Recursive programs in XCAS.

[Han]

Here's a CAS version that works on the Prime. Notice the upper-case MAKELIST command (which is different from the lower-case makelist command). MAKELIST is a non-CAS command. This was a quick conversion from non-CAS to CAS (which does not support CASE as far as I can tell; it does have CASES, though, which seems to be the same as piecewise). You can make the code much cleaner using several return() calls.

PHP Code:

#cas
comboset(s,n):=
BEGIN
local l;
local sl;
local tl;
local k;
local j;
local m;
local t;

m:=size(s);
l:={};
sl:={};
tl:={};

if n==m then l:={s}; else
  if n==1 then l:=MAKELIST({s(X)},X,1,m); else 
    if n>m then l:={}; else
      for k from 2 to m-n+2 do
        sl:=sub(s,k,m); 
        tl:=comboset(sl,n-1);
        for j from 1 to size(tl) do
          tl[j]:=concat({s[k-1]},tl[j]);
        end;
        l:=concat(l,tl); 
      end;
    end;
  end;
end;

return(l);

END;
#end