-- Your existing Compute function
function Compute(operator, op1, op2)
    if operator == "+" then return op1 + op2 end
    if operator == "-" then return op1 - op2 end
    if operator == "*" then return op1 * op2 end
    if operator == "/" then return op1 / op2 end
    error("Unknown operator: " .. operator)
end

-- The recursive evaluation function
function EvaluatePrefix(token_list)
    if #token_list == 0 then
        error("Invalid expression: missing operands")
    end

    local current_token = table.remove(token_list, 1)
    local number_value = tonumber(current_token)
    
    if number_value ~= nil then
        return number_value
    else
        local left_operand = EvaluatePrefix(token_list)
        local right_operand = EvaluatePrefix(token_list)
        return Compute(current_token, left_operand, right_operand)
    end
end

-- Wrapper Function to parse the custom string format
function MainEvaluate(expression_string)
    local token_list = {}
    local i = 1
    local len = string.len(expression_string)

    while i <= len do
        local char = string.sub(expression_string, i, i)

        if char == "+" or char == "-" or char == "*" or char == "/" then
            table.insert(token_list, char)
            i = i + 1
        elseif char == "&" or char == " " then
            -- Skips both ampersands AND empty spaces for safety
            i = i + 1
        else
            local num_str = ""
            while i <= len do
                local next_char = string.sub(expression_string, i, i)
                if next_char == "&" or next_char == " " or next_char == "+" or next_char == "-" or next_char == "*" or next_char == "/" then
                    break
                end
                num_str = num_str .. next_char
                i = i + 1
            end
            
            if num_str ~= "" then
                table.insert(token_list, num_str)
            end
        end
    end

    local final_result = EvaluatePrefix(token_list)

    if #token_list > 0 then
        error("Invalid expression: too many operands")
    end

    return final_result
end

-- --- USER INPUT LOOP ---

while true do
    io.write("\nEnter expression: ")
    local input = io.read()

    -- Exit condition
    if input == nil or input == "exit" or input == "quit" then
        print("Goodbye!")
        break
    end

    -- Skip empty entries
    if input ~= "" then
        -- pcall runs the function safely so the whole program doesn't crash if the user types a bad formula
        local success, result = pcall(MainEvaluate, input)
        
        if success then
            print("Result: " .. result)
        else
            print("Error: " .. result)
        end
    end
end