CC BlockPlacer

--ComputerCraft BlockPlacer by Teki

local filledSlot = 0
local details

local function getfirstFilledSlot()
    for i =1,16 do
        details = turtle.getItemDetail(i)
        if details ~= nil then
            return i
        end
    end
    return 0
end

while true do
    if redstone.getInput("back") then
        filledSlot = getfirstFilledSlot()
        if filledSlot ~= 0 then
            turtle.select(filledSlot)
            turtle.place()
        end
    end
    sleep(0)
end