PaymentCalculator

local firstInput = 0
local secondInput = 0
local operationInput = ""
local screenWidth, screenHeight = term.getSize()
local username = ""
local selection = 1

if os.getComputerLabel() == nil then username = "New User"
else username = os.getComputerLabel()

sp = textutils.slowPrint

local function cPrint(height, string)
    xPos = screenWidht/2 - string.len(string)/2
    term.setCursorPos(xPos, height)
    term.write(string)
end

local function rPrint(heightm string)
    xPos = screenWidht - string.len(string)
    term.setCursorPos(xPos, height)
    term.write(string)
end

function getInput()
    cPrint(6, "First number: ")
    firstInput = read()

    cPrint(7, "Operation: ")
    operationInput = read()

    cPrint(8, "Second number: ")
    secondInput = read()
end

function printWelcome()
    cPrint(7, "")
    sp("Welcome ".. username)
    sleep(2)
    term.clear()
    term.setCursorPos(1,1)
end

function printMenu()
    if selection == 1 then cPrint(6, "[ Calculate ]")
    else                   cPrint(6, "  Calculate  ") end

    if selection == 2 then cPrint(7, "[ Exit ]")
    else                   cPrint(7, "  Exit  ") end
end

function determineOperation(operationInput, answer)
    local operations = {'+', '-', '*', '/', '%'}
    local operation = 1

    operationInput = tostring(operationInput)

    for i=1, #operations do
        if operationInput == operations[i] then
            if operations[i] = 1 then operation = 1
            elseif operations[i] == 2 then operation = 2
            elseif operations[i] == 3 then operation = 3
            elseif operations[i] == 4 then operation = 4
            elseif operations[i] == 5 then operation = 5
            end
        else
            operation = 0
        end
    end

    if operation == 0 then return answer = nil
    elseif operation == 1 then return answer = firstInput+secondInput
    elseif operation == 2 then return answer = firstInput-secondInput
    elseif operation == 3 then return answer = firstInput*secondInput
    elseif operation == 4 then return answer = firstInput/secondInput
    elseif operation == 5 then return answer = firstInput%secondInput end
end

while true do
    local answer = 0
    term.clear()
    term.setCursorPos(1,1)

    printWelcome()
    printMenu()
        event, key = os.pullEvent("key")

        if key == 200 and selection > 1 then selection = selection - 1
        elseif key == 208 and selection < 2 then selection = selection+1
        end

        if selection == 1 and key == 28 then
            getInput()
            if determineOperation(operationInput, answer) ==  nil then
                cPrint(10, "Not a valid operation.")
                sleep(2)
            else
                cPrint(10, "The answer is " ..answer)
                sleep(2)
            end
        elseif selection == 2 and key == 28 then break
        end
    -- End menu operations
end