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Re: Need a function to multiply the elements of an array [message #4143 is a reply to message #4065]

Fri, 28 April 1995 00:00

rlw
Messages: 1
Registered: April 1995

Junior Member

In article <3nm2uk$qqq@pinot.umd.edu>, barsam@Glue.umd.edu (Barsam Marasli) writes:

|> I am looking for a routine to multiply the elements of an array.

Here is one more approach for this problem. I did some tests and found
it to be roughly the same speed as the previous approaches (using logarithms)
for short vectors (< 100 elements), but it is faster by about a factor
of 3 for long vectors. It is about 15 times faster than a direct loop
for long vectors.

function prod,x
; Compute product of elements of x. Returns result as double.
; R. White, 1995 April 28
n = n_elements(x)
y = double(x)
mm = 1.0d0
; takes about log3(n) passes through this loop
; 20 elements is about the right break point where the direct loop becomes
; fastest
while (n GT 20) do begin
mval = n MOD 3
if mval eq 1 then begin
mm = mm*y(n-1)
endif else if mval eq 2 then begin
mm = mm*y(n-1)*y(n-2)
endif
n = n/3
y = y(0:n-1)*y(n:2*n-1)*y(2*n:3*n-1)
endwhile
for i=0,n-1 do mm=mm*y(i)
return, mm
end

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[Message index]

		Need a function to multiply the elements of an array By: barsam on Wed, 26 April 1995 00:00
		Re: Need a function to multiply the elements of an array By: VUKOVIC on Mon, 01 May 1995 00:00
		Re: Need a function to multiply the elements of an array By: isaacman on Fri, 28 April 1995 00:00
		Re: Need a function to multiply the elements of an array By: chase on Fri, 28 April 1995 00:00
		Re: Need a function to multiply the elements of an array By: VUKOVIC on Fri, 28 April 1995 00:00
		Re: Need a function to multiply the elements of an array By: rlw on Fri, 28 April 1995 00:00

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