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Re: Speed-up of code [message #67860 is a reply to message #67858]

Tue, 25 August 2009 08:42

Craig Markwardt
Messages: 1869
Registered: November 1996

Senior Member

On Aug 25, 10:16 am, Philip Elson <philipel...@googlemail.com> wrote:
> Dear All,
>
> I have a question relating to the optimization of some code which
> averages an array based on the values in another array.
> Its much easier to explain in an example:
>
> day = [ 1, 1, 2, 3, 3, 3, 3]
> value = [ 2, 4, 5, 2, 3, 2, 1]
>
> Which should return, depending on which is easier, either
> avg = [ 3, 5, 2]
> or
> avg = [ 3, 3, 5, 2, 2, 2, 2]
>
> This is fairly straightforward using a for loop, but how to do it in
> the IDL way?
>
> You can see two examples of the basic code below:
>
> ; ==================================
> ; FIRST EXAMPLE
> ; ==================================
> unique = uniq(day)
> avg = intarr(n_elements(unique))
> FOR i=0, n_elements(unique) -1 DO BEGIN
> res = WHERE(day EQ day[unique[i]], count)
> if count GT 0 THEN avg[i] = total(value[res],/DOUBLE) / count
> ENDFOR
> print, avg
>
> ; ==================================
> ; SECOND EXAMPLE
> ; ==================================
> h = histogram(day, REVERSE_INDICES=ri)
> avg = h*0
> FOR i=0, n_elements(h)-1 DO BEGIN
> data_inds = ri[ri[i]:ri[i+1]-1]
> avg[i] = total(value[data_inds],/DOUBLE) / h[i]
> ENDFOR
> print, avg
>
> At this stage I open the floor; I essentially want to achieve the
> results as above without the need for the for loop.
>
> My assumption is that the HISTOGRAM function will be helpful, but
> having spent quite some time on this I am beginning to think that it
> cannot be done - though I would love to be proved wrong by any
> histogram guru out there.

Those are the techniques I would have tried! Be careful: in your
second example, you don't handle the case where the histogram bin h[i]
is empty. You just need an "if h[i] GT 0" test there.

Craig

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[Message index]

		Re: Speed-up of code By: JDS on Tue, 25 August 2009 15:30
		Re: Speed-up of code By: philipelson on Tue, 25 August 2009 09:18
		Re: Speed-up of code By: philipelson on Tue, 25 August 2009 09:14
		Re: Speed-up of code By: philipelson on Tue, 25 August 2009 09:12
		Re: Speed-up of code By: Craig Markwardt on Tue, 25 August 2009 08:42
		Re: Speed-up of code By: wlandsman on Tue, 25 August 2009 08:36
		Re: Speed-up of code By: David Fanning on Tue, 25 August 2009 08:01

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