A constant is a variable that helps to improve the readability of a program. Constants are not defined as a specific type of variable, but rather are used as placeholders in the code. You can only define a constant once and it cannot be modified. Use the following statement to define a constant:
常数是一种有助于提高程序可读性的变量，但它其实并不是变量，而是用作代码中的通配符。常数一经定义就无法再修改。可以使用以下语句定义常数：