x=fltarr(n)*0.+1.000001 & p=1 & for i=0.,1.*n_elements(x)-1 do p=p*x[i] & print,p
>
> x=fltarr(n)*0.+1.000001 & p=exp(total(alog(x)))&print,p
>
> You'll get a surprise, I promise.

One way to speed things up is to use some sort of a divide and conquer
algorithm. Which is to say, divide the array into two segments and
multiply them element-by-element. Keep doing this until you get down
to a single element.

FUNCTION CMPRODUCT, ARRAY
X = ARRAY
N = N_ELEMENTS(X)
WHILE N GT 1 DO BEGIN
IF (N MOD 2) EQ 1 THEN X(0) = X(0) * X(N-1) ;; When N is odd!!
N2 = FLOOR(N/2)
X = X(0:N2-1) * X(N2:*) ;; Don't worry if N is odd here.

;; X keeps shrinking by a factor of two each time
N = N2
ENDWHILE
RETURN,X(0)
END

Disadvantages are that it may be slower when n_elements(array) is
small. Also, the round-off error can grow to significance, as I think
Carsten was trying to say, but this will happen with most approaches
unfortunately. Double precision can help.

Craig

--
------------------------------------------------------------ --------------
Craig B. Markwardt, Ph.D. EMAIL: craigmnet@cow.physics.wisc.edu
Astrophysics, IDL, Finance, Derivatives | Remove "net" for better response
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[Message index]

		multiplication By: marc schellens[1] on Tue, 28 March 2000 00:00
		Re: multiplication By: Harald Frey on Thu, 30 March 2000 00:00
		Re: multiplication By: James Kuyper on Wed, 29 March 2000 00:00
		Re: multiplication By: meron on Tue, 28 March 2000 00:00
		Re: multiplication By: John-David T. Smith on Tue, 28 March 2000 00:00
		Re: multiplication By: Craig Markwardt on Tue, 28 March 2000 00:00
		Re: multiplication By: Carsten Dominik on Tue, 28 March 2000 00:00
		Re: multiplication By: James Kuyper on Tue, 28 March 2000 00:00

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