[GAP Forum] The Add function

[Keshav Rao Kini]

Hello,

Each time you use Add, it adds perm as a new list entry in out. It does NOT add the current value of perm as a new list entry in out, but rather inserts a reference to the list perm. The list perm is added to the list out three times, and by the time the function returns, perm = [1,2,3], so out consists of three [1,2,3].

If you want to remember previous states of a list, you'll always need to make a copy, because this is not done automatically when using functions like Add. Try using for example the ShallowCopy function.

Hope that helps.

Yours,
    Keshav

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2011/1/8 Hebert Pérez-Rosés <hebert.perez at gmail.com<mailto:hebert.perez at gmail.com>>
Dear all,

I'm embarrased to say this, but I'm very puzzled by the behaviour of the Add
function. I have this simple test code:

--------------------------------------
 TestFun:= function(n)
  local perm, out, i;

  out:= [];
  perm:= [];
  for i in [1..n] do
      perm[i]:=0;
  od;
  for i in [1..n] do
      perm[i]:= i;
      Add(out, perm);
  od;
  return out;
 end;
----------------------------------

If I now make a call to Test(3), I should get [ [1,0,0], [1,2,0], [1,2,3] ],
right? However, I'm getting [ [1,2,3], [1,2,3], [1,2,3] ]. Do you get the
same thing? Can anybody explain to me what's happening here?

Thank you very much in advance,

Hebert Perez-Roses
The University of Newcastle, Australia
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