Wall

input = {...}
wallLength = tonumber(input[1])
wallHeight = tonumber(input[2])

-- The starting slot to look for building materials (from left to right, top to bottem)
--ie. 1 2 3
--    4 5 6
--    7 8 9
currentSlot = 1

-- This is a function, it is peice of script that we will run later.
-- If this inventory slot runs out of building blocks, move to the next slot.
function gotoNextSlotIfEmpty()
  if turtle.getItemCount(currentSlot) == 0 then
    currentSlot = currentSlot + 1
    turtle.select(currentSlot)
  end
end

-- This is where the code starts running.
turtle.select(currentSlot)
turtle.up()

for height = 1, wallHeight do

  for length = 1, wallLength do
    turtle.placeDown()
    gotoNextSlotIfEmpty() -- This call runs the function above, functions must be placed above where they are used.
    if length ~= wallLength then --  ~= means 'not equal'
      turtle.forward()
    end
  end

  if height ~= wallHeight then
    turtle.turnRight()
    turtle.turnRight()
    turtle.up()
  end
end