编写一个四则运算的程序，要求加减乘除各定义为一个函数来实现

以下是Python语言的实现：

def add(num1, num2):
    return num1 + num2

def subtract(num1, num2):
    return num1 - num2

def multiply(num1, num2):
    return num1 * num2

def divide(num1, num2):
    if num2 == 0:
        print("Error: division by zero")
        return None
    else:
        return num1 / num2

print("Welcome to the calculator program!")

while True:
    operation = input("Please enter an operation (+, -, *, /) or 'quit' to exit: ")
    if operation == "quit":
        break
    elif operation not in ["+", "-", "*", "/"]:
        print("Invalid operation")
        continue
    else:
        num1 = float(input("Please enter the first number: "))
        num2 = float(input("Please enter the second number: "))
        if operation == "+":
            print(add(num1, num2))
        elif operation == "-":
            print(subtract(num1, num2))
        elif operation == "*":
            print(multiply(num1, num2))
        elif operation == "/":
            print(divide(num1, num2))

运行程序后，会提示用户输入一个加减乘除的操作符，然后再输入两个数字。程序根据用户输入的操作符调用相应的函数进行计算，然后输出结果。如果用户输入了无效的操作符，程序会提示用户重新输入。如果用户输入了除数为0的除法操作，程序会输出错误信息。用户输入"quit"可以退出程序。