local C1 = peripheral.wrap("crystal_0")
local m = peripheral.wrap("left")
local itemList = { }        --# place this at or near the beginning of your code

local function drawScreen() --# place this *before* any calls to this function
  m.setBackgroundColor(colors.red)
  m.clear()
  local tmpY = 1
  m.setCursorPos(1,tmpY)
  for i = 1, #itemList do
    m.write(itemList[i].name .. "  QTY: " .. tostring(itemList[i].qty))
    tmpY = tmpY + 1
    m.setCursorPos(1,tmpY)
  end
end

function CheckItems()
  itemList = { }
  local InvSize = C1.getInventorySize()
  for CurrSlot = 1, InvSize-1 do --Checking slots
    local slot = C1.getStackInSlot(CurrSlot)
    if slot ~= nil then -- Slots with stuff in them
      local match = false
      if #itemList == 0 then --# if there are no entries yet, add the first one
        itemList[1] = { name = slot.name }
        itemList[1].slots = { }
        itemList[1].slots[1] = { slot = CurrSlot }
        itemList[1].slots[1].qty = slot.qty
        itemList[1].qty = slot.qty
      else
        for i = 1, #itemList do                 --# cycle through the master item list
          if slot.name == itemList[i].name then --# if the new item's name matches the name of item [i] in the master list then update the info
            itemList[i].slots[#itemList[i].slots+1] = { }
            itemList[i].slots[#itemList[i].slots] = { slot = CurrSlot }
            itemList[i].slots[#itemList[i].slots].qty = slot.qty
            itemList[i].qty = itemList[i].qty + slot.qty
            match = true
            break
          end
        end
        if not match then --# if the item isn't already in the list, add it
          itemList[#itemList+1] = { name = slot.name }
          itemList[#itemList].slots = { }
          itemList[#itemList].slots[1] = { slot = CurrSlot }
          itemList[#itemList].slots[1].qty = slot.qty
          itemList[#itemList].qty = slot.qty
        end
      end
    end
  end
end

CheckItems()
drawScreen()