print("First number:")
let num1 = Double(readLine(strippingNewline: true) ?? "2" ) ?? 1
print("Second number:")
let num2 = Double(readLine(strippingNewline: true) ?? "2" ) ?? 1
let operat = readLine(strippingNewline: true) ?? "+"
print("请输入运算符（+、-、*、/）：\(num1) \(operat) \(num2)")
var result: Double!
switch operat {
case "+":
result = num1 + num2
case "-":
result = num1 - num2
case "*":
result = num1 * num2
case "/":
result = num1 / num2
default:
print("Not valid")
break
}
print("the result is：\(result)")
