信用计算器产生错误的输出

Question

我需要计算在 12 个月内还清信用卡余额所需的最低固定月供。正确答案应该是310，但我得到了340。我编辑了几个小时的代码，但没有找到合适的解决方案。这里有什么问题？如何修复它？

balance = 3329
annualInterestRate = 0.2
payment = 10

def year_balance(init_payment, init_balance):
    """ Calculates total debt after 12 months """
    interest_sum = 0
    for month in range(12):
        # balance after monthly payment
        unpaid = init_balance - init_payment 
        # monthly interest of remaining balance
        interest = unpaid * (annualInterestRate / 12.0) 
        interest_sum += interest
    yearly = init_balance + interest_sum # total debt after 12 months
    return yearly

total = year_balance(payment, balance) # total debt after 12 months

while total - payment * 12 > 0: 
# checks if payment is big enough to fully repay the credit after 12 months 
    payment += 10

print "Lowest payment: ", payment

Answer 1

我们需要运行每个新付款金额的余额计数功能，因为付款越多，利息金额就越小。所以

balance = 3329
annualInterestRate = 0.2
payment = 10


def year_balance(init_payment, init_balance):
    """ Calculates total debt after 12 months """
    interest_sum = 0
    unpaid = init_balance
    for month in range(12):
        # balance after monthly payment
        unpaid -= init_payment
        # monthly interest of remaining balance
        unpaid += unpaid * (annualInterestRate / 12.0)
    return unpaid

while year_balance(payment, balance) > 0:
    payment += 10

print("Lowest payment: ", payment)

注意：这是为了防止您在信用期开始之前付款。如果你一个月后才这样做，你应该先加上月利率。

Answer 2

您实际上不需要迭代来计算每月还款额。相反，您可以使用封闭形式的解决方案：

loan_amount = 3329
annual_interest_rate = 0.2
monthly_repayment = ((loan_amount * annual_interest_rate / 12.) /
                     (1 - (1 + annual_interest_rate / 12.) ** -12))
print monthly_repayment

这假设每月还款和每月复利。对于一般公式，请参见 here.

信用计算器产生错误的输出

Credit calculator generates wrong output

python

python-2.7

calculator

currency