Cactus Builder

So, this script is awesome, but it doesn't work completely. Luckily it works well enough to make it worth sharing. When i wrote it, I totally didn't think about the fact that the turtle has to place a cactus down. I may rewrite this one later if there is a need for it. but it builds the basic structure for you, and all you have to do is add the cactus manually.

-- Clearing Terminal
term.clear()
term.setCursorPos(1,1)

-- Setting up Variables
local tArgs = { ... }
local fuel = 1
local sand = 2
local block = 3
local cactus = 4
local gone = 0
local slots = {"fuel", "sand", "blocks", "cactus"}

if #tArgs ~= 1 then
    -- Missing Height Argument
    print("Usage: Cactus <layers>")
    print("Slot 1: Fuel")
    print("Slot 2: Sand")
    print("Slot 3: Blocks")
    print("Slot 4: Cactus")
    error()
end

local height = tArgs[1]

--Missing Items Check
if (turtle.getItemCount(fuel) < 1 or turtle.getItemCount(sand) < 1 or turtle.getItemCount(block) < 1 or turtle.getItemCount(cactus) < 1) then
    print("Missing Critical Item")
    error()
end

-- Fuel Checking Function
function tFuel(f)
    if turtle.getFuelLevel() < 60 then
        turtle.select(f)
        if not turtle.refuel(1) then
            print("Need Fuel")
            while turtle.getItemCount(f) < 2 do sleep(0.5) end
        end
    end
end

--Place Row
function row(slot)
    for p = 1, 4 do
        turtle.select(slot)
        if turtle.getItemCount(slot) < 3 then
            print("Need more " .. slots[slot])
            while turtle.getItemCount(slot) < 3 do sleep(0.5) end
        end
        turtle.forward()
        turtle.turnLeft()
        turtle.place()
        turtle.turnRight()
        turtle.turnRight()
        turtle.place()
        turtle.turnLeft()
        if slot == cactus then
            turtle.select(block)
            if turtle.getItemCount(block) < 3 then
                print("Need more " .. slots[block])
                while turtle.getItemCount(block) < 3 do sleep(0.5) end
            end
            turtle.placeUp()
        end
        turtle.forward()
        end
end

print("Starting cactus farm of " .. height .. " layers")

for g = 1, height, 2 do
    tFuel(fuel)
    turtle.up()
    row(block)
    turtle.up()
    turtle.turnRight()
    turtle.turnRight()
    row(sand)
    turtle.up()
    turtle.turnRight()
    turtle.turnRight()
    row(cactus)
    turtle.turnRight()
    turtle.turnRight()
    turtle.up()
    turtle.up()
    turtle.up()
    row(block)
    turtle.up()
    turtle.turnRight()
    turtle.turnRight()
    row(sand)
    turtle.up()
    turtle.turnRight()
    turtle.turnRight()
    row(cactus)
    turtle.turnRight()
    turtle.turnRight()
    turtle.up()
    turtle.up()
end