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MathGroup Archive 1997

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Re: List manipulation

  • To: mathgroup at smc.vnet.net
  • Subject: [mg7543] Re: [mg7390] List manipulation
  • From: Dick Zacher <dick at loc3.tandem.com>
  • Date: Fri, 13 Jun 1997 19:37:34 -0400 (EDT)
  • Organization: Tandem Computers
  • Sender: owner-wri-mathgroup at wolfram.com

seanross at worldnet.att.net wrote:
> 
> V. Nandagopal wrote:
> >
> > Hello,
> >
> >         I have a list X={1,2,3,4,5,6,7,8}. How do I get a list of the
> > folllowing kind:
> >
> >         S={{1,2,3,4}, {1,2,3,5}, {1,2,3,6}, .....}.
> >
> > S will have 8-choose-2 = 70 elements. Each element of S should have length
> > 4.
> >
> >         Is there any function which does such list making? Something of
> > the form
> >
> >         ?????[list, n]
> >
> >         Thanks in advance.
> >
> >         V. Nandagopal
> >         School of Mathematics
> >         Tata Institute of Fundamental Research
> >         Colaba, Bombay 400 005, India
> >
> >         e-mail: nandgopa at math.tifr.res.in
> 
> Try using the Table command.  Something like
> 
> Table[Join[Take[mylist,3],{i}],{i,4,whatever}]
> 
> You may have to play around with the curly braces etc or replace the
> start and end values of i with something like
> {i,mylist[[4]],mylist[[Length[mylist]]]} so it would work on any list.
-----------------------------

Or you can use the KSubsets function from the standard package
DiscreteMath`Combinatorica` as follows:

In[1]:=
<<DiscreteMath`Combinatorica`

In[2]:=
x={1,2,3,4,5,6,7,8}

Out[2]=
{1,2,3,4,5,6,7,8}

In[3]:=
s = KSubsets[x,4]

Out[3]=
{{1, 2, 3, 4}, {1, 2, 3, 5}, {1, 2, 3, 6}, {1, 2, 3, 7}, 
  {1, 2, 3, 8}, {1, 2, 4, 5}, {1, 2, 4, 6}, {1, 2, 4, 7}, 
  {1, 2, 4, 8}, {1, 2, 5, 6}, {1, 2, 5, 7}, {1, 2, 5, 8}, 
  {1, 2, 6, 7}, {1, 2, 6, 8}, {1, 2, 7, 8}, {1, 3, 4, 5}, 
  {1, 3, 4, 6}, {1, 3, 4, 7}, {1, 3, 4, 8}, {1, 3, 5, 6}, 
  {1, 3, 5, 7}, {1, 3, 5, 8}, {1, 3, 6, 7}, {1, 3, 6, 8}, 
  {1, 3, 7, 8}, {1, 4, 5, 6}, {1, 4, 5, 7}, {1, 4, 5, 8}, 
  {1, 4, 6, 7}, {1, 4, 6, 8}, {1, 4, 7, 8}, {1, 5, 6, 7}, 
  {1, 5, 6, 8}, {1, 5, 7, 8}, {1, 6, 7, 8}, {2, 3, 4, 5}, 
  {2, 3, 4, 6}, {2, 3, 4, 7}, {2, 3, 4, 8}, {2, 3, 5, 6}, 
  {2, 3, 5, 7}, {2, 3, 5, 8}, {2, 3, 6, 7}, {2, 3, 6, 8}, 
  {2, 3, 7, 8}, {2, 4, 5, 6}, {2, 4, 5, 7}, {2, 4, 5, 8}, 
  {2, 4, 6, 7}, {2, 4, 6, 8}, {2, 4, 7, 8}, {2, 5, 6, 7}, 
  {2, 5, 6, 8}, {2, 5, 7, 8}, {2, 6, 7, 8}, {3, 4, 5, 6}, 
  {3, 4, 5, 7}, {3, 4, 5, 8}, {3, 4, 6, 7}, {3, 4, 6, 8}, 
  {3, 4, 7, 8}, {3, 5, 6, 7}, {3, 5, 6, 8}, {3, 5, 7, 8}, 
  {3, 6, 7, 8}, {4, 5, 6, 7}, {4, 5, 6, 8}, {4, 5, 7, 8}, 
  {4, 6, 7, 8}, {5, 6, 7, 8}}

In[4]:=
Length[%]

Out[4]=
70
-- 
-----------------------------
Dick Zacher                    
Performance Engineering Dept., Tandem Computers
zacher_dick at tandem.com     phone: 408-285-5746     fax:   408-285-7079


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