| Re: Finding the closest value in an array... [message #38858 is a reply to message #38771] |
Wed, 31 March 2004 00:41  |
timrobishaw
Messages: 16
Registered: June 2003
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Junior Member |
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JD Smith <jdsmith@as.arizona.edu> wrote in message
> For monotonic arrays, you know either one or the other of the two
> bracketing values is the closest. VALUE_LOCATE is faster than
> MIN(ABS()) since it relies on the monotonicity to skip rapidly through
> the vector using bisection. This doesn't address your aesthetic
> concerns, but it's much more efficient:
>
> j=value_locate(r,find)
> mn=min(abs(r[j:j+1]-find),pos)
> pos+=j
>
> When compared to:
>
> mn=min(abs(r-find),pos)
>
> the former can be *much* faster, especially for long arrays. While
> the latter is linear in N, the former is logarithmic.
Hi JD. Thanks for the advanced cleverness. That is great! That
factor of 130,000 in speed is wicked awesome! So, if I do a few tests
and find that the MIN(ABS()) method is faster for the case when FIND
only has one element, should I (would you) add an if/then to check for
this case and perform the two-line MIN(ABS()) evaluation so that the
slower SORT/MIN/ABS/REBIN method is avoided? I haven't really been
too aware of efficiency issues, but I'm starting to do LOTS of
reduction on BIG data sets, so I'd better start thinking about this
stuff! Thanks a bunch -Tim.
> ;; Find indices closest to find values in vector, which must be
> ;; monotonically increasing or decreasing, otherwise a sort vector
> ;; should be passed. Find can be a vector itself.
> function closest,vector,find,SORT=s
> nf=n_elements(find)
> sort=keyword_set(s) || arg_present(s)
> if sort && n_elements(s) ne n_elements(vector) then s=sort(vector)
> j=value_locate(sort?vector[s]:vector,find)
> b=[[j>0],[(j+1)<(n_elements(vector)-1)]]
> mn=min(abs((sort?vector[s[b]]:vector[b])- $
> rebin([find],nf,2)),DIMENSION=2,pos)
> pos=j>0+pos/nf
> return,sort?s[pos]:pos
> end
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