CS50 Pset2 Coke Machine程序while循环问题调试求助

需求说明 #

• 机器售卖50美分的可口可乐，仅接受25、10、5美分硬币
• 提示用户逐个插入硬币，每次输入后显示剩余应付款
• 当总投入金额≥50美分时，输出应找零金额；忽略非指定面额的输入

问题代码 #

初始代码 #

coins = [25,10,5]

def main():
    amount_due = 50
    print(f"Amount Due: {amount_due}")
    coin = evaluated(amount_due)


def evaluated(a):
    while True:
        coin = int(input("Insert Coin:").strip())
        if coin != coins:
            print(f"Please enter your coins in the following increments: {coins}")
            return coin
        elif coin == coins:
            continue

def amount_due(coin, amount_due):
   remainder = amount_due-coin
   if remainder <= 0:
        print(f"Change owed: {remainder}")

   if remainder > 0:
        print(f"Amount due: {remainder}")
        return remainder

main()

更新后代码（修正了成员判断逻辑） #

coins = [25,10,5]

def main():
    amount_due = 50
    print(f"Amount Due: {amount_due}")
    coin = evaluated(amount_due)


def evaluated(a):
    while True:
        coin = int(input("Insert Coin:").strip())
        if coin not in coins:
            print(f"Please enter your coins in the following increments: {coins}")
            return coin

        elif coin in coins:
            continue


def amount_due(coin, amount_due):
   remainder = amount_due-coin
   if remainder <= 0:
        print(f"Change owed: {remainder}")

   if remainder > 0:
        print(f"Amount due: {remainder}")
        return remainder

main()

核心问题排查 #

1. evaluated函数逻辑完全错误

   • 遇到有效硬币时用continue，会直接重新进入循环，不会返回有效硬币给主函数
   • 遇到无效硬币时直接return coin，导致主函数拿到无效硬币后没有后续处理
   • 函数参数a完全未使用，属于冗余参数

2. main函数没有循环处理流程

   • 仅调用一次evaluated，无法实现多次插入硬币的需求
   • 未调用amount_due函数来更新剩余应付款或计算找零

3. amount_due函数计算逻辑错误

   • 找零金额应为正数，原代码会输出负数（比如投入60美分，会输出Change owed: -10）
   • 没有处理剩余应付款的循环更新，无法持续接收新硬币

修复后的完整代码 #

coins = [25, 10, 5]

def main():
    amount_due = 50
    print(f"Amount Due: {amount_due}")
    
    while amount_due > 0:
        coin = get_valid_coin()
        amount_due -= coin
        if amount_due <= 0:
            print(f"Change owed: {abs(amount_due)}")
        else:
            print(f"Amount Due: {amount_due}")

def get_valid_coin():
    while True:
        try:
            coin = int(input("Insert Coin: ").strip())
            if coin in coins:
                return coin
            else:
                print(f"Please enter coins in denominations: {coins}")
        except ValueError:
            print("Invalid input. Please enter an integer coin value.")

main()

修复说明 #

• 重命名evaluated为get_valid_coin，语义更清晰，移除冗余参数
• get_valid_coin循环直到获取有效硬币才返回，无效输入仅提示不返回
• main函数通过while循环持续处理硬币插入，直到剩余应付款≤0
• 修正找零计算逻辑，用abs(amount_due)确保输出正数
• 增加try-except捕获非整数输入，提升鲁棒性

内容的提问来源于stack exchange，提问作者Mia