Mathematica 9 is now available
Services & Resources / Wolfram Forums
-----
 /
MathGroup Archive
2002
*January
*February
*March
*April
*May
*June
*July
*August
*September
*October
*November
*December
*Archive Index
*Ask about this page
*Print this page
*Give us feedback
*Sign up for the Wolfram Insider

MathGroup Archive 2002

[Date Index] [Thread Index] [Author Index]

Search the Archive

Re: need a function for sums of subsets

  • To: mathgroup at smc.vnet.net
  • Subject: [mg33012] Re: need a function for sums of subsets
  • From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
  • Date: Tue, 26 Feb 2002 04:34:52 -0500 (EST)
  • Organization: Universitaet Leipzig
  • References: <a5clm3$9gl$1@smc.vnet.net>
  • Reply-to: kuska at informatik.uni-leipzig.de
  • Sender: owner-wri-mathgroup at wolfram.com

Hi,

if you only need pairs

Clear[findsum]

findsum[lst_, val_Rational] := 
  Catch[Outer[If[Plus[#1, #2] == val, Throw[{#1, #2}]] &, lst, lst]]

findsum[lst_, {val__Rational}] := findsum[lst, #] & /@ { val}

Regards
  Jens

Mark Ganson wrote:
> 
> Hello,
> 
> I need a function that will find the first subset of a list that will
> sum up to a given value.
> 
> For example, if I have a list of rationals:
> 
> myrationals = {1/2, 1/3, 1/4, 1/8, 3/10, 12/79, 13/38}
> 
> I would like a function that will return the first subset of
> myrationals that sums up to a given parameter.
> 
> We can call the function "findsum".  It would work like so:
> 
> In: findsum[myrationals, 3/4]
> Out: {1/2, 1/4}
> 
> It would be nice also, but not essential, to be able to use another
> list as the second parameter and have the function return a list of
> lists.
> 
> Example:
> 
> In: findsum[myrationals, {3/4, 3/8}]
> Out: {{1/2, 1/4}, {1/4, 1/8}}
> 
> I need something really fast and memory efficient because my lists
> tend to be quite large (up to 100 elements).
> 
> Many thanks,
> 
> Mark Ganson


  • Prev by Date: Re: Animation in one frame?
  • Next by Date: removing rules from function definitions
  • Previous by thread: Re: need a function for sums of subsets
  • Next by thread: Re: need a function for sums of subsets