Python：对传递到函数中的变量所做的操作无法正常工作

Question

def pay(cost, selection,):
    deposit = 0
    deficit = cost - deposit
    change = deposit - cost
    print("That item will cost you $",cost,".")
    deposit = float(input("Enter your money amount (e.g. 1.5 for $1.50, .50 for $0.50, etc.):\n--\n"))
    if deposit < cost:
        while deficit >= cost:
            deficit -= float(input("Please enter an additional $" + str(deficit) + "."))
            if deficit > cost:
                print("Your change is $",change,".")
                print("Thank you for purchasing item#",selection,". Your change is $",change,".")
            returnyn = input("Would you like to make another purchase? Answer with a y or n\n")
            if returnyn == "Y" or returnyn == "y":

                return
    if deposit == cost:
            print("Thank you for purchasing item#",selection,".")
            returnyn = input("Would you like to make another purchase? Answer with a y or n\n")
            if returnyn == "Y" or returnyn == "y":
                return

    if deposit > cost:
        print("Thank you for purchasing item#",selection,". Your change is $",change,".")
        returnyn = input("Would you like to make another purchase? Answer with a y or n\n")
    if returnyn == "Y" or returnyn == "y":

        return

    else:
            exit()


def main():
    cost = 0
selection = 0
loopCount = 0
flag = 0
print("--- Welcome to the HOWE CO. VENDOTRON ---\n  --- Please make a selection below ---\n")
while loopCount < 3 or flag == 0:
    loopCount +=1
    if loopCount == 1:
        print("You have three transactions left.")
    if loopCount == 2:
        print("You have two transactions left.")
    if loopCount == 3:
        print("You have one transaction left.")
    if loopCount == 4:
        print(("Thank you for your business!"))
        break
    print ("Item#\tCost")
    print("1)\t\t$1.75\n2)\t\t$.75\n3)\t\t$.90\n4)\t\t$.75\n5)\t\t$1.25\n6)\t\t$.75\n7)\t\tExit transaction\n-----\n")
    selection = int(input("Please enter the item number:\n"))
    if selection > 6 or selection < 1:
        print("Invalid input.")
    if selection == 1:
        cost = 1.75
        x = pay(cost,selection,)
    if selection == 2:
        cost = 0.75
        x = pay(cost,selection,)
    if selection == 3:
        cost = 0.90
        x = pay(cost,selection,)
    if selection == 4:
        cost = 0.75
        x = pay(cost,selection,)
    if selection == 5:
        cost = 1.25
        x = pay(cost,selection,)
    if selection == 6:
        cost = 0.75
        x = pay(cost,selection,)
    if selection == 7:
          print(("Thank you for your business!"))
          break






main()

你好。我在做一个自动售货机程序。传递到“pay”函数的变量的行为不稳定。例如，如果您输入的金额不足，并且它请求额外的输入，那么它请求的是项目的成本，而不是成本保证金（赤字）。它也不能正确地计算更改。例如，如果我选择项目1（即1.75）并支付2美元，而不是给我正确的找零金额，它会给我以下信息：

“感谢您购买第1项。找您的零钱是-1.75美元。”

我假设支付功能有问题，但我不知道是什么。我已经花了好几个小时研究这个问题了。一定有不正确的数额之间的联系，额外的钱需要的项目和不正确的数额的变化给予了回报。环路有问题吗？缩进？请原谅，如果这是一件非常简单的事情——我只做了几个月的编程，还有很多东西要学

Answer 1

你的计算顺序不对。在以下代码中：

deposit = 0
deficit = cost - deposit
change = deposit - cost
print("That item will cost you $",cost,".")
deposit = float(input("Enter your money amount (e.g. 1.5 for $1.50, .50 for $0.50, etc.):\n \n"))
if deposit < cost:
    while deficit >= cost:

deficit最初总是等于cost，因为您根据用户输入更改了deposit，但不重新计算deficit（它是从初始化为0的deposit计算出来的），所以deficit仍然等于cost，而且由于deposit从未使用过，它将“吃掉”最初不足的存款，而不会记入您的贷方。您需要在读入deposit之后而不是之前计算deficit。您还需要检查赤字是否是> 0，而不是>= cost，因为您试图消除缺口，而不是使缺口小于成本

print("That item will cost you $",cost,".")
deposit = float(input("Enter your money amount (e.g. 1.5 for $1.50, .50 for $0.50, etc.):\n \n"))
deficit = cost - deposit
change = -deficit
if deficit > 0:
    while deficit > 0:

代码可能会使用许多其他清理，但这正是导致所描述的问题的原因