COMBOSET - Subsets of a set S of size n

COMBOSET - Subsets of a set S of size n - Printable Version

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COMBOSET - Subsets of a set S of size n - Han - 03-07-2017 02:25 AM

Usage: COMBOSET(set, size) where set is a set of objects, and size refers to the desired size of the subsets.

Example: COMBOSET({1, "b", (2,-1), [1,2], 3.14}, 3)

{
{1,"b",2-i},
{1,"b",[1,2]},
{1,"b",3.14},
{1,2-i,[1,2]},
{1,2-i,3.14},
{1,[1,2],3.14},
{"b",2-i,[1,2]},
{"b",2-i,3.14},
{"b",[1,2],3.14},
{2-i,[1,2],3.14}
}

Code:

EXPORT COMBOSET(s,n)

BEGIN

  local l:={};

  local sl:={};

  local tl:={};

  local i,j;

  local m:=size(s);

  n:=IP(n); // ensure n is an integer to avoid infinite loop

  case

    if n==m then l:={s}; end;

    if n==1 then l:=makelist({s(X)},X,1,m); end; 

    if n>m then l:={}; end;

    for i from 2 to m-n+2 do

      sl:=sub(s,i,m); 

      tl:=COMBOSET(sl,n-1);

      for j from 1 to size(tl) do

        tl(j):=concat({s(i-1)},tl(j));

      end;

      l:=concat(l,tl); 

    end;

  end;

END;