| Re: Unique combinations from a 1d array [message #37624] |
Wed, 14 January 2004 14:38  |
David Fanning
Messages: 11724
Registered: August 2001
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Senior Member |
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Darren writes:
> Does anyone know of a more efficient means to determine the set of all
> unique combinations of 2 from a 1d array? The following is an approach
> that works but for large arrays -say 3000 or more elements it is very
> slow. Part of the problem is due to memory because the number of
> paired comparisons becomes very large � i.e. for 3000 elements the
> total number of combinations is 4498500. Writing the paired difference
> results to a temporary file helped considerably, but is still far too
> slow. Any ideas would be much appreciated.
>
> Here is the code I have:
>
> X = [X1, X2, X3�..Xn+1]
> n = n_elements(X)
> d = make_array(1, /float)
> for i=0, n-1 do for j=0, n-1 do begin
> if i le j then begin
> d = [d, X[i] - X[j]]
> endif
> endfor
> d = d[1:n-1]
Here is a method that gets the same answer as
your code. (Although I can't convince myself it
does what you *say* it does!)
x = RandomU(-3L, 10) * 10
Darren's method:
% Compiled module: $MAIN$.
IDL> .go
0.000000 3.39667 1.30986 3.08815 8.37598
0.751965 8.60027 6.79858 7.55522
My method:
IDL> Print, ( x[0] - Shift(x,1) )[1:*]
0.000000 3.39667 1.30986 3.08815 8.37598
0.751965 8.60027 6.79858 7.55522
Cheers,
David
--
David W. Fanning, Ph.D.
Fanning Software Consulting, Inc.
Phone: 970-221-0438, E-mail: david@dfanning.com
Coyote's Guide to IDL Programming: http://www.dfanning.com/
Toll-Free IDL Book Orders: 1-888-461-0155
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