使用ComputerCraft进行Lua编程

我创建了一个小程序，询问您想要一个挖掘乌龟挖掘的长度、宽度和高度。当我在高级计算机上运行它时，它让我提示长度、宽度和高度，但是我会遇到一个错误。错误如下：

miner:39: attempt to index ? ( a nil value)

以下是我的代码：

term.clear()
term.setCursorPos(1,1)

write("Length:")
length = read()
print()
write("Confirm:")
ul = read()
print()

write("Width:")
width = read()
print()
write("Confirm:")
uw = read()
print()

write("Height:")
height = read()
print()
write("Confirm:")
uh = read()
print()

local totcount = ul + uw + uh
local subcount = 0

function Length()

repeat

    turtle.dig()
    turtle.forward()
    length = length - 1
    subcount = subcount + 1

until length == 0
length = ul

end

function Width()

repeat

    turtle.dig()
    turtle.forward()
    width = width - 1
    subcount = subcount + 1

until width == 0
width = uw
end

function Height()
turtle.digDown()
turtle.down()
height = height - 1
subcount = subcount + 1
end

function Turn()

turtle.turnRight()

end

repeat

Length()
Turn()
Width()
Turn()
Length()
Turn()
Width()
Turn()
Height()

until subcount == totcount