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Re: Vector comparison. [message #37040 is a reply to message #37036]

Thu, 20 November 2003 10:55

tam
Messages: 48
Registered: February 2000

Member

Craig Markwardt wrote:

> David Fanning <david@dfanning.com> writes:
>
>> Timm Weitkamp writes:
>>
>>
>>> Darnit! I was so proud of my answer... only to find that Chris Lee already
>>> proposed the same method (the "memory-hogging" one), albeit using more
>>> lines and this "if count eq 0 then ..." after WHERE, which always annoys
>>> me for some stupid reason. It can be avoided, which gives me an excuse to
>>> post my suggestion anyway:
>>>
>>> function Get_Match, a, b
>>> na = n_elements(a) & nb = n_elements(b)
>>> return, where(total((rebin(a,na,nb) - rebin(transpose(b), na, nb)) eq 0, 2))
>>> end
>>
>> Now there you go! That's a solution I can
>> steal for my web page. :-)
>
>
> I think this is the same solution as Tom McGlynn. This is a classic
> solution by brute force *memory* hogging instead of compute hogging.
>
> If the lists are really large, then the arrays will be
> n_elements(a)*n_elements(b) in size, which can be pretty big. For
> example if each list had 10000 elements, then those 100 million
> element arrays would start to suck a large amount of virtual memory.
>
> Craig
>

If the range of elements in b is not too large, then I suppose
something like:

mn = min(a, max=mx)-1
mx = mx+1
h = histogram(a,min=mn,max=mx)
w = where(h[a-mn] ne 0)

would do the trick.... We make the histogram one
to large on both sides so that we can use IDL's quirk
that going beyond the edge of an array is treated as
the last (or first) element. So long as the range in
b is smaller than the product of the number of elements
in the two arrays, this may be faster.

But it could only work on integers.
Tom

P.S. Note that I'm doing my best in all my solutions to ensure
that the histogram function is called.

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[Message index]

		Re: Vector comparison. By: JD Smith on Thu, 20 November 2003 11:38
		Re: Vector comparison. By: tam on Thu, 20 November 2003 10:55
		Re: Vector comparison. By: David Fanning on Thu, 20 November 2003 09:55
		Re: Vector comparison. By: Craig Markwardt on Thu, 20 November 2003 09:35
		Re: Vector comparison. By: tam on Thu, 20 November 2003 09:24
		Re: Vector comparison. By: David Fanning on Thu, 20 November 2003 08:52
		Re: Vector comparison. By: Chris[1] on Thu, 20 November 2003 08:08
		Re: Vector comparison. By: David Fanning on Thu, 20 November 2003 08:08
		Re: Vector comparison. By: R.Bauer on Thu, 20 November 2003 07:11
		Re: Vector comparison. By: tam on Thu, 20 November 2003 07:03
		Re: Vector comparison. By: hunter on Thu, 20 November 2003 06:49
		Re: Vector comparison. By: David Fanning on Thu, 20 November 2003 05:40
		Re: Vector comparison. By: R.Bauer on Thu, 20 November 2003 04:02
		Re: Vector comparison. By: Craig Markwardt on Thu, 20 November 2003 02:19
		Re: Vector comparison. By: Chris Lee on Thu, 20 November 2003 02:09
		Re: Vector comparison. By: R.Bauer on Thu, 20 November 2003 01:01
		Re: Vector comparison. By: Craig Markwardt on Thu, 20 November 2003 00:23
		Re: Vector comparison. By: David Fanning on Wed, 19 November 2003 18:05
		Re: Vector comparison. By: R.Bauer on Wed, 19 November 2003 17:50
		Re: Vector comparison. By: Craig Markwardt on Wed, 19 November 2003 15:48
		Re: Vector comparison. By: Pavel Romashkin on Wed, 19 November 2003 16:09

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