Re: help on calculating sum of combinations

[pkienzle]

On 8 Oct 2003 at 8:05, sanjiv kapur wrote:
> a function 
> from a matrix or an array,i want to calculate the sum of various 
> combinations, i.e
> if A=[2, 3, 4,5]
> how do i get
>       a1=2+3+4+5
>       a2=2*3+2*4+2*5+3*4+3*5+4*5
>       a3=2*3*4+2*3*5+2*4*5+3*4*5
>       a4=2*3*4*5

Here's a solution using the nchoosek function from octave-forge
(http://octave.sf.net):

function a=sumcomb(v)
  for k=length(v):-1:1
    a(k) = sum(prod(nchoosek(v,k),2));
  end
end


nchoosek itself is particularly inefficient (it uses a recursive
definition which expands to a complete matrix of all combinations)
so this solution recommends itself only for brevity, and not for
speed.

Paul Kienzle
address@hidden



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