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Re: majority voting [message #65160 is a reply to message #65079]

Thu, 12 February 2009 01:15

Mort Canty
Messages: 134
Registered: March 2003

Senior Member

JD Smith schrieb:
> You're trying to compute the n-dimensional mode. For the 1D case,
> see:
>
> http://www.dfanning.com/code_tips/mode.html
>
> You could of course use SORT_ND and a 2D version of the "find the
> longest string of the same number" method on this page. This is very
> safe against sparse input distributions -- notice the HISTOGRAM in
> SORT_ND is of the sort indices, not the data themselves. It's also
> easy to recognize a mode length of 1 as "no mode", and (if you like)
> detect multiple modes in the data (aka "tie votes").
>
> However, your problem is a special case, since (I presume) your vote
> labels are always low contiguous integers, you don't have to worry
> about sparseness. In this case, using HIST_ND directly without
> sorting will likely be much faster:
>
> n=n_elements(array)
> s=size(array,/DIMENSIONS)
> h=hist_nd([lindgen(1,n)/s[1],reform(transpose(array),1,n)],1 )
> m=max(h,DIMENSION=2,mode) gt 1
> mode=mode/s[0] * m + m - 1
>
> All I'm doing here is forming a 2d space, with the first dimension the
> integer ID of the vote (aka column number in your example), and the
> second the actual votes cast.
>
> Had you organized your array with the rows as your "vote items" you
> could save the transpose. Also note I specifically test for and set
> to -1 any mode with frequency of 1 (your "don't care"). I do not
> check for multiple modes, but you could do this as well in a
> straightforward way. The fact that the dimensions of the 2D histogram
> are the same as the input array is incidental, by the way.
>
> JD
>

Great newsgroup! Ask for magic and you get ... magic. Thanks very much
for the thorough explanation, JD.

Cheers,

Mort

PS. For some reason HIST_ND isn't in David's zipped COYOTE library
package, although SORT_ND is.

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[Message index]

		majority voting By: Mort Canty on Wed, 11 February 2009 08:14
		Re: majority voting By: Allan Whiteford on Fri, 13 February 2009 01:31
		Re: majority voting By: David Fanning on Sat, 14 February 2009 08:44
		Re: majority voting By: JD Smith on Thu, 12 February 2009 14:45
		Re: majority voting By: Foldy Lajos on Thu, 12 February 2009 12:47
		Re: majority voting By: David Fanning on Thu, 12 February 2009 12:11
		Re: majority voting By: pgrigis on Thu, 12 February 2009 11:58
		Re: majority voting By: JD Smith on Thu, 12 February 2009 10:49
		Re: majority voting By: Mort Canty on Thu, 12 February 2009 07:45
		Re: majority voting By: JD Smith on Thu, 12 February 2009 07:08
		Re: majority voting By: David Fanning on Thu, 12 February 2009 05:43
		Re: majority voting By: Allan Whiteford on Thu, 12 February 2009 04:17
		Re: majority voting By: Mort Canty on Thu, 12 February 2009 01:15
		Re: majority voting By: JD Smith on Wed, 11 February 2009 15:20
		Re: majority voting By: ben.bighair on Wed, 11 February 2009 14:03
		Re: majority voting By: Allan Whiteford on Mon, 16 February 2009 04:11

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