| Re: m choose n [message #67520 is a reply to message #67376] |
Wed, 29 July 2009 06:38   |
pgrigis
Messages: 436
Registered: September 2007
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Senior Member |
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On Jul 28, 7:09 pm, Rob <rob.webina...@gmail.com> wrote:
> Has anyone implemented the combinatorial function which the "n choose
> k" combinations of an input vector, like Matlab's nchoosek? I'm not
> talking about just the binomial coefficient n!/(m!*(n-m)!). I'm
> interested in getting the "n choose k" combinations. Matlab's
> function:
>
> http://www.mathworks.com/access/helpdesk/help/techdoc/index. html?/acc...
>
> Example:
> octave-3.0.5:2> nchoosek([1,2,3,4],2)
> ans =
>
> 1 2
> 1 3
> 1 4
> 2 3
> 2 4
> 3 4
>
> If not, I will just codify Matlab/Octave's nchoosek() and submit to
> ITT Vis or something like that.
>
> R
Yes, I posted this function to the newsgroup a few years ago.
http://tinyurl.com/nra4d8
I report it below.
To reproduce your result:
a=[1,2,3,4]
combind=pgcomb(4,2)
print,a[combind]
or
print,pgcomb(4,2)+1 if you are lazy :)
It's a nice example of a routine that would be
somewhat harder to write without a BREAK statement :)
Ciao,
Paolo
FUNCTION pgcomb,n,j
;;number of combinations of j elements chosen from n
nelres=long(factorial(n)/(factorial(j)*factorial(n-j)))
res=intarr(j,nelres);array for the result
res[*,0]=indgen(j);initialize first combination
FOR i=1,nelres-1 DO BEGIN;go over all combinations
res[*,i]=res[*,i-1];initialize with previous value
FOR k=1,j DO BEGIN;scan numbers from right to left
IF res[j-k,i] LT n-k THEN BEGIN;check if number can be increased
res[j-k,i]=res[j-k,i-1]+1;do so
;if number has been increased, set all numbers to its right
;as low as possible
IF k GT 1 THEN res[j-k+1:j-1,i]=indgen(k-1)+res[j-k,i]+1
BREAK;we can skip to the next combination
ENDIF
ENDFOR
ENDFOR
RETURN,res
END
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