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Re: Approximate convolution - for loop problem [message #64459 is a reply to message #64415]

Tue, 23 December 2008 18:27

Jeremy Bailin
Messages: 618
Registered: April 2008

Senior Member

On Dec 21, 3:01 pm, Sam <samuel.le...@gmail.com> wrote:
> Hi David, unfortunately shift() does not do the business for me, as
> these two examples below show. So I'm still a bit stumped here.
>
> ; Array operation I'm trying to execute.
> a=[1.,2.,3.,4.]
> for ii=1,3 do a[ii] += 0.5*a[ii-1]
> print,a
> 1.00000 2.50000 4.25000 6.12500
>
> ; Attempt to perform this operation with shift()
> a=[1.,2.,3.,4.]
> a += 0.5*shift(a,-1)
> print,a
> 2.00000 3.50000 5.00000 4.50000
>
> On Dec 21, 7:03 pm, David Fanning <n...@dfanning.com> wrote:
>
>> samuel.le...@gmail.com writes:
>>> Hello everyone, I'm trying to execute a 1-d convolution of an array,
>>> signal.
>
>>> Using an analytic approximation, obtaining the convolved bolometer
>>> signal, bolo_signal, at time step ii, is given by the following:
>
>>> nsamp=n_elements(signal)
>>> const1 = exp(-tsamp/taubolo)
>>> const2 = 1.-const1
>
>>> bolo_signal = const2*signal
>>> for ii= 1L,nsamp-1L do begin
>>> bolo_signal[ii] += const1*bolo_signal[ii-1]
>>> endfor
>
>>> where tsamp and taubolo are scalars. Is there any way to avoid the for
>>> loop in this case? The hope is to speed up the execution.
>
>> I think this gives you the same results:
>
>> bolo_signal += const1 * shift(bolo_signal,-1)
>
>> Cheers,
>
>> David
>> --
>> David Fanning, Ph.D.
>> Fanning Software Consulting, Inc.
>> Coyote's Guide to IDL Programming:http://www.dfanning.com/
>> Sepore ma de ni thui. ("Perhaps thou speakest truth.")
>
>

How about this:

a = [1.,2.,3.,4.]
n = n_elements(a)
c = 0.5^reverse(indgen(n))
new_a = total(a*c, /cumulative) / c

-Jeremy.

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[Message index]

		Re: Approximate convolution - for loop problem By: Sam on Mon, 22 December 2008 14:19
		Re: Approximate convolution - for loop problem By: Wout De Nolf on Mon, 22 December 2008 01:31
		Re: Approximate convolution - for loop problem By: David Fanning on Sun, 21 December 2008 12:44
		Re: Approximate convolution - for loop problem By: Sam on Sun, 21 December 2008 12:01
		Re: Approximate convolution - for loop problem By: David Fanning on Sun, 21 December 2008 10:03
		Re: Approximate convolution - for loop problem By: Jeremy Bailin on Tue, 23 December 2008 18:27
		Re: Approximate convolution - for loop problem By: andrews32940 on Tue, 23 December 2008 11:46

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