Slots example (written for LÖVE 11.0.0)

--Friendly reminder that this is only an example of code that should be read and analyzed so that you understand the core concepts behind it
--Pasting it on top of your code will not fix whatever issue you're having :P
function love.load()
    --Decoration
    width = 25
    spacing = 5

    slots = {} --Where your items go
    curSlot = 1 --Selected slot
    numOfSlots = 7 --Total slots size

    for i = 1, numOfSlots do
        slots[i] = {inUse = true} --By default, each slot is in use
    end

    love.window.setMode((width+spacing)*numOfSlots+spacing, spacing*2+width)
end

function love.draw()
    for i, v in ipairs(slots) do
        love.graphics.setColor(.5, .5, .5, 1) --Sets default slot color
        if not v.inUse then --Sets inactive slot color
            love.graphics.setColor(.25, .25, .25, 1)
        end
        if curSlot == i then --Sets highlighted slot color
            love.graphics.setColor(.75, .75, .25, 1)
        end
        love.graphics.rectangle("fill", spacing + (i-1)*(width + spacing), spacing, width, width) --Draw slot

        --This is just the make-shift item representation via colored balls
        love.graphics.setColor(.75, .1, 0, 1)
        if v.inUse then
            love.graphics.setColor(.1, .75, 0, 1)
        end
        love.graphics.circle("fill", spacing + (i-1)*(width + spacing)+width/2, spacing+width/2, width/4, 32)
        love.graphics.setLineWidth(2)
        if curSlot == i then
            love.graphics.setColor(.3, 1, 0, 1)
        end
        love.graphics.circle("line", spacing + (i-1)*(width + spacing)+width/2, spacing+width/2, width/4, 32)

        if curSlot == i then --Highlight slot
            love.graphics.setColor(.25, .25, 0, 1)
            love.graphics.setLineWidth(5)
            love.graphics.rectangle("line", spacing + (i-1)*(width + spacing), spacing, width, width)
        end
    end
end

function love.wheelmoved(x, y)
    if y ~= 0 then
        y = y < 0 and -1 or 1
        local foundSlot = false

        for i = 1, numOfSlots do --Ensures you try all items
            if curSlot then
                curSlot = curSlot + y --Moves the selected slot up or down depending on the mouse wheel movement
            else
                curSlot = 1 --If no slot was selected, make it start at 1
            end

            if curSlot > numOfSlots then --This makes the slot selection wrap around if you reach the end
                curSlot = 1
            elseif curSlot < 1 then
                curSlot = numOfSlots
            end

            if slots[curSlot].inUse then --It'll go on until it finds a valid slot. Once it's found, leave it selected and exit the loop
                foundSlot = true
                break
            end
        end

        if not foundSlot then --In case no slot is in use, we don't highlight any of them
            curSlot = false
        end
    end
end

function love.keypressed(key)
    local number = false

    --This is a cheap way of checking if the key pressed is 1, 2, 3... or kp1, kp2, kp3...
    if string.len(key) == 3 and string.sub(key, 1, 2) == "kp" and tonumber(string.sub(key, -1, -1)) then
        number = tonumber(string.sub(key, -1, -1))
    elseif tonumber(key) then
        number = tonumber(key)
    end

    if number and number >= 1 and number <= numOfSlots then
        slots[number].inUse = not slots[number].inUse

        if slots[number].inUse == false and curSlot == number then --Guarantees that the selection will be over a usable slot
            love.wheelmoved(0, 1) --Kind of a cheat
        elseif slots[number].inUse == true and curSlot == false then
            curSlot = number
        end
    end
end