| Re: totalling data with LONG lists of indices [message #59979 is a reply to message #59926] |
Thu, 24 April 2008 05:36   |
Conor
Messages: 138
Registered: February 2007
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Senior Member |
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On Apr 22, 2:40 pm, Jeremy Bailin <astroco...@gmail.com> wrote:
> In JD's magnificent histogram tutorial, there is a description of how
> to total data using a separate list of indices (with possible
> repeats),
> along with a vague hint that "For large histograms, there are even
> more
> efficient ways to do this with very short or no loops (e.g. using a
> histogram of the histogram)." I have exactly that situation. Because
> about 90% of my indices are not repeated, I have achieved a decent
> speed-up using the following code for the "single" cases ("indices",
> "data", and "hist" are the index list, data list, and final result
> respectively, and hist is already pre-dimensioned).
>
> indhist = histogram(indices, omin=om, reverse_indices=indri)
> dupehist = histogram(indhist, min=1, reverse_indices=duperi)
> ; unique cases, so we can use them to index the LHS:
> if dupehist[0] gt 0 then begin
> just1 = duperi[duperi[0]:duperi[1]-1]
> hist[just1+om] += data[indri[indri[just1]]]
> endif
>
> And going with the brute-force for loop for the rest:
>
> ; loop through the rest
> if n_elements(dupehist) gt 1 then begin
> multiples = duperi[duperi[1]:*]
> for j=0l,n_elements(multiples)-1 do begin
> elements = indri[indri[multiples[j]]:indri[multiples[j]+1]-1]
> hist[multiples[j]+om] += total(data[elements])
> endfor
> endif
>
> However, the loop is still going over hundreds of thousands of entries
> and I can't help but suspect that another histogram and some fancy
> footing with the i-vector would get rid of it. Does anyone have any
> suggestions? Thanks.
I had to do something like this and found this page to be very handy:
http://www.dfanning.com/code_tips/drizzling.html
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