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Re: TOTAL gives totally different result on identical array [message #76896 is a reply to message #76892]

Fri, 08 July 2011 08:44

Foldy Lajos
Messages: 268
Registered: October 2001

Senior Member

On Fri, 8 Jul 2011, Liam Gumley wrote:

> function kahansum, input
> sum = 0.0
> c = 0.0
> for i = 0L, n_elements(input) - 1 do begin
> y = input[i] - c
> t = sum + y
> c = (t - sum) - y
> sum = t
> endfor
> return, sum
> end

It will be very slow. But it's IDL, vectorize it!

; test.pro begin

function kahan_sum, input
sum=0.0
c=0.0
for i=0l, n_elements(input)-1 do begin
y=input[i]-c
t=sum+y
c=(t-sum)-y
sum=t
endfor
return, sum
end

function kahan_sum_1000, input
sum=fltarr(1000)
c=fltarr(1000)
for i=0l, n_elements(input)-1,1000 do begin
y=input[i:i+999]-c
t=sum+y
c=(t-sum)-y
sum=t
endfor
return, kahan_sum(sum)
end

pro test
vec=findgen(100000)

t1=systime(1)
s1=kahan_sum(vec)
t1=systime(1)-t1
print, 't1:', t1

t2=systime(1)
s2=kahan_sum_1000(vec)
t2=systime(1)-t2
print, 't2:', t2
print, 'ratio:', t1/t2
print, 'sums and diff:', s1, s2, s2-s1

end

; test.pro end

IDL> test
t1: 0.056817055
t2: 0.0012319088
ratio: 46.121153
sums and diff: 4.99995e+09 4.99995e+09 0.00000
IDL>

(The general case is left to the reader.)

regards,
Lajos

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[Message index]

		Re: TOTAL gives totally different result on identical array By: Michael Galloy on Fri, 08 July 2011 12:10
		Re: TOTAL gives totally different result on identical array By: Liam Gumley on Fri, 08 July 2011 11:48
		Re: TOTAL gives totally different result on identical array By: Foldy Lajos on Fri, 08 July 2011 11:33
		Re: TOTAL gives totally different result on identical array By: Fabzou on Fri, 08 July 2011 10:46
		Re: TOTAL gives totally different result on identical array By: Liam Gumley on Fri, 08 July 2011 10:25
		Re: TOTAL gives totally different result on identical array By: Foldy Lajos on Fri, 08 July 2011 08:44
		Re: TOTAL gives totally different result on identical array By: Liam Gumley on Fri, 08 July 2011 07:37
		Aw: Re: TOTAL gives totally different result on identical array By: M. Suklitsch on Fri, 08 July 2011 05:55
		Re: TOTAL gives totally different result on identical array By: Fabzou on Fri, 08 July 2011 03:34

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