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MathGroup Archive 2006

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Re: Using a list as a variable

  • To: mathgroup at smc.vnet.net
  • Subject: [mg67077] Re: Using a list as a variable
  • From: "J Siehler" <jsiehler at gmail.com>
  • Date: Fri, 9 Jun 2006 01:06:38 -0400 (EDT)
  • References: <e68qc9$d2i$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Bonny Banerjee wrote:
> I would like to use a list (or set) as a variable. I am working on the
> Subset-Sum problem. Given a set S of integers, the goal is to find a subset
> of S (say Q) whose elements add to a given integer x.

Well, the brute force way to find them is like this:

SubsetSum[s_, x_] := Select[Subsets[s], Total[#] == x &]

ex = {5, 8, 12, 1, 3, 4, 1, 10}
SubsetSum[ex, 10]

This finds all sets with the desired sum but obviously takes
exponential time in the size of set S.  There is a better but more
complicated exponential-time algorithm for subset-sum, but there is no
subexponential algorithm known.

If you really want it to just return one subset - the first one it
finds - then you can do this:

SubsetSum[s_, x_] := Select[Subsets[s], Total[#] == x &,1]


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