func changeString(var str : String)  {   //如果参数str前不加var修饰，函数默认str是一个常量
    
    print(str)                           //修改前输出一次str
    
    let resultString: String = "myString Changed."
    
    str = resultString
    
    print(str)                           //修改后输出一次str
    
}

var myString: String = "myString"        //定义一个String类型的常量myString
print(myString)                          //输出myString
changeString(myString)                   //myString调用changeString函数
print(myString)                          //myString输出的值仍为"string"

myString
myString
myString Changed.
myString
Program ended with exit code: 0