That's what Ada does, yes. You'd let the array (or whatever collection) do the work for you:

  for I in A'Range loop
    A(I) = A(I) + A(I);
  end loop;

Whatever the range is, this will work. If you really need the first and last elements or want to be explicit:

  Start := A'First;
  End := A'Last;

And if the type of the range (since any discrete type can be used) doesn't support simple incrementing with +1 or similar, you can use 'Succ to step through:

  Index := A'First;
  Index := Whatever_Type'Succ(Index);

Also 'Pred to work backwards. Those can be wrapped up in a simpler function if desired.