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Re: multiplication [message #19623 is a reply to message #19427]

Tue, 28 March 2000 00:00

Carsten Dominik
Messages: 45
Registered: February 1998

Member

>>>> > "JK" == James Kuyper <kuyper@wizard.net> writes:

JK> meron@cars3.uchicago.edu wrote:
>>
>> In article <38E03BDC.868B8396@hotmail.com>, marc
>> <m_schellens@hotmail.com> writes:
>>> Is there a function like TOTAL but for multiplication. Like the
>>> big PI symbol in mathematical notation. Or this really something
>>> for the for loop?
>>>
>>> I.E.
>>>
>>> a=[1,2,3,...]
>>>
>>> result=a[1]*a[2]*a[3]...
>>>
>> if all the elements of a are positive then you can simply do
>>
>> result = exp(total(alog(a)))
JK> ...
>> If some of the elements are negative, you can still handle it. do
>>
>> dum = where(a lt 0, ndum) sig = (-1)^ndum result =
>> sig*exp(total(alog(abs(a))))

JK> You can't honestly be suggesting that this is a good technique?
JK> Ignore for a momement what happens if any element of 'a' is
JK> 0. That code performs two transcendental function evaluations per
JK> element of 'a'. IDL would have to be very badly engineered (which
JK> I suppose is possible), for a 'for' loop to execute more slowly
JK> than your code.

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.

- Carsten

--
Carsten Dominik <dominik@astro.uva.nl> \ _ /
Sterrenkundig Instituut "Anton Pannekoek" |X| _
Kruislaan 403; NL-1098 SJ Amsterdam /| |\ _ _ _/ \
phone +31 (20) 525-7477; FAX +31 (20) 525-7484 ___|o|____/ ~~ \___/ ~~~~~

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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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