Python -找到信用卡的每月净现值

Question

我在运行这个程序：

balance = 320000
annualInterestRate = 0.2

monthlyPayment = 0.0
monthlyInterestRate = annualInterestRate/12
low = balance / 12.0
high = ((balance * ((1.0 + monthlyInterestRate)**12.0))/12.0)
monthlyPayment = (high+low)/2
while balance != 0:
    newBalance = balance
    for x in range(1, 13):
        lastMonthBalance = newBalance - monthlyPayment
        newBalance = lastMonthBalance + ((monthlyInterestRate) * lastMonthBalance)
        if -0.002 <= newBalance <= 0.002:
            balance = 0

    if newBalance < 0.0:
        low = 0
        high = monthlyPayment
    elif newBalance > 0.0:
        low = monthlyPayment

    monthlyPayment = (low + high) / 2

print "Lowest Payment: %.2f" % monthlyPayment

最初，我试图找到使我的newBalance等于0的月付款，但是由于处理时间的限制，我选择了一种不太准确的方法。

然而，当我把界限从-0.2/0.2改为-0.002/0.002时，我的答案从14578.55变为29157.09，我不明白为什么？

任何帮助都是非常感谢的。

Answer 1

因此，我不完全确定为什么，但通过删除low = 0的行，代码才能正常工作。

工作守则是：

#Write a program that calculates the minimum fixed monthly payment needed in order pay off a credit card balance within 12 months. By a fixed monthly payment, we mean a single number which does not change each month, but instead is a constant amount that will be paid each month.

balance = 999999
annualInterestRate = 0.18

monthlyPayment = 0.0
monthlyInterestRate = annualInterestRate/12
low = balance / 12.0
high = ((balance * ((1.0 + monthlyInterestRate)**12.0))/12.0)
monthlyPayment = (high+low)/2
while balance != 0:
    newBalance = balance
    for x in range(1, 13):
        lastMonthBalance = newBalance - monthlyPayment
        newBalance = lastMonthBalance + ((monthlyInterestRate) * lastMonthBalance)
        if -0.05 <= newBalance <= 0.05:
            balance = 0

    if newBalance < 0.0:
        high = monthlyPayment
    elif newBalance > 0.0:
        low = monthlyPayment

    monthlyPayment = (low + high) / 2

print "Lowest Payment: %.2f" % monthlyPayment

如果有人能向我解释为什么会发生这种情况，我将非常感激。