Index expected, got number.

args = {...}
width = tonumber(args[2])
cells = {}
for i = 1,width do
  cells[i] = i
end
function range(index)
  ret = -1
  i = index
  v = cells[i]
  while i <= width and v == cells[i] do
	i = i + 1
	ret = ret + 1
  end
  return ret
end
function selectVertical()
  ret = {}
  dug = {}
  for i = 1,width do ret[i] = 0 end
  for i = 1,width do dug[i] = 0 end
  for i = width,1,-1 do
	ret[i] = 0
	if dug[cells[i]] == 0 then
	  if (i == 1) or (cells[i-1] ~= cells[i]) then
		j = math.random(i, i + range(i))
		ret[j] = 1
		dug[cells[i]] = 1
	  end
	end
  end
  return ret
end