How to design a "Dynamic inventory system" for a point and click game?

Question

I have done lots of research on invetory system for point and click game in Lua and corona. I have come across this example,I am doing something similar to this,but I need a dynamic inventory system. I mean if I have 4 slots,and all them are full the fifth object go to next slot,so there will be an arrow to the right so I can click on ;and go to the next page. And imagine there are 5 items,and I have 4 slots,the fifth slot would be on the next page. I use the third item,and third slot would then be empty,so I want the fourth and fifth item automatically move back to third and fourth slot. I have hard time figuring this out. Thanks for advance.

local myInventoryBag={}
local maxItems = 10 -- change this to how many you want

myInventoryBag[5]=3 -- Hammer for instance
myInventoryBag[4]=7 -- A metal Pipe for instance

local function getImageForItem(thisItem)
    local itemNumber = tonumber(thisItem)
    local theImage=""

    if itemNumber==3 then
        theImage="hammer.png"
    elseif itemNumber == 7 then
        theImage="metalpipe.png"
    elseif ... -- for other options
        ...
    else
        return nil
    end

    local image = display.newImage(theImage)
    return image
end

local function displayItems()
    local i
    for i=1,#myInventoryBag do
        local x = 0  -- calculate based on the i
        local y = 0 --  calculate based on the i

        local image = getImageForItem(myInventoryBag[i])

        if image==nil then return end

        image.setReferencePoint(display.TopLeftReferencePoint)
        image.x = x
        image.y = y
    end
end

Answer 1

local itemImages =
{
    [0] = display.newImage('MISSING_ITEM_IMAGE.PNG'),
    [3] = display.newImage('hammer.png'),
    [7] = display.newImage('metalpipe.png'),
}

function getImageForItem(itemId)
    return itemImages[itemId] or itemImages[0]
end

local myInventoryBag={}
local maxItems = 10 -- change this to how many you want
local visibleItems = 4 -- show this many items at a time (with arrows to scroll to others)

-- show inventory items at index [first,last]
local function displayInventoryItems(first,last)
    local x = 0 -- first item goes here
    local y = 0 -- top of inventory row
    for i=first,last do
        image = getImageForItem(myInventoryBag[i])
        image.x = x
        image.y = y
        x = x + image.width
    end
end

-- show inventory items on a given "page"
local function displayInventoryPage(page)
    page = page or 1 -- default to showing the first page
    if page > maxItems then
        -- error! handle me!
    end
    local first = (page - 1) * visibleItems + 1
    local last = first + visibleItems - 1
    displayInventoryItems(first, last)
end

myInventoryBag[5] = 3 -- Hammer for instance
myInventoryBag[4] = 7 -- A metal Pipe for instance

displayInventoryPage(1)
displayInventoryPage(2)